Can I set a value on a struct through reflection without boxing?

Question

Actually, I should've asked: how can I do this and remain CLS Compliant? Because the only way I can think of doing this is as follows, but using either __makeref, FieldInfo.SetValueDirect or just System.TypedReference in general invalidates CLS Compliance.

// code illustrating the issue:
TestFields fields = new TestFields { MaxValue = 1234 };  // test struct with one field

FieldInfo info = fields.GetType().GetField("MaxValue");  // get the FieldInfo

// actual magic, no boxing, not CLS compliant:
TypedReference reference = __makeref(fields);
info.SetValueDirect(reference, 4096);

The compliant counterpart of SetValueDirect is SetValue, but it takes an object as the target, hence my struct will be boxed, making me setting a value on a copy, not the original variable.

A generic counterpart for SetValue doesn't exist as far as I know. Is there any other way of setting the field of a (reference to a) struct through reflection?

Answer 1

Make cls-compliant wrapper on SetValueDirect:

  var item = new MyStruct { X = 10 };

  item.GetType().GetField("X").SetValueForValueType(ref item, 4);


[CLSCompliant(true)]
static class Hlp
{
  public static void SetValueForValueType<T>(this FieldInfo field, ref T item, object value) where T : struct
  {
    field.SetValueDirect(__makeref(item), value);
  }
}

Answer 2

For properties, if you have the struct and property types, you can create a delegate from the property setter. As you point out, fields don't have setters, but you can create one that behaves exactly the same:

delegate void RefAction<T1, T2>(ref T1 arg1, T2 arg2);

struct TestFields
{
    public int MaxValue;

    public int MaxValueProperty
    {
        get { return MaxValue; }
        set { MaxValue = value; }
    }
};

static class Program
{
    static void Main(string[] args)
    {
        var propertyInfo = typeof(TestFields).GetProperty("MaxValueProperty");
        var propertySetter = (RefAction<TestFields, int>)Delegate.CreateDelegate(typeof(RefAction<TestFields, int>), propertyInfo.GetSetMethod());

        var fieldInfo = typeof(TestFields).GetField("MaxValue");

        var dynamicMethod = new DynamicMethod(String.Empty, typeof(void), new Type[] { fieldInfo.ReflectedType.MakeByRefType(), fieldInfo.FieldType }, true);
        var ilGenerator = dynamicMethod.GetILGenerator();
        ilGenerator.Emit(OpCodes.Ldarg_0);
        ilGenerator.Emit(OpCodes.Ldarg_1);
        ilGenerator.Emit(OpCodes.Stfld, fieldInfo);
        ilGenerator.Emit(OpCodes.Ret);
        var fieldSetter = (RefAction<TestFields, int>)dynamicMethod.CreateDelegate(typeof(RefAction<TestFields, int>));

        var fields = new TestFields { MaxValue = 1234 };
        propertySetter(ref fields, 5678);
        fieldSetter(ref fields, 90);
        Console.WriteLine(fields.MaxValue);
    }
}

Answer 3

Not sure if this will fit into your constraints, but by declaring the struct instance as ValueType, SetValue will work as expected.

    ValueType fields = new TestFields { MaxValue = 1234 };  // test struct with one field
    FieldInfo info = typeof(TestFields).GetField("MaxValue");  // get the FieldInfo
    info.SetValue(fields, 4096);
    Console.WriteLine(((TestFields)fields).MaxValue);  // 4096

See this answer for some more info.