Using IL Emit to generate a virtual function which returns an existed object

Question

I want to generate a derived class of BasicModel dynamically in my C# code. And I want to override the virtual property in the derived class to return an existed object.

public class BasicModel
{
    [IgnoreProperty]
    public virtual CloudStorageAccount StorageAccount { get; }
}

And here is the IL part. But I constantly get null from calling that method.

var IlGen2 = newMethod2.GetILGenerator();
Func<CloudStorageAccount> getStoredObj = () => parentModel.StorageAccount;
IlGen2.Emit(OpCodes.Call, getStoredObj.GetMethodInfo());
IlGen2.Emit(OpCodes.Ldobj);
IlGen2.Emit(OpCodes.Ret);

What's the problem? Or is there a better way?

Thanks a lot.

Answer 1

Since the delegate is the same for all instances of the type, I would define a static field that will contain the delegate and use that:

static void Main(string[] args)
{

    var parentModel = new ContainerCloudStorageAccount { StorageAccount = new CloudStorageAccount() } ;
    var type = CreateType("MyOverride", () => parentModel.StorageAccount);
    var value = (BasicModel)type.GetConstructors().First().Invoke(new object[0]);
    Console.WriteLine(value.StorageAccount);
}

public static Type CreateType(string name, Func<CloudStorageAccount> getter)
{ 
    AppDomain myDomain = Thread.GetDomain();
    AssemblyBuilder myAsmBuilder = myDomain.DefineDynamicAssembly(new AssemblyName("dynamicTypes"), AssemblyBuilderAccess.Run);
    ModuleBuilder interfaceImplementationModule = myAsmBuilder.DefineDynamicModule("overrrides");

    TypeBuilder typeBuilder = interfaceImplementationModule.DefineType(name,
        TypeAttributes.Public | TypeAttributes.Class,
        typeof(BasicModel));

    var newMethod2 = typeBuilder.DefineMethod("get_StorageAccount", MethodAttributes.Virtual | MethodAttributes.Public,
            typeof(CloudStorageAccount), Type.EmptyTypes
    );
    typeBuilder.DefineMethodOverride(newMethod2, typeof(BasicModel).GetProperty("StorageAccount").GetGetMethod());

    var fieldInfo = typeBuilder.DefineField("getter", typeof(Func<CloudStorageAccount>), FieldAttributes.Static | FieldAttributes.Public);

    var IlGen2 = newMethod2.GetILGenerator();
    IlGen2.Emit(OpCodes.Ldsfld, fieldInfo);
    IlGen2.Emit(OpCodes.Call, typeof(Func<CloudStorageAccount>).GetMethod("Invoke"));
    IlGen2.Emit(OpCodes.Ret);

    typeBuilder.DefineDefaultConstructor(MethodAttributes.Public);
    var type = typeBuilder.CreateType();
    type.GetField("getter").SetValue(null, getter);

    return type;
}

You original solution does not work because the delegate is more then a method, it is also a target object for that method. So if you just call the method associated with the delegate it will not have the data that you captured in the delegate.