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Generic Type Parameters

Posted on 2009-05-15
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Last Modified: 2012-05-07
As an example, if I inspect an object the following type:

public class LookupList<K, T> : IList<T>, IDictionary<K, T>

How would I look up the type corresponding to the generic argument for IList<>? In this case just retrieving GetGenericArguments will return an array of the two types K and T, and it's unclear which is the argument to IList. So what I'd really like to be able to do is test the object to see if it implements IList<> ( i.e. typeof(IList<>).IsInstanceOfType(myObject) ), and then if so retrieve just the IList<> type so I can then retrieve the generic type.

Is that clear? What's the right call?

Thanks in advance!

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Question by:ebyrne
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Expert Comment

by:tculler
ID: 24396073
It's really not a good idea to implement both of these interfaces, as they will surely conflict with each other (even though it is possible...). Why implement both, anyway? Due to polymorphism, I'm not sure how to do this--or why it would even be useful.
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Author Comment

by:ebyrne
ID: 24396104
And I suppose I could call GetInterfaces() in this case and iterate through them testing each for it's GenericTypeDefinition, but it seems to me there should be a more direct way (and/or a way that would work with all base classes/interfaces). Is there?
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Author Comment

by:ebyrne
ID: 24396142
tculler,

Thanks for responding, but that's not really an answer or even an attempt. :) To respond though, it seems perfectly reasonable to me. In this case the class is a list that provides the lookup functionality of a dictionary and the normal iteration functionality of a list (and it was just an example I cooked up, not an actual class)...

Thanks for stopping in though!
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Author Comment

by:ebyrne
ID: 24396251
And he does bring up a good point, perhaps it's not an ideal example, but really all I'm getting at is that the class implements several generic interfaces with varying generic parameters. I know what interface I'm interested in, I just want to know how to get the parameter for that interface/class...
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Accepted Solution

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ebyrne earned 0 total points
ID: 24396310
Okay, so this is the best I've got so far, supporting only interfaces and a single argument. If you can either provide the more direct route I'm looking for or generalize this to classes and multiple arguments the points are yours.

Thanks Experts!
        static public Type GetGenericArgument(Type type, Type genericTypeDefinition)
        {
            if (type == null || genericTypeDefinition == null)
            {
                throw new ArgumentNullException();
            }
            if (!genericTypeDefinition.IsGenericTypeDefinition)
            {
                throw new ArgumentException("The generic type must be a generic type definition");
            }
            if (!genericTypeDefinition.IsInterface || genericTypeDefinition.GetGenericArguments().Length != 1)
            {
                throw new NotImplementedException("Must be a single argument interface type currently!");
            }
 
            var baseTypes = type.GetInterfaces();
 
            foreach (Type t in baseTypes)
            {
                if (t.IsGenericType && t.GetGenericTypeDefinition() == genericTypeDefinition)
                {
                    var args = t.GetGenericArguments();
 
                    if (args.Length > 0)
                    {
                        return args[0];
                    }
                }
            }
 
            return null;
        }
 
Usage:
        type = GetGenericArgument(dataSource.GetType(), typeof(IEnumerable<>));

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