How to mutate a boxed struct using IL

Question

Imagine we have a mutable struct (yes, don\'t start):

public struct MutableStruct
{
    public int Foo { get; set; }
    public override string          


        

Answer 1

Well, that was fun.

Using Ldflda and Stind_* seems to work. Actually, it's mostly Unbox (see history for version that works with Ldflda and Stind_*).

Here's what I hacked together in LinqPad to prove it out.

public struct MutableStruct
{
    public int Foo { get; set; }

    public override string ToString()
    {
        return Foo.ToString();
    }
}

void Main()
{
    var foo = typeof(MutableStruct).GetProperty("Foo");
    var setFoo = foo.SetMethod;

    var dynMtd = new DynamicMethod("Evil", typeof(void), new [] { typeof(object), typeof(int) });
    var il = dynMtd.GetILGenerator();
    il.Emit(OpCodes.Ldarg_0);                       // object
    il.Emit(OpCodes.Unbox, typeof(MutableStruct));  // MutableStruct&
    il.Emit(OpCodes.Ldarg_1);                       // MutableStruct& int
    il.Emit(OpCodes.Call, setFoo);                  // --empty--
    il.Emit(OpCodes.Ret);                           // --empty--

    var del = (Action)dynMtd.CreateDelegate(typeof(Action));

    var mut = new MutableStruct { Foo = 123 };

    var boxed= (object)mut;

    del(boxed, 456);

    var unboxed = (MutableStruct)boxed;
    // unboxed.Foo = 456, mut.Foo = 123
}