Create a constructor call using Reflection Emit that passes a Func<> as a parameter

Question

I'm hoping someone can point me in the right direction with the following problem.

I am working on a project where the types are generated using Reflection.Emit, all has been working fine until a requirement arose where I needed to pass a Func<> into a constructor of a new object as below.

public class SearchTerm : IEntity
{
   private readonly NavigationProperty<Item> _item;

   public SearchTerm()
   {
       _item = new NavigationProperty<Item>(() => ItemIds);
   }
   public string[] ItemIds { get; set; }
}

Using Linqpad I can see the IL output is as follows:

SearchTerm.<.ctor>b__0:
IL_0000:  ldarg.0     
IL_0001:  call        UserQuery+SearchTerm.get_ItemIds
IL_0006:  stloc.0     // CS$1$0000
IL_0007:  br.s        IL_0009
IL_0009:  ldloc.0     // CS$1$0000
IL_000A:  ret         

SearchTerm..ctor:
IL_0000:  ldnull      
IL_0001:  stloc.0     
IL_0002:  ldarg.0     
IL_0003:  call        System.Object..ctor
IL_0008:  nop         
IL_0009:  nop         
IL_000A:  ldarg.0     
IL_000B:  ldloc.0     
IL_000C:  brtrue.s    IL_001D
IL_000E:  ldarg.0     
IL_000F:  ldftn       UserQuery+SearchTerm.<.ctor>b__0
IL_0015:  newobj      System.Func<System.Collections.Generic.IEnumerable<System.String>>..ctor
IL_001A:  stloc.0     
IL_001B:  br.s        IL_001D
IL_001D:  ldloc.0     
IL_001E:  newobj      UserQuery<UserQuery+Item>+NavigationProperty`1..ctor
IL_0023:  stfld       UserQuery+SearchTerm._item
IL_0028:  nop         
IL_0029:  ret  

The problem I have is that I'm not sure how to define the delegate within IL.

I have tried the following

var method = typeBuilder.DefineMethod("func", MethodAttributes.Public | MethodAttributes.SpecialName | MethodAttributes.HideBySig | MethodAttributes.Virtual, typeof(IEnumerable<string>), Type.EmptyTypes);

var methodIl = method.GetILGenerator();
methodIl.Emit(OpCodes.Ldarg_0);
methodIl.Emit(OpCodes.Call, dictionary["get_ItemIds"]);
methodIl.Emit(OpCodes.Ret);

Then trying to create the delegate the following throws "Not Supported Exception" with the error "Derived classes must provide an implementation."

var funcType = typeof(Func<,>).MakeGenericType(typeBuilder, typeof(IEnumerable<string>));
method.CreateDelegate(funcType);

I have also tried using Delegate.CreateDelegate and DynamicMethod which both require the type to have been created before using them.

Any suggestion on what I might be doing wrong are greatly appreciated.

Answer 1

If you want to invoke your code, you have to work with Type s and MethodInfo s, not TypeBuilder s and MethodBuilder s.

So, you need to CreateType() and then use that Type and its method:

var generatedType = typeBuilder.CreateType();

var funcType = typeof(Func<,>).MakeGenericType(
    generatedType, typeof(IEnumerable<string>));
var d = generatedType.GetMethod("func").CreateDelegate(funcType);