Reflection.Emit: property with function call in getter/setter

Question

I am trying to use Reflection.Emit to create a dynamic type which inherits from a given type, and adds a new property whose getter/setter call methods of the base type.

Suppose my base type looks as follows:

class Test
{
    private int _val1;

    public int GetVal(int fld)
    {
        if (fld == 1) return _val1;
        return 0;
    }

    public void SetVal(int fld, int val)
    {
        if (fld == 1) _val1 = val;
    }
}

I want to create a subtype which has a new property, defined as follows:

public int NewProp { get { return GetVal(1); } set { SetVal(1, value); } }

Seems simple enough.

I came up with the following (which is the working answer):

PropertyBuilder pbNewProp = tb.DefineProperty("NewProp", PropertyAttributes.HasDefault, typeof(int), null);
MethodAttributes getSetAttr = MethodAttributes.Public | MethodAttributes.SpecialName | MethodAttributes.HideBySig;

// Define the "get" accessor method
MethodBuilder mbNewPropGetAccessor = tb.DefineMethod(
    "get_NewProp", 
    getSetAttr, 
    typeof(int), 
    Type.EmptyTypes);

ILGenerator NewPropGetIL = mbNewPropGetAccessor.GetILGenerator();
NewPropGetIL.Emit(OpCodes.Ldarg_0);
NewPropGetIL.Emit(OpCodes.Ldc_I4_1);
NewPropGetIL.Emit(OpCodes.Call, typeof(Test).GetMethod("GetVal"));
NewPropGetIL.Emit(OpCodes.Ret);

// Define the "set" accessor method 
MethodBuilder mbNewPropSetAccessor = tb.DefineMethod(
    "set_NewProp", 
    getSetAttr, 
    null, 
    new Type[] { typeof(int) });

ILGenerator NewPropSetIL = mbNewPropSetAccessor.GetILGenerator();
NewPropSetIL.Emit(OpCodes.Ldarg_0);
NewPropSetIL.Emit(OpCodes.Ldc_I4_1);
NewPropSetIL.Emit(OpCodes.Ldarg_1);
NewPropSetIL.Emit(OpCodes.Call, typeof(Test).GetMethod("SetVal"));
NewPropSetIL.Emit(OpCodes.Ret);

// Map the accessor methods
pbNewProp.SetGetMethod(mbNewPropGetAccessor);
pbNewProp.SetSetMethod(mbNewPropSetAccessor);

I did compare this to the compiler generated IL (using ildasm) based on a hardcoded sample and can't spot a difference.

Here is what I did to test whether above code would work:

var inst = Activator.CreateInstance(myType);

var p = inst.GetType().GetProperty("NewProp");
p.GetValue(inst, null);
p.SetValue(inst, 1, null);

For reference, here is what ildasm says about "set_NewProp":

.method public hidebysig specialname instance void 
    set_NewProp(int32 'value') cil managed
{
    // Code size       9 (0x9)
        .maxstack  8
        IL_0000:  ldarg.0
        IL_0001:  ldc.i4.1
        IL_0002:  ldarg.1
        IL_0003:  call       instance void ConsoleApplication2.Test::SetVal(int32, int32)
        IL_0008:  ret
    } // end of method TestSub::set_NewProp

And here "get_NewProp":

.method public hidebysig specialname instance int32 
    get_NewProp() cil managed
    {
        // Code size       8 (0x8)
        .maxstack  8
        IL_0000:  ldarg.0
        IL_0001:  ldc.i4.1
        IL_0002:  call       instance int32 ConsoleApplication2.Test::GetVal(int32)
        IL_0007:  ret
    } // end of method TestSub::get_NewProp

In the original version of the question, the call to GetValue threw a TargetInvocationException, whose InnerException was an InvalidProgramException that says "Common Language Runtime detected an invalid program." It was due to a typo (since corrected); d'oh!

Answer 1

You'll kick yourself:

NewPropSetIL.Emit(OpCodes.Ldarg_1);       
NewPropGetIL.Emit(OpCodes.Callvirt, typeof(Test).GetMethod("SetVal"));
NewPropSetIL.Emit(OpCodes.Ret);

Look closely. Closer. Closer. Still not there?

Line 2 talks to NewPropGetIL

If that still doesn't work, make sure that you declared the base-type in the TypeBuilder, and that the class Test is public, and not internal as shown in the question. It works fine for me now.

How I investigated it

I started by adding the following preamble:

AssemblyName aName = new AssemblyName("SomeAssembly");
AssemblyBuilder ab =
    AppDomain.CurrentDomain.DefineDynamicAssembly(
        aName,
        AssemblyBuilderAccess.RunAndSave);

// For a single-module assembly, the module name is usually 
// the assembly name plus an extension.
ModuleBuilder mb =
    ab.DefineDynamicModule(aName.Name, aName.Name + ".dll");

TypeBuilder tb = mb.DefineType(
    "SomeType",
     TypeAttributes.Public, typeof(Test));

and the following foot:

RunTest(typeof(ManualTest));
RunTest(tb.CreateType());

where:

private static void RunTest(Type type)
{
    Console.WriteLine(type.Name);
    Console.WriteLine();
    dynamic obj = Activator.CreateInstance(type);

    int i = obj.NewProp;
    Console.WriteLine(i);
    obj.NewProp = 123;
    i = obj.NewProp;
    Console.WriteLine(i);

    Console.WriteLine();
}

I also added some logging to the base methods:

public class Test
{
    private int _val1;

    public int GetVal(int fld)
    {
        Console.WriteLine("GetVal:" + fld);
        if (fld == 1) return _val1;
        return 0;
    }

    public void SetVal(int fld, int val)
    {
        Console.WriteLine("SetVal:" + fld);
        if (fld == 1) _val1 = val;
    }
}

and a manual test for comparison (the expected result):

class ManualTest : Test
{
    public int NewProp { get { return GetVal(1); } set { SetVal(1, value); } }
}

With this in place, it was clear that there is a problem in the set:

ManualTest

GetVal:1
0
SetVal:1
GetVal:1
123

SomeType

GetVal:1
0
GetVal:1
0