MyStruct myStruct = new MyStruct();

myStruct.myField = 3;

​

FieldInfo fi = typeof(MyStruct).GetField("myField", BindingFlags.Public | BindingFlags.Instance);

TypedReference reference = __makeref(myStruct);

fi.SetValueDirect(reference, 111);

Console.WriteLine(myStruct.myField); //prints 111