#779 – Methods in struct that Modify Elements Can Be Dangerous

If you have a method in a struct that modifies a data member of the struct, you can run into unexpected results, due to the value type semantics of the struct.

Assume that we have a struct that includes a method that can change the value of one of its data members.  The method works just fine for a locally defined instance of the struct.

            // Method that modifies struct works if local
            DogCollarInfo collar = new DogCollarInfo(0.5, 8.0);
            collar.Dump();
            collar.DoubleLength();
            collar.Dump();

But if you have a property of some class whose type is this struct, it’s no longer safe to call this method. Because the property’s get accessor returns a copy of the struct, the data in the original struct won’t get modified.

            // Does not work if struct is property
            Dog d = new Dog("Kirby");
            d.Collar = new DogCollarInfo(0.5, 8.0);
            d.Collar.Dump();
            d.Collar.DoubleLength();
            d.Collar.Dump();   // Length not doubled!