Attributes and Named/Optional constructor parameters not working

Question

I have custom attribute defined like so:

  [AttributeUsage(AttributeTargets.Field)]
  public class EnumDisplayAttribute : Attribute
  {
    public string Des         


        

Answer 1

Optional parameters were added to C# after optional values for attributes already existed in C#. Therefore, for optional attribute parameters, you should fall back to the attribute-specific syntax:

[AttributeUsage(AttributeTargets.Field)]
public class EnumDisplayAttribute : Attribute
{
    public string Description { get; set; }
    public string Code { get; set; }

    public EnumDisplayAttribute()
    {
    }
}

public enum TransactionType
{
    [EnumDisplay(Code = "B")] 
    Bill,
    [EnumDisplay(Description = null, Code = "C")]
    CashReceipt,
}

As you see, the end-result is effectively the same, but instead of using named arguments, you are using named properties (where syntax like [EnumDisplay(Description = null, Code = "C")] is only possible in attribute declarations).

Another way to think of it is that attribute declarations "borrowed" its syntax from method/constructor invocations, but attribute declarations are not in themselves method invocations, so they don't get all the same features as methods.

Answer 2

Optional parameters are not really optional, the method signature has all arguments in it and attributes are special (existed before optional parameters and have different rules when applied as an attribute (eg consider who calls the attribute constructor)). I imagine however that support will be added in the future.

For now, if you wish to achieve the optional effect try the following:

[AttributeUsage(AttributeTargets.Field)]
public class EnumDisplayAttribute : Attribute
{
  public string Description { get; set; }
  public string Code { get; set; }

}

And apply as so:

[EnumDisplay(Description = null, Code = "C")]
private object _aField;

Answer 3

If you do want to push values into your attribute using a constructor (e.g. if some of your attribute's properties are mandatory or to perform some kind of processing on them) you can always go old school and overload the constructor.

For example:

[AttributeUsage(AttributeTargets.Field)]
public class SampleAttribute : Attribute
{
    public string MandatoryProperty { get; private set; }
    public string OptionalProperty { get; private set; }

    // we use an overload here instead of optional parameters because 
    // C# does not currently support optional constructor parameters in attributes
    public SampleAttribute(string mandatoryProperty)
        : this(mandatoryProperty, null)
    {
    }

    public SampleAttribute(string mandatoryProperty, string optionalProperty)
    {
        MandatoryProperty = mandatoryProperty;
        OptionalProperty = optionalProperty;
    }
}