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.

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// Create struct DogCollarInfo collar1 = new DogCollarInfo(0.5, 14.0); collar1.Dump();   // Modify data in struct collar1.Length = 20.0; collar1.Dump();

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

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// 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:

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// Do this instead d.Collar = new DogCollarInfo(<span class="skimlinks-unlinked">d.Collar.Width</span>, 10.0); Console.WriteLine("Dog's collar:"); <span class="skimlinks-unlinked">d.Collar.Dump</span>();

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