lowleveldesign.org

I’m currently working on a simple debugger in VB.NET. I’m using mdbg assemblies to manage all the magic stuff happening between debugger and debuggee. I wrote following code in VB.NET just to handle process execution:

Imports System
Imports System.Reflection
Imports Microsoft.Samples.Debugging.MdbgEngine

Public Class vdbg
  Public Shared Sub Main(ByVal args() As String) 
    Dim debugger = New MDbgEngine()
    debugger.Options.CreateProcessWithNewConsole = True
    debugger.Options.StopOnModuleLoad = True
    
    Dim process = debugger.CreateProcess(args(0), "", DebugModeFlag.Debug, Nothing)
    While process.IsAlive
      process.Go().WaitOne()
    End While
  End Sub
End Class

and compiled it:

vbc /r:mdbgeng.dll /vbruntime- /debug+ /debug:full /t:exe vdbg.vb

As you can see this code is rather straightforward. New console window is opened for a “debuggee” and after debuggee finishes its execution both processes should finish. What happened was a big surprise to me. The new console window was opened and then both processes hanged. I wrote an equivalent in C# and it worked as expected. So the only thing that rested was to analyze the generated IL for both versions:

ildasm /item=vdbg::Main /out=vdbg.il vdbg.exe

I highlighted the controversial line of IL below:

.class public auto ansi vdbg
       extends [mscorlib]System.Object
{
  .method public static void  Main(string[] args) cil managed
  {
    .entrypoint
    .custom instance void [mscorlib]System.STAThreadAttribute::.ctor() = ( 01 00 00 00 ) 
    // Code size       86 (0x56)
    .maxstack  5

It appears that the VB compiler by default puts STAThreadAttribute above the Main method unless you don’t choose a different COM threading model. The C# compiler is not so eager in changing your code :). Be aware of that fact when writing multithreaded application. In my application adding the highlighted line resolved all problems:

...
Public Class vdbg
  <MTAThread()> _
  Public Shared Sub Main(ByVal args() As String) 
     ...