Enum from type and calling an extension method

Question

 static void Main(string[] args)
    {
        List<string> tests = new List<string>() { "TypeTester.NonExist", "TypeTester.Thing", "TypeTester.Fruit", "TypeTester.Color" };
        PrintEnums();
        foreach (var item in tests)
        {
            try
            {
                ConvertFromTypeName(item);
            }
            catch (TypeLoadException)
            {
                Console.WriteLine("Could not load {0}", item);
            }
            catch (ArgumentException e)
            {
                Console.WriteLine(e.Message);
            }    
        }
        Console.ReadLine();
    }

    static void PrintEnums()
    {
        Console.WriteLine("Printing Fruit");
        foreach (Fruit thing in Enum.GetValues(typeof(Fruit)))
        {
            Console.WriteLine("Value: {0}, Description: {1}", (int)thing, thing.GetDescription());
        }
        Console.WriteLine("Printing Color");
        foreach (Color thing in Enum.GetValues(typeof(Color)))
        {
            Console.WriteLine("Value: {0}, Description: {1}", (int)thing, thing.GetDescription());
        }
    }

    static void ConvertFromTypeName(string typeName)
    {
        Type type = Type.GetType(typeName, true);
        if (!type.IsEnum)
            throw new ArgumentException(typeName + " is not an enum.");

        Console.WriteLine("UnderlingType: {0}", type.UnderlyingSystemType);
        string description = string.Empty;
        foreach ( var thing in Enum.GetValues((type.UnderlyingSystemType)))
            {
//This will not compile                
//Console.WriteLine("Value: {0}, Description: {1}", (int)thing, thing.GetDescription());
                Console.WriteLine("Value: {0}, Description: {1}", (int)thing, thing.ToString());
        }
    }
}

public class Thing
{
    public int Id { get; set; }
    public string MyName { get; set; }
}

public enum Fruit
{
    [System.ComponentModel.DescriptionAttribute("I am an apple")]
    Apple = 8,
    [System.ComponentModel.DescriptionAttribute("I am an Banana")]
    Banana,
    [System.ComponentModel.DescriptionAttribute("I am an Grape")]
    Grape,
    [System.ComponentModel.DescriptionAttribute("I am an Kiwi")]
    Kiwi,
    [System.ComponentModel.DescriptionAttribute("I am an Orange")]
    Orange
}

public enum Color
{
    [System.ComponentModel.DescriptionAttribute("I am the color Red")]
    Red = 56,
    [System.ComponentModel.DescriptionAttribute("I am the color Green")]
    Green,
    [System.ComponentModel.DescriptionAttribute("I am the color Blue")]
    Blue,
    [System.ComponentModel.DescriptionAttribute("I am the color Black")]
    Black,
    [System.ComponentModel.DescriptionAttribute("I am the color White")]
    White
}

public static class Helper
{
    public static string GetDescription(this Enum value)
    {
        System.Reflection.FieldInfo field = value.GetType().GetField(value.ToString());
        System.ComponentModel.DescriptionAttribute attribute = Attribute.GetCustomAttribute(field, typeof(System.ComponentModel.DescriptionAttribute)) as System.ComponentModel.DescriptionAttribute;
        return attribute == null ? value.ToString() : attribute.Description;
    }
}

Why in ConvertFromTypeName if the underlying type is Fruit or Color I cannot call the GetDescription extension method? It seems to not to be able to infer that from the var. I would like this to work just like PrintEnums, the GetDescription extension.

Printing Fruit
Value: 8, Description: I am an apple
Value: 9, Description: I am an Banana
Value: 10, Description: I am an Grape
Value: 11, Description: I am an Kiwi
Value: 12, Description: I am an Orange
Printing Color
Value: 56, Description: I am the color Red
Value: 57, Description: I am the color Green
Value: 58, Description: I am the color Blue
Value: 59, Description: I am the color Black
Value: 60, Description: I am the color White
Could not load TypeTester.NonExist
TypeTester.Thing is not an enum.
UnderlingType: TypeTester.Fruit
Value: 8, Description: Apple
Value: 9, Description: Banana
Value: 10, Description: Grape
Value: 11, Description: Kiwi
Value: 12, Description: Orange
UnderlingType: TypeTester.Color
Value: 56, Description: Red
Value: 57, Description: Green
Value: 58, Description: Blue
Value: 59, Description: Black
Value: 60, Description: White

Please see the FAQ regarding the use of signatures in questions. — M.Babcock
– M.Babcock, Commented Jan 13, 2012 at 2:48

Eric · Accepted Answer · 2012-01-13 17:24:35Z

1

Got it. In the foreach of ConvertFromTypeName method I changed

Console.WriteLine("Value: {0}, Description: {1}", (int)thing, thing.ToString());

to

Console.WriteLine("Value: {0}, Description: {1}", (int)thing, ((Enum)thing).GetDescription());

Looks like all I needed to do was cast thing to an Enum to get @ the extension method

answered Jan 13, 2012 at 17:24

Eric

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Comments

Jon Skeet · Accepted Answer · 2012-01-13 02:34:46Z

0

You're mistaking the enum type with its underlying type. Look at this code:

Type type = Type.GetType(typeName, true);
if (!type.IsEnum)
    throw new ArgumentException(typeName + " is not an enum.");

Console.WriteLine("UnderlingType: {0}", type.UnderlyingSystemType);
string description = string.Empty;
foreach ( var thing in Enum.GetValues((type.UnderlyingSystemType)))
{
    ...
}

Here, type is the enum type - so why are you fetching the underlying type? That would be int in both the cases you've given. That isn't an enum type, so Enum.GetValues will clearly fail.

You just want:

foreach (var thing in Enum.GetValues(type))

answered Jan 13, 2012 at 2:34

Jon Skeet

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1 Comment

Eric Over a year ago

I still get an error if I uncomment out //Console.WriteLine("Value: {0}, Description: {1}", (int)thing, thing.GetDescription()); I get Error 2 'object' does not contain a definition for 'GetDescription' and the best extension method overload 'TypeTester.Helper.GetDescription(System.Enum)' has some invalid arguments

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