IDictionary`2 is not assignable from IDictionary?

Question

I have a property on my ViewModel that is of type IDictionary<string, string>. I am going through the list of properties on that ViewModeland using reflection to determine if it is a dictionary.

Currently I have:

if (typeof(IDictionary).IsAssignableFrom(propDescriptor.PropertyType))

However, that is always false because propDescriptor.PropertyType is IDictionary`2. Any ideas how I can get that to work? Also, why isn't that working?

I just changed my property to be IDictionary instead of IDictionary.

EDIT: Not sure where my generics went, but the second IDictionary in the sentence above has string, string.

Should it not then be if (typeof(IDictionary).IsAssignableFrom(propDescriptor.PropertyType)) — stuartd, Jan 17 '13 at 13:59
How about http://stackoverflow.com/questions/3765859/is-there-a-way-to-determine-if-a-generic-type-is-built-from-a-specific-generic-t — stuartd, Jan 17 '13 at 14:05
Don't change the property type to the non-generic interface. Actually, don't use non-generic interfaces from .NET 1 when there are new generic interfaces that supersede them. — Jeppe Stig Nielsen, Jan 17 '13 at 14:15

Jeppe Stig Nielsen · Answer 1 · 2013-01-17T14:34:19.900

The reason why it's not working is that the generic IDictionary<,> interface does not have the non-generic IDictionary as a base interface.

Maybe this is what you want:

var type = propDescriptor.PropertyType;
if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(IDictionary<,>))
{ // ...

Edit: The above code will only check if type is declared as IDictionary<X, Y> for some X and Y. If you also want to handle cases where type represents a class or struct that implements IDictionary<X, Y> (or even an interface derived from IDictionary<X, Y>), then try this:

Func<Type, bool> isGenericIDict =
  t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(IDictionary<,>);
var type = propDescriptor.PropertyType;
if (isGenericIDict(type) || type.GetInterfaces().Any(isGenericIDict))
{ // ..

score 1 · Answer 2 · edited Jan 17 '13 at 14:04

1

It's not IDictionary`2. It's the compiler-generated class name for a generic IDictionary<TKey,TValue>, which is not directly castable to an IDictionary.

edited Jan 17 '13 at 14:04

Jeppe Stig Nielsen

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answered Jan 17 '13 at 13:57

D Stanley

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The `2 is saying that the type has an arity of two, ie it has two type parameters.. – stuartd Jan 17 '13 at 13:58

Adi Lester · Answer 3 · 2013-01-17T14:01:39.970

1

IDictionary`2 is not derived from IDictionary as can be seen in its definition:

public interface IDictionary<TKey, TValue> :
    ICollection<KeyValuePair<TKey, TValue>>,
    IEnumerable<KeyValuePair<TKey, TValue>>,
    IEnumerable

Therefore the generic IDictionary<TKey, TValue> is not castable to IDictionary.

edited Jan 17 '13 at 14:01

answered Jan 17 '13 at 14:00

Adi Lester

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I guess my question is, why is it doing IDictionary`2? Also, when I do this: var dictionary = propDescriptor.GetValue(instance) as IDictionary; It works and gives me the value. I don't understand why that would work and the type check wouldn't. – g.t.w.d Jan 17 '13 at 14:02
You should differentiate between the `IDictionary` interface, which doesn't derive from `IDictionary` and the `Dictionary` class which does implement `IDictionary`. – Adi Lester Jan 17 '13 at 14:06
That only works because `Dictionary<,>` implements `IDictionary` and `IDictionary<,>`, but `IDictionary<,>` does not implement `IDictionary`. – odyss-jii Jan 17 '13 at 14:09

score 0 · Answer 4 · answered Jan 17 '13 at 13:59

0

IDictionary is different from IDictionary< T, K >.

Use:

if (typeof(IDictionary<string, string>).IsAssignableFrom(propDescriptor.PropertyType))

answered Jan 17 '13 at 13:59

lante

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IDictionary`2 is not assignable from IDictionary?

4 Answers4