Use System.Reflection.Emit to show a MessageBox in Windows.Forms

Question

I am trying to do a msgbox with ilgenerator.emit but I have an exception when I run the code:

exception generated from destination of a call

This is my code:

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim assemblyName As AssemblyName = New AssemblyName("SamAsm")
    Dim assemblyBuilder As AssemblyBuilder = Thread.GetDomain().DefineDynamicAssembly(assemblyName, AssemblyBuilderAccess.RunAndSave)

    'Creating module
    Dim moduleBuilder As ModuleBuilder = assemblyBuilder.DefineDynamicModule("SamAsm.exe")

    Creating class file
    Dim typeBuilder As TypeBuilder = moduleBuilder.DefineType("SamAsmType", TypeAttributes.Public Or TypeAttributes.Class)

    Creating Main Method
    Dim methodBuilder1 As MethodBuilder = typeBuilder.DefineMethod("Main", MethodAttributes.Static Or MethodAttributes.Public, GetType(Void), New System.Type() {GetType(String)})

    'Generating Code for method builder in IL
    Dim ilGenerator As ILGenerator = methodBuilder1.GetILGenerator
    ilGenerator.Emit(OpCodes.Ldstr, "test")
    ilGenerator.EmitCall(OpCodes.Call, GetType(System.Windows.Forms.MessageBox).GetMethod("Show", {GetType(String)}), {GetType(String)})
    ilGenerator.Emit(OpCodes.Ret)

    'Creating Class object (type) and accessing main method
    Dim SamAsmType As Type = typeBuilder.CreateType
    SamAsmType.GetMethod("Main").Invoke(Nothing, New String() {Nothing})

    'Creating Entry Point and saving assembly
    assemblyBuilder.SetEntryPoint(methodBuilder1, PEFileKinds.WindowApplication)
    assemblyBuilder.Save("SamAsm.exe")
End Sub

Answer 1

You have to ensure the stack is empty when returning from a Sub/void function.

The Show-Method returns a DialogResult, so you have to call Pop to remove the DialogResult from the stack.

...
ilGenerator.EmitCall(OpCodes.Call, GetType(System.Windows.Forms.MessageBox).GetMethod("Show", {GetType(String)}), {GetType(String)})
ilGenerator.Emit(OpCodes.Pop) ' remove the return value of Show from the stack
ilGenerator.Emit(OpCodes.Ret)
...

---

Nonetheless, you can always use peverify to trace bugs in your dynamically created assemblies.

So in your case, running peverify over SamAsm.exe would give you following error:

[IL]: Error: [c:\yourpath\SamAsm.exe : SamAsmType::Main][offset 0x0000000A] Stack must be empty on return from a void function. 1 Error(s) Verifying SamAsm.exe