How to determine if a type implements a specific generic interface type

Question

By using the answer from TcKs it can also be done with the following LINQ query:

bool isBar = foo.GetType().GetInterfaces().Any(x =>
  x.IsGenericType &&
  x.GetGenericTypeDefinition() == typeof(IBar<>));

You have to go up through the inheritance tree and find all the interfaces for each class in the tree, and compare typeof(IBar<>) with the result of calling Type.GetGenericTypeDefinition if the interface is generic. It's all a bit painful, certainly.

See this answer and these ones for more info and code.

public interface IFoo<T> : IBar<T> {}
public class Foo : IFoo<Foo> {}

var implementedInterfaces = typeof( Foo ).GetInterfaces();
foreach( var interfaceType in implementedInterfaces ) {
    if ( false == interfaceType.IsGeneric ) { continue; }
    var genericType = interfaceType.GetGenericTypeDefinition();
    if ( genericType == typeof( IFoo<> ) ) {
        // do something !
        break;
    }
}

As a helper method extension

public static bool Implements<I>(this Type type, I @interface) where I : class
{
    if(((@interface as Type)==null) || !(@interface as Type).IsInterface)
        throw new ArgumentException("Only interfaces can be 'implemented'.");

    return (@interface as Type).IsAssignableFrom(type);
}

Example usage:

var testObject = new Dictionary<int, object>();
result = testObject.GetType().Implements(typeof(IDictionary<int, object>)); // true!

I'm using a slightly simpler version of @GenericProgrammers extension method:

public static bool Implements<TInterface>(this Type type) where TInterface : class {
    var interfaceType = typeof(TInterface);

    if (!interfaceType.IsInterface)
        throw new InvalidOperationException("Only interfaces can be implemented.");

    return (interfaceType.IsAssignableFrom(type));
}

Usage:

    if (!featureType.Implements<IFeature>())
        throw new InvalidCastException();

To tackle the type system completely, I think you need to handle recursion, e.g. IList<T> : ICollection<T> : IEnumerable<T>, without which you wouldn't know that IList<int> ultimately implements IEnumerable<>.

    /// <summary>Determines whether a type, like IList&lt;int&gt;, implements an open generic interface, like
    /// IEnumerable&lt;&gt;. Note that this only checks against *interfaces*.</summary>
    /// <param name="candidateType">The type to check.</param>
    /// <param name="openGenericInterfaceType">The open generic type which it may impelement</param>
    /// <returns>Whether the candidate type implements the open interface.</returns>
    public static bool ImplementsOpenGenericInterface(this Type candidateType, Type openGenericInterfaceType)
    {
        Contract.Requires(candidateType != null);
        Contract.Requires(openGenericInterfaceType != null);

        return
            candidateType.Equals(openGenericInterfaceType) ||
            (candidateType.IsGenericType && candidateType.GetGenericTypeDefinition().Equals(openGenericInterfaceType)) ||
            candidateType.GetInterfaces().Any(i => i.IsGenericType && i.ImplementsOpenGenericInterface(openGenericInterfaceType));

    }

You have to check against a constructed type of the generic interface.

You will have to do something like this:

foo is IBar<String>

because IBar<String> represents that constructed type. The reason you have to do this is because if T is undefined in your check, the compiler doesn't know if you mean IBar<Int32> or IBar<SomethingElse>.