Allow complex default values

[floatas]

Hi, I'm just starting to use this library and wanted to discuss one issue before making any real changes and pull requests. I wanted to set temp directory path as default value. I come up with solution, but it's not prettiest solution and can be improved by making few small changes to PowerArgs. Maybe there is different way to achieve same result ?

Complex default value now:

    [AttributeUsage(AttributeTargets.Property | AttributeTargets.Parameter)]
    public class TempDirPathAsDefaultValue : DefaultValueAttribute
    {
        static string TempDir = "";
        static TempDirPathAsDefaultValue()
        {
            TempDir = System.IO.Path.GetTempPath();
        }

        public TempDirPathAsDefaultValue() : base(TempDir)
        {
        }
    }

Two small changes are required (see comments in code):

    [AttributeUsage(AttributeTargets.Property | AttributeTargets.Parameter)]
    public class DefaultValueAttribute : ArgHook, ICommandLineArgumentMetadata
    {
        public object Value { get; protected set; }//<- Protected setter

        public DefaultValueAttribute(object value)
        {
            Value = value;
        }

        protected DefaultValueAttribute()//<- protected constructor
        {
        }

        public override void BeforePopulateProperty(HookContext Context)
        {
            if (Context.ArgumentValue == null) Context.ArgumentValue = Value.ToString();
        }
    }

Complex default value after changes:

    [AttributeUsage(AttributeTargets.Property | AttributeTargets.Parameter)]
    public class TempDirPathAsDefaultValue : DefaultValueAttribute
    {
        public TempDirPathAsDefaultValue() : base()
        {
            Value = System.IO.Path.GetTempPath();
        }
    }

Let me know what you think, I can make pull request for this + tests if needed.

[adamabdelhamed]

For complex default values you can always just make the getter of your property do the work like this. No need to add more complexity to PowerArgs.

private string _tempPath;
public string TempPath
{
    get
    {
        return _tempPath || System.IO.Path.GetTempPath();
    }
    set
    {
        _tempPath = value;
    }
}