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

Here you go:

Just use unsafe :)

static void Main(string[] args)
{
  object foo = new MutableStruct {Foo = 123};
  Console.WriteLine(foo);
  Bar(foo);
  Console.WriteLine(foo);
}

static unsafe void Bar(object foo)
{
  GCHandle h = GCHandle.Alloc(foo, GCHandleType.Pinned);

  MutableStruct* fp = (MutableStruct*)(void*)  h.AddrOfPinnedObject();

  fp->Foo = 789;
}

IL implementation is left as an exercise to the reader.

Update:

Based on the Kevin's answer, here is a minimal working example:

ldarg.0
unbox      MutableStruct
ldarg.1
call       instance void MutableStruct::set_Foo(int32)
ret