How can I pass an attribute parameter type with List<string> in C#?

Question

How can I pass a List to a construtor?

It shows a message:

Error 14 An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type

public class CustomAuthorize : AuthorizeAttribute {
    private List<string> multipleProgramID;

    //constructor
    public CustomAuthorize(List<string> _multipleProgramID) {
        multipleProgramID = _multipleProgramID;
    }
}

[CustomAuthorize(new List<string>(new string[] { ProgramList.SURVEY_INPUT, ProgramList.SURVEY_OUTPUT } ))]
[HttpPost]
public ActionResult DeleteWaterQualityItems(string sourceID, string wqID) {
    // ..other code...
}

public class ProgramList {
    public const string SURVEY_INPUT = "A001";
    public const string SURVEY_INPUT = "A002";
}

Jon Skeet · Accepted Answer · 2014-10-09 10:42:06Z

19

The problem isn't passing a List<string> to a constructor in general - the problem is that you're trying to use it for an attribute. You basically can't do that, because it's not a compile-time constant.

It looks like ProgramList is effectively an enum - so you might want to make it an enum instead:

 [Flags]
 public enum ProgramLists
 {
     SurveyInput,
     SurveyOutput,
     ...
 }

Then make your CustomAuthorizeAttribute (which should be named like that, with a suffix of Attribute) accept a ProgramLists in the constructor. You'd specify it as:

[CustomAuthorize(ProgramLists.SurveyInput | ProgramLists.SurveyOutput)]

You can then have a separate way of mapping each ProgramLists element to a string such as "A001". This could be done by applying an attribute to each element, or maybe having a Dictionary<ProgramLists, string> somewhere.

If you really want to keep using strings like this, you could make CustomAuthorizeAttribute accept a single comma-separated list, or make it an array instead of a list and use a parameter array:

[AttributeUsage(AttributeTargets.Method)]
public class FooAttribute : Attribute
{
    public FooAttribute(params string[] values)
    {
        ...
    }
}

[Foo("a", "b")]
static void SomeMethod()
{
}

edited Oct 9, 2014 at 10:42

answered Oct 9, 2014 at 10:36

Jon Skeet

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3 Comments

curiousBoy Over a year ago

in 2020, is this still to way to go?

Jon Skeet Over a year ago

@curiousBoy: Yes, attributes still can't use lists.

LordDraagon Over a year ago

And in 2021 I presume it's still the case. If anyone has a better way please add it as an answer. Thank You.

score 8 · Accepted Answer · 2018-06-19 09:20:31Z

8

You can't use List<T>.

Attributes have restrictions on parameters and property types, because they have to be available at compile-time. Using attributes in C#

Use an array instead:

//constructor
public CustomAuthorize(string[] _multipleProgramID) 
{
    ...
}

// usage
[CustomAuthorize(new string[] { ProgramList.SURVEY_INPUT, ProgramList.SURVEY_OUTPUT })]

edited Jun 19, 2018 at 9:20

answered Oct 9, 2014 at 10:52

user1023602

3 Comments

Ian G Over a year ago

The compiler suggests this is possible - but for some reason this exact code (with string rather than enum) didn't work for me :( I got the same error about only being able to use constant expressions, types and array initializers - which is exactly what I was doing! Had to use a string split, as suggested by @blues_driven.

user1023602 Over a year ago

just tried it and it does compile in VS 2017, but it doesn't have to work for me - it has to work for you :-)

Ian G Over a year ago

I was trying to pass a string array argument as a NUnut TestCase parameter. [TestCase(new string[] {"a"})] public void TestMethod(string[] ab) - this gives me

Error	CS0182	An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type

. And I've just worked out why. It couldn't work out which parameter I was trying to pass - If I make an object array WHICH CONTAINS my string array it works: [TestCase(new object[] { new string[] { "a" } })]

blues_driven · Accepted Answer · 2015-07-28 10:56:13Z

2

I was trying to do something similar and ended up passing a comma separated string and using string.Spilt(',') in the attribute constructor to convert it to an array.

answered Jul 28, 2015 at 10:56

blues_driven

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Comments

Abubakar Riaz · Accepted Answer · 2017-06-12 10:02:36Z

2

You just need to use "params" keyword in your custom attribute constructor and make your programList "enum"

[CustomAttribute(ProgramList.SURVEY_INPUT, ProgramList.SURVEY_OUTPUT)]
[HttpPost]
public ActionResult DeleteWaterQualityItems(string sourceID, string wqID) {
    // ..other code...
}

in you custom attribute class use this

public class CustomAuthorize : AuthorizeAttribute {

//Constructor
public CustomAuthorize (params ProgramList[] programListTypes) {

            multipleProgramID= programListTypes;
        }

private ProgramList[] multipleProgramID;

}

and you enum class

public enum ProgramList : int
{
SURVEY_INPUT = 001;
SURVEY_OUTPUT =002;

}

answered Jun 12, 2017 at 10:02

Abubakar Riaz

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Comments

user117499 · Accepted Answer · 2021-02-01 03:48:30Z

0

One possible option:

   public class ConstraintExpectedValues : ConstraintAttribute
    {

        public ConstraintExpectedValues(object[] expectedValues)
        {
            this.ExpectedValues = expectedValues;
        }

        public object[] ExpectedValues { get; set; }
    }

Usage:

[ConstraintExpectedValues(new object[] {5,7,9})]

answered Feb 1, 2021 at 3:48

user117499

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