Setting the value of a static field using Reflection.Emit fails in Unity

Question

I'm trying to set the value of a static field using Reflection.Emit (I don't have access to .NET 4's Expression.Assign because I'm stuck with Unity's .NET 3.5).

My current code is as follows:

public Action<TTarget, TField> GetSetter<TTarget, TField>(FieldInfo fieldInfo)
{
    DynamicMethod setterMethod = new DynamicMethod
    (
        "setter",
        typeof(void),
        new Type[] { typeof(TTarget), typeof(TField) },
        typeof(TTarget)
    );

    var setterIL = setterMethod.GetILGenerator();

    if (fieldInfo.IsStatic)
    {
        setterIL.Emit(OpCodes.Ldnull);
    }
    else
    {
        setterIL.Emit(OpCodes.Ldarg_0);
    }

    setterIL.Emit(OpCodes.Ldarg_1);
    setterIL.Emit(OpCodes.Stfld, fieldInfo);
    setterIL.Emit(OpCodes.Ret);

    return (Action<TTarget, TField>)setterMethod.CreateDelegate(typeof(Action<TTarget, TField>));
}

And then, I invoke the setter using:

public class Static
{
    public static int x;
}

var fieldInfo = typeof(Static).GetField("x");

var setter = GetSetter<Static, int>(fieldInfo);

setter.Invoke(null, 123);

I get this error message:

NullReferenceException: Object reference not set to an instance of an object (wrapper dynamic-method) setter (...,int)

I thought loading null as the first argument ( Ldnull opcode) would fix it, but it doesn't seem to work. What am I doing wrong?

Update: it seems the exception is only triggered when the code runs from within Unity (latest, 5.5.0p4). In a .NET 3.5 console application created from Visual Studio, there is no issue. Could there be an issue with Unity's Mono compiler?

Here is the full code to test from a Tools > Debug IL menu item in Unity.

using System;
using System.Reflection;
using System.Reflection.Emit;
using UnityEditor;

class Program
{
    public static Action<TTarget, TField> GetSetter<TTarget, TField>(FieldInfo fieldInfo)
    {
        DynamicMethod setterMethod = new DynamicMethod
        (
            "setter",
            typeof(void),
            new Type[] { typeof(TTarget), typeof(TField) },
            typeof(TTarget)
        );

        var setterIL = setterMethod.GetILGenerator();

        setterIL.Emit(OpCodes.Ldarg_0);
        setterIL.Emit(OpCodes.Ldarg_1);
        setterIL.Emit(OpCodes.Stfld, fieldInfo);
        setterIL.Emit(OpCodes.Ret);

        return (Action<TTarget, TField>)setterMethod.CreateDelegate(typeof(Action<TTarget, TField>));
    }

    public class Static
    {
        public static int x;
    }

    [MenuItem("Tools/Debug IL")]
    static void Debug()
    {
        var fieldInfo = typeof(Static).GetField("x");

        var setter = GetSetter<Static, int>(fieldInfo);

        setter.Invoke(null, 123);

        Debug.Log("Static field assignment succeeded.");
    }
}

Answer 1

Got it working with OpCodes.Stsfld (set static field) instead:

if (fieldInfo.IsStatic)
{
    setterIL.Emit(OpCodes.Ldarg_0);
    setterIL.Emit(OpCodes.Stsfld, fieldInfo);
    setterIL.Emit(OpCodes.Ret);
}
else
{
    setterIL.Emit(OpCodes.Ldarg_0);
    setterIL.Emit(OpCodes.Ldarg_1);
    setterIL.Emit(OpCodes.Stfld, fieldInfo);
    setterIL.Emit(OpCodes.Ret);
}

The .NET runtime is probably more lenient than the Mono runtime behind the scenes (ie: it allows Stfld even for a static field, just ignoring the first parameter, whereas Mono doesn't), which would explain why the issue only occurred in Unity.