C# – An attribute argument must be a constant expression, …- Create an attribute of type array

ccustom-attributes

Here is my custom attribute and a class I'm using it on:

[MethodAttribute(new []{new MethodAttributeMembers(), new MethodAttributeMembers()})]
public class JN_Country
{

}

public class MethodAttribute : Attribute
{
    public MethodAttributeMembers[] MethodAttributeMembers { get; set; }

    public MethodAttribute(MethodAttributeMembers[] methodAttributeMemberses)
    {
        MethodAttributeMembers = methodAttributeMemberses;
    }
}

public class MethodAttributeMembers
{
    public string MethodName { get; set; }
    public string Method { get; set; }
    public string MethodTitle { get; set; }
}

The syntax error, displayed on the first line above:

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

Why does this error occur?

Best Answer

This supplements the information Simon already gave.

I found some documentation here: Attribute parameter types:

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 (Attribute specification).
  • Single-dimensional arrays of the above types. (emphasis added by me)

A constructor argument or public field which does not have one of these types, cannot be used as a positional or named parameter in an attribute specification.

The last bullet point explains your syntax error. You've defined a one-dimensional array, but it should only be of primitive types, string, etc. as listed in the previous bullet points.

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