.NET Internals Cookbook Part 6 — Object internals

This is the sixth part of the .NET Internals Cookbook series. For your convenience you can find other parts in the table of contents in Part 0 – Table of contents

Table of Contents

36. What is a shim?

.NET applications are started directly, the same way as native applications are. The entry point of the binary points to mscoree.dll (execution engine) which then checks the metadata of the file, loads correct .NET runtime and passes the execution using _CorExeMain function. The part of mscoree is sometimes called “loading shim”. See this for more details.

This means that in native code you can load .NET manually and execute some managed code. See this to get some more code.

37. How many bytes does an empty structure have?

“Zero” makes perfect sense as the structure is empty so it should occupy no space. However, this would make pointer arithmetic a little weird — you could allocate two structures, one next to another and they would all be pointing to the same address. Because of that the empty structure takes one byte. See this code:

using System;

public class Program
{
	public static void Main()
	{
		Console.WriteLine(System.Runtime.InteropServices.Marshal.SizeOf(typeof(Foo)));
		Console.WriteLine(System.Runtime.InteropServices.Marshal.SizeOf(typeof(Bar)));
	}
}

struct Foo{
}

struct Bar{
	int x;
}

using System;

public class Program

{

public static void Main()

{

Console.WriteLine(System.Runtime.InteropServices.Marshal.SizeOf(typeof(Foo)));

Console.WriteLine(System.Runtime.InteropServices.Marshal.SizeOf(typeof(Bar)));

}

struct Foo{

}

struct Bar{

int x;

}

Output:

1
4

You could also ask how many bytes would empty class have — it is 3 pointers (so 12 bytes on x86 or 24 bytes on x64). This includes space for sync block, space for Method Table pointer, and some more bytes so the GC can work properly.

38. What is a sync block?

Information that the object is locked must be stored somewhere. Whenever you have a reference to an object, it does not point to the object beginning. Actually, before the place pointed by the reference there is a header called “sync block”. You can verify it with the following code and WinDBG:

using System;

namespace ConsoleApp
{
    class Program
    {
        public static void Main()
        {
            var foo = new Foo();
            Console.WriteLine("Foo created");
            Console.ReadLine();

            lock (foo)
            {
                Console.WriteLine("Foo locked");
                Console.ReadLine();
            }
        }
    }

    class Foo
    {
    }
}

using System;

namespace ConsoleApp

{

class Program

{

public static void Main()

{

var foo = new Foo();

Console.WriteLine("Foo created");

Console.ReadLine();

lock (foo)

{

Console.WriteLine("Foo locked");

Console.ReadLine();

}

class Foo

{

}

Attach WinDBG after seeing “Foo created” but before typing anything. You should get this:

0:006> .loadby sos clr

1	0:006> .loadby sos clr

0:006> ~0s
eax=00000000 ebx=00000090 ecx=00000000 edx=00000000 esi=010ff3b4 edi=00000000
eip=7782265c esp=010ff29c ebp=010ff2fc iopl=0         nv up ei pl nz na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000206
ntdll!NtReadFile+0xc:
7782265c c22400          ret     24h

0:006> ~0s

eax=00000000 ebx=00000090 ecx=00000000 edx=00000000 esi=010ff3b4 edi=00000000

eip=7782265c esp=010ff29c ebp=010ff2fc iopl=0 nv up ei pl nz na pe nc

cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00000206

ntdll!NtReadFile+0xc:

7782265c c22400 ret 24h

0:000> !clrstack -a -i
Loaded C:\tmp\mscordbi.dll\5B88589711f000\mscordbi.dll
Loaded C:\Windows\Microsoft.NET\Framework\v4.0.30319\mscordacwks.dll



Dumping managed stack and managed variables using ICorDebug.
=============================================================================
Child SP IP       Call Site
010ff29c 7782265c [NativeStackFrame]
010ff318 72a631df 010ff31c (null) [Managed to Unmanaged transition: 010ff31c]
010ff380 731a0163 [DEFAULT] I4 System.IO.__ConsoleStream.ReadFileNative(Class Microsoft.Win32.SafeHandles.SafeFileHandle,SZArray UI1,I4,I4,Boolean,Boolean,ByRef I4) (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:
  + (Error 0x80131304 retrieving parameter 'hFile')
  + (Error 0x80131304 retrieving parameter 'bytes')
  + (Error 0x80131304 retrieving parameter 'offset')
  + (Error 0x80131304 retrieving parameter 'count')
  + (Error 0x80131304 retrieving parameter 'useFileAPIs')
  + (Error 0x80131304 retrieving parameter 'isPipe')
  + (Error 0x80131304 retrieving parameter 'bytesRead')

LOCALS:
  + (Error 0x80004005 retrieving local variable 'readSuccess')
  + (Error 0x80004005 retrieving local variable 'errorCode')
  + byte& p  = 0
  + (Error 0x80004005 retrieving local variable 'local_3')
  + (Error 0x80004005 retrieving local variable 'p')
  + (Error 0x80004005 retrieving local variable 'charsRead')

010ff3b4 731a0072 [DEFAULT] [hasThis] I4 System.IO.__ConsoleStream.Read(SZArray UI1,I4,I4) (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:
  + (Error 0x80131304 retrieving parameter 'this')
  + (Error 0x80131304 retrieving parameter 'buffer')
  + (Error 0x80131304 retrieving parameter 'offset')
  + (Error 0x80131304 retrieving parameter 'count')

LOCALS:
  + (Error 0x80004005 retrieving local variable 'bytesRead')
  + (Error 0x80004005 retrieving local variable 'errCode')

010ff3d4 72994dd7 [DEFAULT] [hasThis] I4 System.IO.StreamReader.ReadBuffer() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:
  + System.IO.StreamReader this @ 0x3284174

LOCALS:
  + (Error 0x80004005 retrieving local variable 'len')
  + (Error 0x80004005 retrieving local variable 'local_1')

010ff3e4 72a14b18 [DEFAULT] [hasThis] String System.IO.StreamReader.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:
  + System.IO.StreamReader this @ 0x3284174

LOCALS:
  + (Error 0x80004005 retrieving local variable 'sb')
  + (Error 0x80004005 retrieving local variable 'i')
  + (Error 0x80004005 retrieving local variable 'ch')
  + (Error 0x80004005 retrieving local variable 's')

010ff400 732e1c61 [DEFAULT] [hasThis] String System.IO.TextReader+SyncTextReader.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:
  + System.IO.TextReader+SyncTextReader this @ 0x32844f4

LOCALS: (none)

010ff410 73148310 [DEFAULT] String System.Console.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS: (none)

LOCALS: (none)

010ff418 013304a1 [DEFAULT] Void ConsoleApp.Program.Main() (C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe)

PARAMETERS: (none)

LOCALS:
  + ConsoleApp.Foo foo @ 0x32823fc
  + (Error 0x80004005 retrieving local variable 'local_1')
  + (Error 0x80004005 retrieving local variable 'local_2')

010ff450 73a9ebb6 [NativeStackFrame]
Stack walk complete.
=============================================================================

0:000> !clrstack -a -i

Loaded C:\tmp\mscordbi.dll\5B88589711f000\mscordbi.dll

Loaded C:\Windows\Microsoft.NET\Framework\v4.0.30319\mscordacwks.dll

Dumping managed stack and managed variables using ICorDebug.

=============================================================================

Child SP IP Call Site

010ff29c 7782265c [NativeStackFrame]

010ff318 72a631df 010ff31c (null) [Managed to Unmanaged transition: 010ff31c]

010ff380 731a0163 [DEFAULT] I4 System.IO.__ConsoleStream.ReadFileNative(Class Microsoft.Win32.SafeHandles.SafeFileHandle,SZArray UI1,I4,I4,Boolean,Boolean,ByRef I4) (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:

+ (Error 0x80131304 retrieving parameter 'hFile')

+ (Error 0x80131304 retrieving parameter 'bytes')

+ (Error 0x80131304 retrieving parameter 'offset')

+ (Error 0x80131304 retrieving parameter 'count')

+ (Error 0x80131304 retrieving parameter 'useFileAPIs')

+ (Error 0x80131304 retrieving parameter 'isPipe')

+ (Error 0x80131304 retrieving parameter 'bytesRead')

LOCALS:

+ (Error 0x80004005 retrieving local variable 'readSuccess')

+ (Error 0x80004005 retrieving local variable 'errorCode')

+ byte& p = 0

+ (Error 0x80004005 retrieving local variable 'local_3')

+ (Error 0x80004005 retrieving local variable 'p')

+ (Error 0x80004005 retrieving local variable 'charsRead')

010ff3b4 731a0072 [DEFAULT] [hasThis] I4 System.IO.__ConsoleStream.Read(SZArray UI1,I4,I4) (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:

+ (Error 0x80131304 retrieving parameter 'this')

+ (Error 0x80131304 retrieving parameter 'buffer')

+ (Error 0x80131304 retrieving parameter 'offset')

+ (Error 0x80131304 retrieving parameter 'count')

LOCALS:

+ (Error 0x80004005 retrieving local variable 'bytesRead')

+ (Error 0x80004005 retrieving local variable 'errCode')

010ff3d4 72994dd7 [DEFAULT] [hasThis] I4 System.IO.StreamReader.ReadBuffer() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:

+ System.IO.StreamReader this @ 0x3284174

LOCALS:

+ (Error 0x80004005 retrieving local variable 'len')

+ (Error 0x80004005 retrieving local variable 'local_1')

010ff3e4 72a14b18 [DEFAULT] [hasThis] String System.IO.StreamReader.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:

+ System.IO.StreamReader this @ 0x3284174

LOCALS:

+ (Error 0x80004005 retrieving local variable 'sb')

+ (Error 0x80004005 retrieving local variable 'i')

+ (Error 0x80004005 retrieving local variable 'ch')

+ (Error 0x80004005 retrieving local variable 's')

010ff400 732e1c61 [DEFAULT] [hasThis] String System.IO.TextReader+SyncTextReader.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:

+ System.IO.TextReader+SyncTextReader this @ 0x32844f4

LOCALS: (none)

010ff410 73148310 [DEFAULT] String System.Console.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS: (none)

LOCALS: (none)

010ff418 013304a1 [DEFAULT] Void ConsoleApp.Program.Main() (C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe)

PARAMETERS: (none)

LOCALS:

+ ConsoleApp.Foo foo @ 0x32823fc

+ (Error 0x80004005 retrieving local variable 'local_1')

+ (Error 0x80004005 retrieving local variable 'local_2')

010ff450 73a9ebb6 [NativeStackFrame]

Stack walk complete.

=============================================================================

So we can see there is an object. Let’s examine it:

0:000> !do 0x32823fc
Name:        ConsoleApp.Foo
MethodTable: 01274da4
EEClass:     012716f8
Size:        12(0xc) bytes
File:        C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe
Fields:
None

0:000> !do 0x32823fc

Name: ConsoleApp.Foo

MethodTable: 01274da4

EEClass: 012716f8

Size: 12(0xc) bytes

File: C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe

Fields:

None

And now let’s see raw bytes:

0:000> dd 0x32823fc-4
032823f8  00000000 01274da4 00000000 00000000
03282408  72aac29c 00000000 00000000 00000000
03282418  00000000 00000000 00000000 72aa14c8
03282428  00000000 03282454 03282464 000004b0
03282438  00000100 00000000 00000000 72a9eb40
03282448  00000001 0000fffd 00000000 72aa1768
03282458  00000000 03282444 00000000 72aa17b8
03282468  00000000 03282444 00000000 72a9eee0

0:000> dd 0x32823fc-4

032823f8 00000000 01274da4 00000000 00000000

03282408 72aac29c 00000000 00000000 00000000

03282418 00000000 00000000 00000000 72aa14c8

03282428 00000000 03282454 03282464 000004b0

03282438 00000100 00000000 00000000 72a9eb40

03282448 00000001 0000fffd 00000000 72aa1768

03282458 00000000 03282444 00000000 72aa17b8

03282468 00000000 03282444 00000000 72a9eee0

Notice that I subtracted 4 bytes (this is 32-bit application). You can see Method Table pointer and 4 zeros just before that.

Let’s now continue our application and take a lock.

0:000> g

0:000> g

Type anything, hit enter, see “Foo locked”, break again and repeat last action:

0:008> dd 0x32823fc-4
032823f8  00000001 01274da4 00000000 00000000
03282408  72aac29c 00000000 00000000 00000000
03282418  00000000 00000000 00000000 72aa14c8
03282428  00000000 03282454 03282464 000004b0
03282438  00000100 00000000 00000000 72a9eb40
03282448  00000001 0000fffd 00000000 72aa1768
03282458  00000000 03282444 00000000 72aa17b8
03282468  00000000 03282444 00000000 72a9eee0

0:008> dd 0x32823fc-4

032823f8 00000001 01274da4 00000000 00000000

03282408 72aac29c 00000000 00000000 00000000

03282418 00000000 00000000 00000000 72aa14c8

03282428 00000000 03282454 03282464 000004b0

03282438 00000100 00000000 00000000 72a9eb40

03282448 00000001 0000fffd 00000000 72aa1768

03282458 00000000 03282444 00000000 72aa17b8

03282468 00000000 03282444 00000000 72a9eee0

You can see that now there is value one before MT pointer. This is called “thin lock” which embeds locking information as a bit pattern.

Sync block is also used for caching hash code, COM interop and other things. Because there are only 4 bytes and we cannot put everything there, at some point it will get inflated so the sync block will be a pointer to the sync blocks table. You can read more here.

39. How many app domains are there by default?

3, as shown by WinDBG:

0:008> !dumpdomain
--------------------------------------
System Domain:      7411d5a8
LowFrequencyHeap:   7411d8cc
HighFrequencyHeap:  7411d918
StubHeap:           7411d964
Stage:              OPEN
Name:               None
--------------------------------------
Shared Domain:      7411d258
LowFrequencyHeap:   7411d8cc
HighFrequencyHeap:  7411d918
StubHeap:           7411d964
Stage:              OPEN
Name:               None
Assembly:           01391f10 [C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll]
ClassLoader:        01392df0
  Module Name
725f1000    C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll

--------------------------------------
Domain 1:           01365180
LowFrequencyHeap:   013655ec
HighFrequencyHeap:  01365638
StubHeap:           01365684
Stage:              OPEN
SecurityDescriptor: 013667c8
Name:               ConsoleApp3.exe
Assembly:           01391f10 [C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll]
ClassLoader:        01392df0
SecurityDescriptor: 01391dc0
  Module Name
725f1000    C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll

Assembly:           0139e6b0 [C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe]
ClassLoader:        01391a50
SecurityDescriptor: 0139e628
  Module Name
01274024    C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe

0:008> !dumpdomain

--------------------------------------

System Domain: 7411d5a8

LowFrequencyHeap: 7411d8cc

HighFrequencyHeap: 7411d918

StubHeap: 7411d964

Stage: OPEN

Name: None

--------------------------------------

Shared Domain: 7411d258

LowFrequencyHeap: 7411d8cc

HighFrequencyHeap: 7411d918

StubHeap: 7411d964

Stage: OPEN

Name: None

Assembly: 01391f10 [C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll]

ClassLoader: 01392df0

Module Name

725f1000 C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll

--------------------------------------

Domain 1: 01365180

LowFrequencyHeap: 013655ec

HighFrequencyHeap: 01365638

StubHeap: 01365684

Stage: OPEN

SecurityDescriptor: 013667c8

Name: ConsoleApp3.exe

Assembly: 01391f10 [C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll]

ClassLoader: 01392df0

SecurityDescriptor: 01391dc0

Module Name

725f1000 C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll

Assembly: 0139e6b0 [C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe]

ClassLoader: 01391a50

SecurityDescriptor: 0139e628

Module Name

01274024 C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe

These are:

System app domain — loads mscorlib, creates OutOfMemoryException, StackoverflowException and ExecutionEngineException (so they can always be thrown, even if there is not enough memory) and manages string interning
Shared app domain — contains mscorlib, types in System namespace and some code
Default — contains user code and resources

40. Does a value type have a method table?

Of course it does, somehow you need to be able to call virtual methods and box objects. See this code:

using System;

namespace ConsoleApp
{
    class Program
    {
        public static void Main()
        {
            object foo = new Foo();
            Bar(foo);
        }

        static void Bar(object foo)
        {
            Console.WriteLine(foo.ToString());
            Console.ReadLine();
        }
    }

    struct Foo
    {
        public override string ToString()
        {
            return "Some string";
        }
    }
}

using System;

namespace ConsoleApp

{

class Program

{

public static void Main()

{

object foo = new Foo();

Bar(foo);

}

static void Bar(object foo)

{

Console.WriteLine(foo.ToString());

Console.ReadLine();

}

struct Foo

{

public override string ToString()

{

return "Some string";

}

Using WinDBG:

0:006> ~0s
eax=00000000 ebx=00000090 ecx=00000000 edx=00000000 esi=0095eec4 edi=00000000
eip=7782265c esp=0095edac ebp=0095ee0c iopl=0         nv up ei pl nz na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000206
ntdll!NtReadFile+0xc:
7782265c c22400          ret     24h

0:006> ~0s

eax=00000000 ebx=00000090 ecx=00000000 edx=00000000 esi=0095eec4 edi=00000000

eip=7782265c esp=0095edac ebp=0095ee0c iopl=0 nv up ei pl nz na pe nc

cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00000206

ntdll!NtReadFile+0xc:

7782265c c22400 ret 24h

0:000> .loadby sos clr

1	0:000> .loadby sos clr

0:000> !name2ee * Program
Module:      725f1000
Assembly:    mscorlib.dll
--------------------------------------
Module:      00c94024
Assembly:    ConsoleApp3.exe

0:000> !name2ee * Program

Module: 725f1000

Assembly: mscorlib.dll

--------------------------------------

Module: 00c94024

Assembly: ConsoleApp3.exe

0:000> !dumpmodule -mt 00c94024
Name:       C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe
Attributes: PEFile SupportsUpdateableMethods
Assembly:   00b9d508
LoaderHeap:              00000000
TypeDefToMethodTableMap: 00c90038
TypeRefToMethodTableMap: 00c90048
MethodDefToDescMap:      00c90094
FieldDefToDescMap:       00c900a8
MemberRefToDescMap:      00000000
FileReferencesMap:       00c900b0
AssemblyReferencesMap:   00c900b4
MetaData start address:  005c20ac (1632 bytes)

Types defined in this module

      MT  TypeDef Name
------------------------------------------------------------------------------
00c94d54 0x02000002 ConsoleApp.Program
00c94dbc 0x02000003 ConsoleApp.Foo

Types referenced in this module

      MT    TypeRef Name
------------------------------------------------------------------------------
72a9eee0 0x02000010 System.Object
72aa0234 0x02000011 System.ValueType
72aac514 0x02000012 System.Console

0:000> !dumpmodule -mt 00c94024

Name: C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe

Attributes: PEFile SupportsUpdateableMethods

Assembly: 00b9d508

LoaderHeap: 00000000

TypeDefToMethodTableMap: 00c90038

TypeRefToMethodTableMap: 00c90048

MethodDefToDescMap: 00c90094

FieldDefToDescMap: 00c900a8

MemberRefToDescMap: 00000000

FileReferencesMap: 00c900b0

AssemblyReferencesMap: 00c900b4

MetaData start address: 005c20ac (1632 bytes)

Types defined in this module

MT TypeDef Name

------------------------------------------------------------------------------

00c94d54 0x02000002 ConsoleApp.Program

00c94dbc 0x02000003 ConsoleApp.Foo

Types referenced in this module

MT TypeRef Name

------------------------------------------------------------------------------

72a9eee0 0x02000010 System.Object

72aa0234 0x02000011 System.ValueType

72aac514 0x02000012 System.Console

0:000> !dumpmt -md 00c94dbc 
EEClass:         00c916f8
Module:          00c94024
Name:            ConsoleApp.Foo
mdToken:         02000003
File:            C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe
BaseSize:        0xc
ComponentSize:   0x0
Slots in VTable: 5
Number of IFaces in IFaceMap: 0
--------------------------------------
MethodDesc Table
   Entry MethodDe    JIT Name
00d20058 00c94da8   NONE ConsoleApp.Foo.ToString()
72989dd0 725f85bc PreJIT System.ValueType.Equals(System.Object)
7290c578 725f85e4   NONE System.ValueType.GetHashCode()
729ebf8c 725f80b0 PreJIT System.Object.Finalize()
00d20508 00c94db0    JIT ConsoleApp.Foo.ToString()

0:000> !dumpmt -md 00c94dbc

EEClass: 00c916f8

Module: 00c94024

Name: ConsoleApp.Foo

mdToken: 02000003

File: C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe

BaseSize: 0xc

ComponentSize: 0x0

Slots in VTable: 5

Number of IFaces in IFaceMap: 0

--------------------------------------

MethodDesc Table

Entry MethodDe JIT Name

00d20058 00c94da8 NONE ConsoleApp.Foo.ToString()

72989dd0 725f85bc PreJIT System.ValueType.Equals(System.Object)

7290c578 725f85e4 NONE System.ValueType.GetHashCode()

729ebf8c 725f80b0 PreJIT System.Object.Finalize()

00d20508 00c94db0 JIT ConsoleApp.Foo.ToString()

However, WinDBG is clever. See this:

0:000> !clrstack -a -i
Loaded C:\tmp\mscordbi.dll\5B88589711f000\mscordbi.dll
Loaded C:\Windows\Microsoft.NET\Framework\v4.0.30319\mscordacwks.dll



Dumping managed stack and managed variables using ICorDebug.
=============================================================================
Child SP IP       Call Site
0095edac 7782265c [NativeStackFrame]
0095ee28 72a631df 0095ee2c (null) [Managed to Unmanaged transition: 0095ee2c]
0095ee90 731a0163 [DEFAULT] I4 System.IO.__ConsoleStream.ReadFileNative(Class Microsoft.Win32.SafeHandles.SafeFileHandle,SZArray UI1,I4,I4,Boolean,Boolean,ByRef I4) (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:
  + (Error 0x80131304 retrieving parameter 'hFile')
  + (Error 0x80131304 retrieving parameter 'bytes')
  + (Error 0x80131304 retrieving parameter 'offset')
  + (Error 0x80131304 retrieving parameter 'count')
  + (Error 0x80131304 retrieving parameter 'useFileAPIs')
  + (Error 0x80131304 retrieving parameter 'isPipe')
  + (Error 0x80131304 retrieving parameter 'bytesRead')

LOCALS:
  + (Error 0x80004005 retrieving local variable 'readSuccess')
  + (Error 0x80004005 retrieving local variable 'errorCode')
  + byte& p  = 0
  + (Error 0x80004005 retrieving local variable 'local_3')
  + (Error 0x80004005 retrieving local variable 'p')
  + (Error 0x80004005 retrieving local variable 'charsRead')

0095eec4 731a0072 [DEFAULT] [hasThis] I4 System.IO.__ConsoleStream.Read(SZArray UI1,I4,I4) (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:
  + (Error 0x80131304 retrieving parameter 'this')
  + (Error 0x80131304 retrieving parameter 'buffer')
  + (Error 0x80131304 retrieving parameter 'offset')
  + (Error 0x80131304 retrieving parameter 'count')

LOCALS:
  + (Error 0x80004005 retrieving local variable 'bytesRead')
  + (Error 0x80004005 retrieving local variable 'errCode')

0095eee4 72994dd7 [DEFAULT] [hasThis] I4 System.IO.StreamReader.ReadBuffer() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:
  + System.IO.StreamReader this @ 0x2be4150

LOCALS:
  + (Error 0x80004005 retrieving local variable 'len')
  + (Error 0x80004005 retrieving local variable 'local_1')

0095eef4 72a14b18 [DEFAULT] [hasThis] String System.IO.StreamReader.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:
  + System.IO.StreamReader this @ 0x2be4150

LOCALS:
  + (Error 0x80004005 retrieving local variable 'sb')
  + (Error 0x80004005 retrieving local variable 'i')
  + (Error 0x80004005 retrieving local variable 'ch')
  + (Error 0x80004005 retrieving local variable 's')

0095ef10 732e1c61 [DEFAULT] [hasThis] String System.IO.TextReader+SyncTextReader.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:
  + System.IO.TextReader+SyncTextReader this @ 0x2be44d0

LOCALS: (none)

0095ef20 73148310 [DEFAULT] String System.Console.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS: (none)

LOCALS: (none)

0095ef28 00d204eb [DEFAULT] Void ConsoleApp.Program.Bar(Object) (C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe)

PARAMETERS:
  + ConsoleApp.Foo foo @ 0x2be23b8

LOCALS: (none)

0095ef3c 00d20498 [DEFAULT] Void ConsoleApp.Program.Main() (C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe)

PARAMETERS: (none)

LOCALS:
  + ConsoleApp.Foo foo @ 0x2be23b8
  + (Error 0x80004005 retrieving local variable 'local_1')

0095ef50 73a9ebb6 [NativeStackFrame]
Stack walk complete.
=============================================================================

0:000> !clrstack -a -i

Loaded C:\tmp\mscordbi.dll\5B88589711f000\mscordbi.dll

Loaded C:\Windows\Microsoft.NET\Framework\v4.0.30319\mscordacwks.dll

Dumping managed stack and managed variables using ICorDebug.

=============================================================================

Child SP IP Call Site

0095edac 7782265c [NativeStackFrame]

0095ee28 72a631df 0095ee2c (null) [Managed to Unmanaged transition: 0095ee2c]

0095ee90 731a0163 [DEFAULT] I4 System.IO.__ConsoleStream.ReadFileNative(Class Microsoft.Win32.SafeHandles.SafeFileHandle,SZArray UI1,I4,I4,Boolean,Boolean,ByRef I4) (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:

+ (Error 0x80131304 retrieving parameter 'hFile')

+ (Error 0x80131304 retrieving parameter 'bytes')

+ (Error 0x80131304 retrieving parameter 'offset')

+ (Error 0x80131304 retrieving parameter 'count')

+ (Error 0x80131304 retrieving parameter 'useFileAPIs')

+ (Error 0x80131304 retrieving parameter 'isPipe')

+ (Error 0x80131304 retrieving parameter 'bytesRead')

LOCALS:

+ (Error 0x80004005 retrieving local variable 'readSuccess')

+ (Error 0x80004005 retrieving local variable 'errorCode')

+ byte& p = 0

+ (Error 0x80004005 retrieving local variable 'local_3')

+ (Error 0x80004005 retrieving local variable 'p')

+ (Error 0x80004005 retrieving local variable 'charsRead')

0095eec4 731a0072 [DEFAULT] [hasThis] I4 System.IO.__ConsoleStream.Read(SZArray UI1,I4,I4) (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:

+ (Error 0x80131304 retrieving parameter 'this')

+ (Error 0x80131304 retrieving parameter 'buffer')

+ (Error 0x80131304 retrieving parameter 'offset')

+ (Error 0x80131304 retrieving parameter 'count')

LOCALS:

+ (Error 0x80004005 retrieving local variable 'bytesRead')

+ (Error 0x80004005 retrieving local variable 'errCode')

0095eee4 72994dd7 [DEFAULT] [hasThis] I4 System.IO.StreamReader.ReadBuffer() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:

+ System.IO.StreamReader this @ 0x2be4150

LOCALS:

+ (Error 0x80004005 retrieving local variable 'len')

+ (Error 0x80004005 retrieving local variable 'local_1')

0095eef4 72a14b18 [DEFAULT] [hasThis] String System.IO.StreamReader.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:

+ System.IO.StreamReader this @ 0x2be4150

LOCALS:

+ (Error 0x80004005 retrieving local variable 'sb')

+ (Error 0x80004005 retrieving local variable 'i')

+ (Error 0x80004005 retrieving local variable 'ch')

+ (Error 0x80004005 retrieving local variable 's')

0095ef10 732e1c61 [DEFAULT] [hasThis] String System.IO.TextReader+SyncTextReader.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS:

+ System.IO.TextReader+SyncTextReader this @ 0x2be44d0

LOCALS: (none)

0095ef20 73148310 [DEFAULT] String System.Console.ReadLine() (C:\WINDOWS\Microsoft.Net\assembly\GAC_32\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll)

PARAMETERS: (none)

LOCALS: (none)

0095ef28 00d204eb [DEFAULT] Void ConsoleApp.Program.Bar(Object) (C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe)

PARAMETERS:

+ ConsoleApp.Foo foo @ 0x2be23b8

LOCALS: (none)

0095ef3c 00d20498 [DEFAULT] Void ConsoleApp.Program.Main() (C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe)

PARAMETERS: (none)

LOCALS:

+ ConsoleApp.Foo foo @ 0x2be23b8

+ (Error 0x80004005 retrieving local variable 'local_1')

0095ef50 73a9ebb6 [NativeStackFrame]

Stack walk complete.

=============================================================================

0:000> !do 0x2be23b8
<Note: this object has an invalid CLASS field>
Invalid object

0:000> !do 0x2be23b8

<Note: this object has an invalid CLASS field>

Invalid object

So when we try to dump the object it indicates that it is incorrect. Why? Well, let’s see raw content:

0:000> dd 0x2be23b8
02be23b8  00000000 80000000 72a9eb40 0000000b
02be23c8  006f0053 0065006d 00730020 00720074
02be23d8  006e0069 00000067 00000000 72aac29c
02be23e8  00000000 00000000 00000000 00000000
02be23f8  00000000 00000000 72aa14c8 00000000
02be2408  02be2430 02be2440 000004b0 00000100
02be2418  00000000 00000000 72a9eb40 00000001
02be2428  0000fffd 00000000 72aa1768 00000000

0:000> dd 0x2be23b8

02be23b8 00000000 80000000 72a9eb40 0000000b

02be23c8 006f0053 0065006d 00730020 00720074

02be23d8 006e0069 00000067 00000000 72aac29c

02be23e8 00000000 00000000 00000000 00000000

02be23f8 00000000 00000000 72aa14c8 00000000

02be2408 02be2430 02be2440 000004b0 00000100

02be2418 00000000 00000000 72a9eb40 00000001

02be2428 0000fffd 00000000 72aa1768 00000000

So it shows value, not the reference. The pointer is a bit misleading. Let’s dump the whole stack around the function:

0:000> dd 0095ef3c 
0095ef3c  02be23b4 02be23b4 00000000 0095ef54
0095ef4c  73a9ebb6 00b69bd8 0095efa8 73aa1e10
0095ef5c  0095efe4 0095ef98 73b79b20 0095f0b8
0095ef6c  73aa1dc9 a60effde 0095f104 0095f078
0095ef7c  0095f02c 00c94d34 0095efe4 00000000
0095ef8c  a60effde 0095ef60 0095f078 0095f12c
0095ef9c  73b889e0 d5310e5e 00000001 0095f010
0095efac  73aa7994 00000000 00000000 00000001

0:000> dd 0095ef3c

0095ef3c 02be23b4 02be23b4 00000000 0095ef54

0095ef4c 73a9ebb6 00b69bd8 0095efa8 73aa1e10

0095ef5c 0095efe4 0095ef98 73b79b20 0095f0b8

0095ef6c 73aa1dc9 a60effde 0095f104 0095f078

0095ef7c 0095f02c 00c94d34 0095efe4 00000000

0095ef8c a60effde 0095ef60 0095f078 0095f12c

0095ef9c 73b889e0 d5310e5e 00000001 0095f010

0095efac 73aa7994 00000000 00000000 00000001

And now let’s dump the raw pointer (did you notice that the address is 0x2be23b8 – 4 ?)

0:000> !do 02be23b4 
Name:        ConsoleApp.Foo
MethodTable: 00c94dbc
EEClass:     00c916f8
Size:        12(0xc) bytes
File:        C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe
Fields:
None

0:000> !do 02be23b4

Name: ConsoleApp.Foo

MethodTable: 00c94dbc

EEClass: 00c916f8

Size: 12(0xc) bytes

File: C:\Users\adafurma\source\repos\ConsoleApp3\ConsoleApp3\bin\Debug\ConsoleApp3.exe

Fields:

None

And now you can clearly see that the MT address is the same as shown earlier.

There is a lot of trickery around value types, especially when it comes to enums. See this post describing some of it.

41. What is the base class of the delegate?

See this.

using System;

public class Program
{
	delegate void X();
	
	public static void Main()
	{
		X x = Foo;
		Console.WriteLine(x.GetType().BaseType);
		Console.WriteLine(x.GetType().BaseType.BaseType);
		Console.WriteLine(x.GetType().BaseType.BaseType.BaseType);
	}
	
	static void Foo(){}
}

using System;

public class Program

{

delegate void X();

public static void Main()

{

X x = Foo;

Console.WriteLine(x.GetType().BaseType);

Console.WriteLine(x.GetType().BaseType.BaseType);

Console.WriteLine(x.GetType().BaseType.BaseType.BaseType);

}

static void Foo(){}

}

Output:

System.MulticastDelegate
System.Delegate
System.Object

System.MulticastDelegate

System.Delegate

System.Object

It is MulticastDelegate which in turn inherits from Delegate which inherits from object. MulticastDelegate contains a list of delegates.

42. What is the base class of an array?

See this code:

using System;

public class Program
{
	public static void Main()
	{
		Foo[] foo = new Foo[5];
		Console.WriteLine(foo.GetType().BaseType);
		foreach(var type in foo.GetType().GetInterfaces()){
			Console.WriteLine(type);
		}
	}
}

class Foo{
}

using System;

public class Program

{

public static void Main()

{

Foo[] foo = new Foo[5];

Console.WriteLine(foo.GetType().BaseType);

foreach(var type in foo.GetType().GetInterfaces()){

Console.WriteLine(type);

}

class Foo{

}

It shows the following:

System.Array
System.ICloneable
System.Collections.IList
System.Collections.ICollection
System.Collections.IEnumerable
System.Collections.IStructuralComparable
System.Collections.IStructuralEquatable
System.Collections.Generic.IList`1[Foo]
System.Collections.Generic.ICollection`1[Foo]
System.Collections.Generic.IEnumerable`1[Foo]
System.Collections.Generic.IReadOnlyList`1[Foo]
System.Collections.Generic.IReadOnlyCollection`1[Foo]

System.Array

System.ICloneable

System.Collections.IList

System.Collections.ICollection

System.Collections.IEnumerable

System.Collections.IStructuralComparable

System.Collections.IStructuralEquatable

System.Collections.Generic.IList`1[Foo]

System.Collections.Generic.ICollection`1[Foo]

System.Collections.Generic.IEnumerable`1[Foo]

System.Collections.Generic.IReadOnlyList`1[Foo]

System.Collections.Generic.IReadOnlyCollection`1[Foo]

So whenever you create an array, .NET creates a class for you and implements quite a lot of interfaces. But is it always the same? Check this code:

using System;

public class Program
{
	public static void Main()
	{
		Foo[,] foo = new Foo[5,4];
		Console.WriteLine(foo.GetType().BaseType);
		foreach(var type in foo.GetType().GetInterfaces()){
			Console.WriteLine(type);
		}
	}
}

class Foo{
}

using System;

public class Program

{

public static void Main()

{

Foo[,] foo = new Foo[5,4];

Console.WriteLine(foo.GetType().BaseType);

foreach(var type in foo.GetType().GetInterfaces()){

Console.WriteLine(type);

}

class Foo{

}

It shows

System.Array
System.ICloneable
System.Collections.IList
System.Collections.ICollection
System.Collections.IEnumerable
System.Collections.IStructuralComparable
System.Collections.IStructuralEquatable

System.Array

System.ICloneable

System.Collections.IList

System.Collections.ICollection

System.Collections.IEnumerable

System.Collections.IStructuralComparable

System.Collections.IStructuralEquatable

We don’t have generic interfaces here. What about pointers?

using System;

public class Program
{
	public static void Main()
	{
		unsafe
		{
			int *[] foo = new int *[5];
			Console.WriteLine(foo.GetType().BaseType);
			foreach (var type in foo.GetType().GetInterfaces())
			{
				Console.WriteLine(type);
			}
		}
	}
}

using System;

public class Program

{

public static void Main()

{

unsafe

{

int *[] foo = new int *[5];

Console.WriteLine(foo.GetType().BaseType);

foreach (var type in foo.GetType().GetInterfaces())

{

Console.WriteLine(type);

}

System.Array
System.ICloneable
System.Collections.IList
System.Collections.ICollection
System.Collections.IEnumerable
System.Collections.IStructuralComparable
System.Collections.IStructuralEquatable

System.Array

System.ICloneable

System.Collections.IList

System.Collections.ICollection

System.Collections.IEnumerable

System.Collections.IStructuralComparable

System.Collections.IStructuralEquatable

Again, no generic interfaces.

43. Can you make a 2-based array?

Use this code:

using System;

public class Program
{
	public static void Main()
	{
		Array array = Array.CreateInstance(typeof(int), new int[]{1}, new int[]{2});
		array.SetValue(123, 2);
		Console.WriteLine(array.GetValue(2));
		Console.WriteLine(array.GetValue(0));
	}
}

using System;

public class Program

{

public static void Main()

{

Array array = Array.CreateInstance(typeof(int), new int[]{1}, new int[]{2});

array.SetValue(123, 2);

Console.WriteLine(array.GetValue(2));

Console.WriteLine(array.GetValue(0));

}

Output:

123
Run-time exception (line -1): Index was outside the bounds of the array.

Stack Trace:

[System.IndexOutOfRangeException: Index was outside the bounds of the array.]
   at System.Array.InternalGetReference(Void* elemRef, Int32 rank, Int32* pIndices)
   at System.Array.GetValue(Int32 index)
   at Program.Main()

123

Run-time exception (line -1): Index was outside the bounds of the array.

Stack Trace:

[System.IndexOutOfRangeException: Index was outside the bounds of the array.]

at System.Array.InternalGetReference(Void* elemRef, Int32 rank, Int32* pIndices)

at System.Array.GetValue(Int32 index)

at Program.Main()

You can see that accessing index 0 throws exception.

44. What is a string interning?

It’s a way of deduplicating strings in your application. You can ask .NET to check if a string of given content was already created and reuse it. See this:

using System;
using System.Text;

public class Program
{
	public static void Main()
	{
		String s1 = "MyTest";
		String s2 = new StringBuilder().Append("My").Append("Test").ToString(); 
		String s3 = String.Intern(s2); 
		Console.WriteLine("s1 == '{0}'", s1);
		Console.WriteLine("s2 == '{0}'", s2);
		Console.WriteLine("s3 == '{0}'", s3);
		Console.WriteLine("Is s2 the same reference as s1?: {0}", (Object)s2==(Object)s1); 
		Console.WriteLine("Is s3 the same reference as s1?: {0}", (Object)s3==(Object)s1);
	}
}

using System;

using System.Text;

public class Program

{

public static void Main()

{

String s1 = "MyTest";

String s2 = new StringBuilder().Append("My").Append("Test").ToString();

String s3 = String.Intern(s2);

Console.WriteLine("s1 == '{0}'", s1);

Console.WriteLine("s2 == '{0}'", s2);

Console.WriteLine("s3 == '{0}'", s3);

Console.WriteLine("Is s2 the same reference as s1?: {0}", (Object)s2==(Object)s1);

Console.WriteLine("Is s3 the same reference as s1?: {0}", (Object)s3==(Object)s1);

}

This prints:

s1 == 'MyTest'
s2 == 'MyTest'
s3 == 'MyTest'
Is s2 the same reference as s1?: False
Is s3 the same reference as s1?: True

s1 == 'MyTest'

s2 == 'MyTest'

s3 == 'MyTest'

Is s2 the same reference as s1?: False

Is s3 the same reference as s1?: True