CallingBaseMethodWorkaround.cs

// use in LINQPad

delegate string Test1Delegate(int n, ref float f);

void Main()
{
	var bar = new Bar();

	var baseTest1 = (Test1Delegate)GetBaseMethod(typeof(Foo).GetMethod("Test1"), bar);
	var f = 789f;
	var res2 = baseTest1(456, ref f);
	Console.WriteLine(res2);

	var baseTest2 = (Action<int, float>)GetBaseMethod(typeof(Foo).GetMethod("Test2"), bar);
	baseTest2(1456, 1789f);
}

static Delegate GetBaseMethod(MethodInfo method, object instance)
{
	var ptr = method.MethodHandle.GetFunctionPointer();
	var pTypes = method.GetParameters().Select(p => p.ParameterType).ToList();

	if (method.ReturnType == typeof(void))
	{
		var aType = Type.GetType($"System.Action`{pTypes.Count}", true, false);
		var actionType = aType.MakeGenericType(pTypes.ToArray());
		return (Delegate)Activator.CreateInstance(actionType, instance, ptr);
	}

	pTypes.Add(method.ReturnType);
	var fType = Type.GetType($"System.Func`{pTypes.Count}", true, false);
	var functionType = fType.MakeGenericType(pTypes.ToArray());
	return (Delegate)Activator.CreateInstance(functionType, instance, ptr);
}

public class Foo
{
	public virtual string Test1(int n, ref float f)
	{
		Console.WriteLine($"base test {n}");
		return "base result";
	}

	public virtual void Test2(int n, float f)
	{
		Console.WriteLine($"base test {n}");
	}
}

public class Bar : Foo
{
	public override string Test1(int n, ref float f)
	{
		Console.WriteLine($"sub test {n}");
		return "sub result";
	}

	public override void Test2(int n, float f)
	{
		Console.WriteLine($"sub test {n}");
	}
}