#777 – A struct Isn’t Mutable When Used as a Property

A struct is normally mutable, i.e. you can modify the values of its members directly.  Assume that we have a DogCollarInfo struct with Width and Length members.  We can create the struct and then later modify it.

            // Create struct
            DogCollarInfo collar1 = new DogCollarInfo(0.5, 14.0);
            collar1.Dump();

            // Modify data in struct
            collar1.Length = 20.0;
            collar1.Dump();

777-001

However, if a struct is used as a property in another object, we can’t modify its members.

            // Create Dog object and set Collar property
            Dog d = new Dog("Kirby");
            d.Collar = collar1;

            // Compile-time error: Can't modify Collar because it's not a variable
            d.Collar.Length = 10.0;

Because the struct is a value type, the property accessor (get) returns a copy of the struct.  It wouldn’t make sense to change the copy, so the compiler warns us.

You can instead create a new copy of the struct:

            // Do this instead
            d.Collar = new DogCollarInfo(d.Collar.Width, 10.0);
            Console.WriteLine("Dog's collar:");
            d.Collar.Dump();

777-002

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About Sean
Software developer in the Twin Cities area, passionate about .NET technologies. Equally passionate about my own personal projects related to family history and preservation of family stories and photos.

One Response to #777 – A struct Isn’t Mutable When Used as a Property

  1. Pingback: Dew Drop – February 12, 2013 (#1,497) | Alvin Ashcraft's Morning Dew

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