Is there a way to set properties on struct instances using reflection?

Ever heard of SetValueDirect? There's a reason they made it. :)

struct MyStruct { public int Field; }

static class Program
{
    static void Main()
    {
        var s = new MyStruct();
        s.GetType().GetField("Field").SetValueDirect(__makeref(s), 5);
        System.Console.WriteLine(s.Field); //Prints 5
    }
}

There's other methods than the undocumented __makeref which you could use (see System.TypedReference) but they're more painful.

The value of rectangle is being boxed - but then you're losing the boxed value, which is what's being modified. Try this:

Rectangle rectangle = new Rectangle();
PropertyInfo propertyInfo = typeof(Rectangle).GetProperty("Height");
object boxed = rectangle;
propertyInfo.SetValue(boxed, 5, null);
rectangle = (Rectangle) boxed;