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Defining Attributes

In the previous example, note that the syntax used to attach an attribute to a type or member looks a bit like the instantiation of a class. This is because an attribute is actually a class derived from the System.Attribute base class.

Now let's flesh out the RegistryKey attribute a little bit: -

public enum RegistryHives
{
    HKEY_CLASSES_ROOT,
    HKEY_CURRENT_USER,
    HKEY_LOCAL_MACHINE,
    HKEY_USERS,
    HKEY_CURRENT_CONFIG
}

public class RegistryKeyAttribute : Attribute
{
    public RegistryKeyAttribute(RegistryHives Hive, String ValueName)
    {
        this.Hive = Hive;
        this.ValueName = ValueName;
    }

    protected RegistryHives hive;
    public RegistryHives Hive
    {
        get { return hive; }
        set { hive = value; }
    }

    protected String valueName;
    public String ValueName
    {
        get { return valueName; }
        set { valueName = value; }
    }
}

What I've done here is add an enum for the different Registry types, a constructor for the attribute class (which takes a Registry type and a value name), and two properties for the Registry hive and value name. There's much more you can do when defining attributes, but at this point, because we know how to define and attach attributes, let's go ahead and learn how to query for attributes at run time. That way, we'll have a fully working example to play with. Once we've done that, we'll move on to some of the more advanced issues with defining and attaching attributes.

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