Attribute argument must be a constant expression

Question

Could someone please explain to me why the following interface definition compiles with error in Visual Studio 2010?

   [IncompleteCodePort(SourceOriginType.Other, "This should be a GL abstraction depending on what OpenGL API will be used")]
    public interface IGL
    {
        /// <summary>
        /// Returns true if provided function is available or supported by graphics API
        /// </summary>
        /// <param name="funcName"></param>
        /// <returns></returns>
        bool IsFunctionAvailable(string funcName);

        /// <summary>
        /// Returns true if provided function is supported as extension by graphics API
        /// </summary>
        /// <param name="funcName"></param>
        /// <returns></returns>
        bool IsExtensionAvailable(string funcName);
    }



public class IncompleteCodePortAttribute : Attribute
    {
        public SourceOriginType SourceOriginType { get; private set; }
        public string SourceUrl { get; private set; }
        public string Reason { get; private set; }


        public IncompleteCodePortAttribute(SourceOriginType originType, string reason, string sourceUrl = null)
        {
            SourceOriginType = originType;
            SourceUrl = sourceUrl;
            Reason = reason;
        }
    }

    public enum SourceOriginType
    {
        CodePlex,
        WorldWindJdk,
        StackOverflow,
        Other
    }

and error I am getting is:

An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type

If I remove the custom attribute, I get no compile errors.

I was able to compile with a dummy IncompleteCodePort class. I believe you are missing some information here. — Manish, Feb 24 '13 at 05:16
If you are using `Enum` in the `IncompleteCodePort` class, don't. Use `SourceOriginType` instead. — Corey, Feb 24 '13 at 05:27
My bad. Posted this way too late. I have added the IncompleteCodePortAttribute definition. — Klaus Nji, Feb 24 '13 at 15:04
I take what I said before back, it seems the code actually *doesn't* compile fine under VS2010 (but it does under VS2012). — svick, Feb 24 '13 at 15:15
It's a bug in the c# compiler. It has been fixed. See http://stackoverflow.com/questions/8290853/attribute-argument-must-be-a-constant-error-when-using-an-optional-parameter-in — Jonathan Moffatt, Jul 04 '13 at 05:22

score 2 · Accepted Answer · answered Feb 24 '13 at 15:28

This seems to be a bug in the C# compiler in VS2010 (the code compiles fine under VS2012). It looks like the compiler doesn't treat the null in the default value of the parameter as constant.

This code doesn't compile:

[IncompleteCodePort()]
interface IGL
{}

class IncompleteCodePortAttribute : Attribute
{
    public IncompleteCodePortAttribute(string sourceUrl = null)
    {}
}

with the mentioned error message (“An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type”), but confusingly without a source code location.

Some examples of declaration of the attribute that do work:

class IncompleteCodePortAttribute : Attribute
{
    public IncompleteCodePortAttribute(string sourceUrl = "")
    {}
}

class IncompleteCodePortAttribute : Attribute
{
    private const string Null = null;

    public IncompleteCodePortAttribute(string sourceUrl = Null)
    {}
}

class IncompleteCodePortAttribute : Attribute
{
    public IncompleteCodePortAttribute()
        : this(null)
    {}

    public IncompleteCodePortAttribute(string sourceUrl)
    {}
}

Thanks. Strangely enough, I opened solution in VS2012, went back into VS2010 and reloaded, now it compiles. — Klaus Nji, Feb 25 '13 at 12:36
It did, now it does not compile. I just removed the optional constructor parameter and overloaded the constructor. Thanks. — Klaus Nji, Feb 25 '13 at 12:55
See http://stackoverflow.com/questions/8290853/attribute-argument-must-be-a-constant-error-when-using-an-optional-parameter-in — Jonathan Moffatt, Jul 04 '13 at 05:23
Good to know we are getting a service pack with this fixed... O wait. — Sellorio, Dec 16 '13 at 22:55

Attribute argument must be a constant expression

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