Why doesn't reflection set a property in a Str

问题:

   class PriceClass {

        private int value;
        public int Value
        {
            get { return this.value; }
            set { this.value = value; }
        }           
    }


    struct PriceStruct
    {

        private int value;
        public int Value
        {
            get { return this.value; }
            set { this.value = value; }
        }
    }
    static void Main(string[] args)
    {
        PriceClass _priceClass = new PriceClass();
        Type type = typeof(PriceClass);
        PropertyInfo info = type.GetProperty("Value");
        info.SetValue(_priceClass, 32, null);
        Console.WriteLine(_priceClass.Value);

        PriceStruct _priceStruct = new PriceStruct();
        type = typeof(PriceStruct);
        info = type.GetProperty("Value");
        info.SetValue(_priceStruct, 32, null);
        Console.WriteLine(_priceStruct.Value);

        Debugger.Break();
    }

The first value printed is 32 while the second is 0. No exception thrown

回答1:

It's because boxing your struct makes a copy of it, so you should box it earlier so you call the getter from the same data that you modified. The following code works:

    object _priceStruct = new PriceStruct(); //Box first
    type = typeof(PriceStruct);
    info = type.GetProperty("Value");
    info.SetValue(_priceStruct, 32, null);
    Console.WriteLine(((PriceStruct)_priceStruct).Value); //now unbox and get value

    Debugger.Break();

回答2:

structs are ValueTypes, which are passed by value, that means you only pass around copies of the entire struct, not a reference to the original object.

So when you pass it into info.SetValue(_priceStruct, 32, null), a copy is passed to the method and mutated, so the original object doesn't get changed at all. Another reason why mutable structs are evil.

回答3:

You can still change them using reflection but it is a bit long winded.

See this example: http://social.msdn.microsoft.com/Forums/en/netfxbcl/thread/2dd4315c-0d0d-405c-8d52-b4b176997472