Rory Primrose

Learn from my mistakes, you don't have time to make them yourself

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VS2008 read-only automatic properties

Posted on December 13, 2007

I have started using the very nice automatic properties in VS2008. As I was using these, I was thinking about how they work when you define the property as read-only or write-only. Without a backing field, you wouldn’t be able to read from or write to the backing field, rendering the property useless.

I coded an automatic property in this way and didn’t get any error indication from the IDE, but I didn’t actually compile it. I have since run some code analysis that suggests that my collection properties should be readonly. As these properties are automatic properties, I then removed the setter and compiled.

Boom! Now there is a compiler error CS0840 that includes the message "Automatically implemented properties must define both get and set accessors". It is unfortunate that this wasn’t indicated with those helpful squiggly red lines in the code editor, but not a major problem.

This does highlight an issue though. How can you implement read-only automatic properties? The answer is quite simple and is provided by the help description of the compiler error. It says "To create a read-only auto-implemented property, make the set accessor private".

So the code will look something like this:

public class SomeClass
    public SomeClass()
        SomeProperty = new Collection<SomeOtherClass>();
    public Collection<SomeOtherClass> SomeProperty
        private set;

That is very neat.