C# – Attribute argument must be a constant error when using an optional parameter in the attribute constructor

cnetvisual studio 2010

Can anyone explain why this code works:

public class AdministratorSettingValidationAttribute : Attribute
{
    public AdministratorSettingValidationAttribute(AdministratorSettingDataType administratorSettingDataType)
    {
        DataType = administratorSettingDataType;
    }

    public AdministratorSettingValidationAttribute(AdministratorSettingDataType administratorSettingDataType, Type enumerationType)
    {
        DataType = administratorSettingDataType;
        EnumerationType = enumerationType;
    }
}

…but refactoring it to use an optional parameter instead:

    public AdministratorSettingValidationAttribute(AdministratorSettingDataType administratorSettingDataType, Type enumerationType = null)
    {
        DataType = administratorSettingDataType;
        EnumerationType = enumerationType;
    }

…causes a compile time error:
"An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type".

Best Answer

UPDATE

The bug was reported in July of last year and is already fixed. The fix will appear in the next version of C#. See this Connect feedback item for details:

http://connect.microsoft.com/VisualStudio/feedback/details/574497/optional-parameter-of-type-string-in-a-attribute-constructor-cannot-be-null

That's clearly a compiler bug. Thanks for bringing it to my attention.

What is supposed to happen here is the compiler is supposed to realize that the optional value expression is implicitly converted to the formal parameter type, and then treat the expression as a constant expression of that type. What it is actually doing is treating the expression as the typeless null literal, which is wrong.

You can work around the bug by turning the constant into an explicitly typed one:

public AdministratorSettingValidationAttribute(AdministratorSettingDataType administratorSettingDataType, Type enumerationType = (Type)null)

The fix is probably straightforward but I cannot promise that the fix will be in the next version of C#; I'm not sure what the schedule is like for taking non-critical bug fixes at this point.

Thanks again, and apologies for the inconvenience.

Related Solutions

C# – How to pass objects into an attribute constructor

The values into attributes are limited to simple types; for example, basic constants (including strings) and typeof... you can't use new or other more complex code. In short; you can't do this. You can give it the type though:

[PropertyValidation(typeof(NullOrEmptyValidatorScheme)]

i.e. the PropertyValidation ctor takes a Type, and use Activator.CreateInstance inside the code to create the object. Note that you should ideally just store the string internally (AssemblyQualifiedName).

From ECMA 334v4:

§24.1.3 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, short, string.

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.

and

§24.2 Attribute specification

...

An expression E is an attribute-argument-expression if all of the following statements are true:

The type of E is an attribute parameter type (§24.1.3).

At compile-time, the value of E can be resolved to one of the following:

A constant value.

A typeof-expression (§14.5.11) specifying a non-generic type, a closed constructed type (§25.5.2), or an unbound generic type (§25.5).

A one-dimensional array of attribute-argument-expressions.

C# – DisplayName attribute from Resources

If you use MVC 3 and .NET 4, you can use the new Display attribute in the System.ComponentModel.DataAnnotations namespace. This attribute replaces the DisplayName attribute and provides much more functionality, including localization support.

In your case, you would use it like this:

public class MyModel
{
    [Required]
    [Display(Name = "labelForName", ResourceType = typeof(Resources.Resources))]
    public string name{ get; set; }
}

As a side note, this attribute will not work with resources inside App_GlobalResources or App_LocalResources. This has to do with the custom tool (GlobalResourceProxyGenerator) these resources use. Instead make sure your resource file is set to 'Embedded resource' and use the 'ResXFileCodeGenerator' custom tool.

(As a further side note, you shouldn't be using App_GlobalResources or App_LocalResources with MVC. You can read more about why this is the case here)

Best Answer

UPDATE

Related Solutions

C# – How to pass objects into an attribute constructor

C# – DisplayName attribute from Resources

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