Enum and generics

N a v a n e e t h

I have a generic method which looks for a specific attribute and returns value of "Text" property. My attribute is named "DetailsAttribute". Here is my generic method

public static string GetDescription<EnumType>(EnumType enumType) {

string description = string.Empty;

Type type = enumType.GetType();
if (type != null) {
   // Getting filed info
    FieldInfo info = type.GetField(enumType.ToString());
    if (info != null) {
        // getting the attributes
          DetailsAttribute\[\] attributes = info.GetCustomAttributes(typeof(DetailsAttribute), false)  as DetailsAttribute\[\];
          if (attributes != null && attributes.Length > 0)
              description = attributes\[0\].Text;
    }
}
return description;

}

This works fine. But I am looking for applying a constraint to the generic parameter "enumType" which should allow only enum types. I am not able to write something like

public static string GetDescription<EnumType>(EnumType enumType) : where enumType : enum // error

How can I set such kind of restriction ? Also is there any better method than what I provided to retrieve attribute values from fields ? any help would be appreciated.

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leppie

N a v a n e e t h wrote:

How can I set such kind of restriction ?

You cant. Best will be to have where EnumType : struct and have a check inside the method.

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xibeifeijian

Maybe you should check it by yourself. Just like this: public static string GetDescription(EnumType enumType) { string description = string.Empty; Type type = enumType.GetType(); Type typParent=type.BaseType; bool blIsEnum=false; while(typParent!=null) { if(typParent==typeof(System.Enum)) { blIsEnum=true; break; } typParent=typParent.BaseType; } if(blIsEnum==false) { throw new Exception("Haha,game over!^_^"); } ......

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N a v a n e e t h

Yeah, I guessed it. Thanks leppie. Do you think there is any better method to take the attribute value other than what I did ?

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N a v a n e e t h

Thanks.

xibeifeijian wrote:

Type type = enumType.GetType(); Type typParent=type.BaseType; bool blIsEnum=false; while(typParent!=null)

Why not make it simple like

if (enumType.GetType() != typeof(System.Enum))
throw new InvalidOperationException("An enum is expected");

or am I missing something ?

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xibeifeijian

Ha,Sorry,my error. As the enum type cann't inherit,so you can judge it in this way: if (enumType.GetType().BaseType != typeof(System.Enum)) You must use basetype to judge them,because the son type won't equals the parent type.System.Enum is a root type for all the enum types.

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leppie

How about: enumType is Enum ... :doh: You guys certainly like to do things the hard way! ;P

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N a v a n e e t h

Ya.. BaseType is required. Thanks :)

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N a v a n e e t h

leppie wrote:

enumType is Enum

Looks simple, but I cannot make it working. AFAIK, BaseType checking is required. When you use is, it throws exception for enum types too.

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leppie

N a v a n e e t h wrote:

Looks simple, but I cannot make it working. AFAIK, BaseType checking is required. When you use is, it throws exception for enum types too.

Then you are doing something wrong! The following prints 'Friday':

object f = DayOfWeek.Friday;

if (f is Enum)
{
Console.WriteLine(f);
}
else
{
Console.WriteLine("Not enum");
}

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PIEBALDconsult

Does no one read my articles? :~ I use:

System.Type basetype = typeof(T) ;

if ( !basetype.IsEnum )
{
throw ( new System.ArgumentException ( "T must be an Enum" ) ) ;
}

I suspect that all that checking for null in your example is needless. And please read this[^].

xibeifeijian

Haha ,this is the best way ! :)

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N a v a n e e t h

PIEBALDconsult wrote:

Does no one read my articles?

Sorry, I just missed it.

PIEBALDconsult wrote:

I suspect that all that checking for null in your example is needless.

I am not getting you fully. Are you saying that the methods I used to get "FieldInfo", attributes will never return NULL ?

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Scott Dorman

You can't apply a generic constraint on an enum type. The best you can get is struct. Also, take a look at this article[^] for a way to work with enums and a description attribute.

Scott Dorman

Microsoft® MVP - Visual C# | MCPD President - Tampa Bay IASA [Blog][Articles][Forum Guidelines]

Hey, hey, hey. Don't be mean. We don't have to be mean because, remember, no matter where you go, there you are. - Buckaroo Banzai

Scott Dorman

PIEBALDconsult wrote:

Does no one read my articles?

Hmmm...I missed that one somehow. Interesting approach to things. Did you see my article[^]? I have a similar GetDescription method.

Scott Dorman

Microsoft® MVP - Visual C# | MCPD President - Tampa Bay IASA [Blog][Articles][Forum Guidelines]

Hey, hey, hey. Don't be mean. We don't have to be mean because, remember, no matter where you go, there you are. - Buckaroo Banzai

PIEBALDconsult

Correct. GetType() won't and GetCustomAttributes won't, GetField won't because you're passing in a known field name (in this case).

modified on Tuesday, July 8, 2008 12:41 AM

PIEBALDconsult

I probably did, I definitely remember the picture of someone blocking the Green Monster.

PIEBALDconsult

Oh, and... a few months ago I asked about it on MSDN and Jon Skeet said he'd ask whether or not an enum constraint might be in the future, he later reported... "maybe". I suspect there are a lot more important things concerning them. I think maybe we should start a letter campaign.

N a v a n e e t h · modified on Tuesday, July 8, 2008 12:41 AM

Thanks. I will remove the NULL checking. Thanks

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N a v a n e e t h

Scott, Thanks. I figured it out. Great article though

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