Question

Is it possible in an C# attribute to explicitly check what named parameters are set?

The problem is that I have a couple of parameters of type bool in an attribute, and I want to explicitly check which one of them are set. I know I could probably make them nullable and check against that in the code, but is there a better way?

Answer 1

This appears to do what you want, assuming you control the attribute itself:

using System;

[AttributeUsage(AttributeTargets.All)]
class SampleAttribute : Attribute
{
    private bool hasFlag = false;
    public bool HasFlag { get { return hasFlag; } }

    private bool flag = false;
    public bool Flag
    {
        get { return flag; }
        set
        {
            flag = value;
            hasFlag = true;
        }
    }
}

class Test
{
    static void Main()
    {
        foreach (var method in typeof(Test).GetMethods())
        {
            var attributes = (SampleAttribute[])
                method.GetCustomAttributes(typeof(SampleAttribute), false);
            if (attributes.Length > 0)
            {
                Console.WriteLine("{0}: Flag={1} HasFlag={2}",
                                  method.Name,
                                  attributes[0].Flag,
                                  attributes[0].HasFlag);
            }
        }
    }

    [Sample(Flag = true)]
    public static void WithFlagTrue() {}

    [Sample(Flag = false)]
    public static void WithFlagFalse() {}

    [Sample]
    public static void WithoutFlag() {}
}

Results:

WithFlagTrue: Flag=True HasFlag=True
WithFlagFalse: Flag=False HasFlag=True
WithoutFlag: Flag=False HasFlag=False

I’m not sure whether it’s really a good idea, mind you…