“方法未在泛型类型定义中声明”错误

Question

I am trying to generate dynamically method like that:

MyList<T> CreateList<T>(T arg) => new MyList<T>(){arg};

Here is modification of program from documentation (original program from https://docs.microsoft.com/en-us/dotnet/framework/reflection-and-codedom/how-to-define-a-generic-method-with-reflection-emit ). All I want to invoke Add method, defined within base class List . I am getting this method info via TypeBuilder.GetMethod method:

public class MyList<T> : List<T>
{
}
public static void Main()
{
    var asmName = new AssemblyName("DemoMethodBuilder1");
    var demoAssembly = AppDomain.CurrentDomain.DefineDynamicAssembly(asmName, AssemblyBuilderAccess.RunAndSave);

    var demoModule = demoAssembly.DefineDynamicModule(asmName.Name, asmName.Name + ".dll");
    var demoType = demoModule.DefineType("DemoType", TypeAttributes.Public);

    var create_list_method = demoType.DefineMethod("CreateList", MethodAttributes.Public | MethodAttributes.Static);

    var TInput = create_list_method.DefineGenericParameters(new string[] { "TInput" })[0];
    var t_list_type = typeof(MyList<>).MakeGenericType(TInput);

    create_list_method.SetParameters(new Type[] { TInput });
    create_list_method.SetReturnType(t_list_type);

    var ilgen = create_list_method.GetILGenerator();

    ilgen.Emit(OpCodes.Newobj, TypeBuilder.GetConstructor(t_list_type, typeof(MyList<>).GetConstructors()[0]));
    ilgen.Emit(OpCodes.Dup);
    ilgen.Emit(OpCodes.Ldarg_0);
    ilgen.Emit(OpCodes.Callvirt, TypeBuilder.GetMethod(t_list_type, typeof(MyList<>).GetMethod("Add")));
    ilgen.Emit(OpCodes.Ret);

    demoType.CreateType();
    demoAssembly.Save(asmName.Name + ".dll");
}

Execution of program is failed at ilgen.Emit(OpCodes.Callvirt, TypeBuilder.GetMethod(t_list_type, typeof(MyList<>).GetMethod("Add"))); with message:

The specified method cannot be dynamic or global and must be declared on a generic type definition

I cannot understand, why does such code fail? As for me, I think that it should work - I intentionally used static method TypeBuilder.GetMethod

Answer 1

I recommend you to always have a decompiler at hand when trying to create IL. You are nearly there, but the Add method is not definied in MyList but in List as you can see here:

IL_0000: newobj instance void class MyList`1<!!T>::.ctor()
IL_0005: dup
IL_0006: ldarg.1
IL_0007: callvirt instance void class [mscorlib]System.Collections.Generic.List`1<!!T>::Add(!0)
IL_000c: ret

So that is what you have to tell the static TypeBuilder method:

var add = TypeBuilder.GetMethod(typeof(List<>).MakeGenericType(TInput), typeof(List<>).GetMethod("Add"))
ilgen.Emit(OpCodes.Callvirt, add);