Return values get lost with Reflection.Emit

[Matthias Klöpper]

Hi there,
I'm currently trying to call some API-Functions via Reflection.Emit since I
need to bind to different dlls based on user input. The dynamic creation of
the required PInvoke-Methods works just fine and when invoked the functions
are executed perfectly well. The only problem is that I seem to be unable to
get back any return values. To simplify the whole case I created a
test-setup that binds to user32.dll, dynamically creates a PInvokeMethod to
call the MessageBox function and invokes that function. Same effect ... the
return value always is 0. Quite impossible to detect which button was
clicked this way :-(
The example provided in the MSDN is not really helpful since it simply
ignores the return value since they a simple OkOnly-MessageBox in contrast
to my Cancel/Retry/Ignore-box.
Anybody out there who can reproduce my scenario or can point me to where I
screwed up? Any help would be greatly appreciated.

Greeting,
Matti

public class Manager
{
AssemblyBuilder assembly;

public Manager()
{
assembly = CreateDLLAccess ();
TestApiCall(ass embly);
}

private AssemblyBuilder CreateDLLAccess ()
{
AppDomain currentDomain;
AssemblyName name;
MethodBuilder method=null;
AssemblyBuilder assembly;
ModuleBuilder module;

// Get the current application domain for the current thread.
currentDomain = AppDomain.Curre ntDomain;
name = new AssemblyName();
name.Name = "TEST";

// Define a dynamic assembly in the 'currentDomain' .
assembly = currentDomain.D efineDynamicAss embly(name,
AssemblyBuilder Access.Run);

// Define a dynamic module in "TEST" assembly.
module = assembly.Define DynamicModule(" ApiWrapper");

// Define a PInvoke method giving the name of the entrypoint in the
dll.
Type[] myType = new Type[] {typeof(int), typeof(string),
typeof(string), typeof(int)};
method = module.DefinePI nvokeMethod("Ms gBox", "user32.dll ",
"MessageBox ", MethodAttribute s.Public | MethodAttribute s.Static |
MethodAttribute s.PinvokeImpl, CallingConventi ons.Standard, typeof(int),
myType,CallingC onvention.Winap i, CharSet.Auto);
module.CreateGl obalFunctions() ;

return assembly;
}

private void TestApiCall(Ass emblyBuilder assembly)
{
Module module = assembly.GetMod ule("ApiWrapper ");
MethodInfo method;
method = module.GetMetho d("MsgBox");
object[] args = new object[] {(int) 0, "a", "b", (int) 0x2};
int i = (int) method.Invoke(n ull, args);
}
}

Whatever I try (fiddling around with Attributes, CallingConventi ons,
dynamically creating a type for the PInvokeMethod and substituting
MethodInfo.Invo ke() with Type.InvokeMeth od() or Type.InvokeMemb er()) i will
always be 0 :-(

[Matthias Klöpper]

Hi again,
Mattias Sögren over at framework.inter op just posted the solution which
works absolutely perfect:

Matthias,

You have to set the preservesig metadata flag on the method. For some
reason it defaults to off for DefinePInvokeMe thod, whereas it defaults
to on when you use the DllImport attribute.

method.SetImple mentationFlags( MethodImplAttri butes.PreserveS ig |
method.GetMeth odImplementatio nFlags() );

Mattias

--
Mattias Sjögren [MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/
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