Calling a method by enum Attribute in C#

Question

First, I'm so sorry because my bellow stupid question. But I hope someone can help me on this approach.

I have an Enum that I want to be add new magic attribute as described:

public enum FunctionType
    {
        [CallMethod(ExecuteFunction.DOPLUS)] //How to implement CallMethod magic attribute
        PLUS,
        [CallMethod(ExecuteFunction.DOMINUS)]
        MINUS,
        [CallMethod(ExecuteFunction.DOMULTIPLY)]
        MULTIPLY,
        [CallMethod(ExecuteFunction.DODIVIDE)]
        DIVIDE
    }

My class has a FunctionType property like this:

public class Function
    {
        private FunctionType _functionType;

        public List<object> Params
        { get; set; }

        public FunctionType FunctionType
        {
            get { return _functionType; }
            set { _functionType = value; }
        }

        public string Execute()
        {
            return SomeMagicMethod(this.FunctionType); //How to implement this method to return my result as expected
        }
    }

Last, my calculate class has some functions return result:

public static class ExecuteFunction
    {
        public static string DOPLUS(int a, int b)
        {
            return (a + b).ToString();
        }

        public static string DOMINUS(int a, int b)
        {
            return (a - b).ToString();
        }

        public static string DOMULTIPLY(int a, int b)
        {
            return (a * b).ToString();
        }

        public static string DODIVIDE(int a, int b)
        {
            return (a / b).ToString();
        }
    }

My stupid question is: How can I implement CallMethodAttribute in enum and SomeMagicMethod above to run specified method without using switch case as normal ?

Forget the calling part for the moment - your attribute simply isn't going to be compilable right now, as the argument for the attribute has to be a compile-time constant value of one of a few supported types. You could have a string specifying the name of the method though. As an aside, I would strongly suggest following .NET naming conventions - lose the shouty names. — Jon Skeet
– Jon Skeet, Commented Aug 15, 2014 at 8:32

darkl · Accepted Answer · 2014-08-15 08:52:26Z

You can't put a reference to a method in an attribute as you wrote (it is not compile-time).

Your approach is wrong - You should decorate the methods with an attribute referring their corresponding enum, like this:

public static class ExecuteFunction
{
    [CallMethod(FunctionType.PLUS)]
    public static string DOPLUS(int a, int b)
    {
        return (a + b).ToString();
    }

    [CallMethod(FunctionType.MINUS)]
    public static string DOMINUS(int a, int b)
    {
        return (a - b).ToString();
    }

    [CallMethod(FunctionType.MULTIPLY)]
    public static string DOMULTIPLY(int a, int b)
    {
        return (a * b).ToString();
    }

    [CallMethod(FunctionType.DIVIDE)]
    public static string DODIVIDE(int a, int b)
    {
        return (a / b).ToString();
    }
}

The attribute code:

[AttributeUsage(AttributeTargets.Method, Inherited = true, AllowMultiple = false)]
public class CallMethodAttribute : Attribute
{
    private readonly FunctionType mFunctionType;

    public CallMethodAttribute(FunctionType functionType)
    {
        mFunctionType = functionType;
    }

    public FunctionType FunctionType
    {
        get { return mFunctionType; }
    }
}

And then detect the corresponding method for a given enum value type with reflection and invoke it:

public class YourMagicClass
{
    private static readonly Dictionary<FunctionType, MethodInfo> FunctionTypeToMethod =
        typeof (ExecuteFunction).
            GetMethods(BindingFlags.Public | BindingFlags.Static)
                                .Where(x => x.IsDefined(typeof (CallMethodAttribute)))
                                .Select(x => new
                                    {
                                        Method = x,
                                        FunctionType = x.GetCustomAttribute<CallMethodAttribute>().FunctionType
                                    })
                                .ToDictionary(x => x.FunctionType, x => x.Method);


    public static string SomeMagicMethod(FunctionType functionType, int a, int b)
    {
        MethodInfo method;

        if (!FunctionTypeToMethod.TryGetValue(functionType, out method))
        {
            throw new ArgumentException("Could not find a handler for the given function type", "functionType");
        }
        else
        {
            string result = (string)method.Invoke(null, new object[] { a, b });

            return result;
        }
    }
}

Of course, optimizations can be done such as caching a compiled delegate using Delegate.CreateDelegate.

You're right. I was wrong in my approach. Thanks you for helping me.

Douglas · Accepted Answer · 2014-08-15 08:38:25Z

2

If you're ready to replace your attributes with a mapping dictionary:

public class Function 
{
    private static readonly IDictionary<FunctionType, Func<int, int, string>> functionMappings = 
        new Dictionary<FunctionType, Func<int, int, string>>
    {
        { FunctionType.PLUS, ExecuteFunction.DOPLUS },
        { FunctionType.MINUS, ExecuteFunction.DOMINUS },
        { FunctionType.MULTIPLY, ExecuteFunction.DOMULTIPLY },
        { FunctionType.DIVIDE, ExecuteFunction.DODIVIDE },
    };

    public string Execute()
    {
        return functionMappings[_functionType]((int)Params[0], (int)Params[1]);
    }
}

answered Aug 15, 2014 at 8:38

Douglas

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1 Comment

Tan Le Over a year ago

Your answer is so great. It's maybe alternative way to solve my problem now. You also have my accepted.

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