Question

I had this solution working happily and i added this attribute:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = false, Inherited = true)]
public class metadata : Attribute
{
    string mid;
    string mdescription;
    Dictionary<string, string> mdata;
    public string id
    {
        get
        {
            return mid;
        }
    }
    public string description
    {
        get
        {
            return mdescription;
        }
    }
    public Dictionary<string, string> data
    {
        get
        {
            return mdata;
        }
    }
    public metadata(string thisid, string thisdescription, params KeyValuePair<string, string>[] thisdataarray)
    {
        mid = thisid;
        mdescription = thisdescription;
        mdata = new Dictionary<string, string>();
        if (thisdataarray != null)
        {
            foreach (KeyValuePair<string, string> thisvaluepair in thisdataarray)
            {
                mdata.Add(thisvaluepair.Key, thisvaluepair.Value);
            }
        }
    }
}

I added a few declarations of this attribute, all [metadata(null, null)] with no errors. Somehow, when I compile, for each [metadata(null, null)] an “error CS0182: An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type” occurs, however with no line or column or file, only the project. What went wrong? Thanks.

Answer 1

The problem is that KeyValuePair is not supported as attribute parameter type.
According to the C# Language specification:

The types of positional and named parameters for an attribute class are limited to the attribute parameter types, which are:

• One of the following types: bool, byte, char, double, float, int,
  long, sbyte, short, string, uint, ulong, ushort.
• The type object.
• The type System.Type.
• An enum type, provided it has public accessibility and the types in
  which it is nested (if any) also have public accessibility.
• Single-dimensional arrays of the above types.

As a workaround you can pass an array of strings where the odd members are keys and even members are values. Then inside the attribute constructor you can create your Dictionary.