Finding out if a type implements a generic interface

// this conditional is necessary if myType can be an interface,
// because an interface doesn't implement itself: for example,
// typeof (IList<int>).GetInterfaces () does not contain IList<int>!
if (myType.IsInterface && myType.IsGenericType && 
    myType.GetGenericTypeDefinition () == typeof (IList<>))
    return myType.GetGenericArguments ()[0] ;

foreach (var i in myType.GetInterfaces ())
    if (i.IsGenericType && i.GetGenericTypeDefinition () == typeof (IList<>))
        return i.GetGenericArguments ()[0] ;

Edit: Even if myType implements IDerivedFromList<> but not directly IList<>, IList<> will show up in the array returned by GetInterfaces().

Update: added a check for the edge case where myType is the generic interface in question.

Using reflection (and some LINQ) you can easily do this:

public static IEnumerable<Type> GetIListTypeParameters(Type type)
{
    // Query.
    return
        from interfaceType in type.GetInterfaces()
        where interfaceType.IsGenericType
        let baseInterface = interfaceType.GetGenericTypeDefinition()
        where baseInterface == typeof(IList<>)
        select interfaceType.GetGenericArguments().First();
}

First, you are getting the interfaces on the type and filtering out only for those that are a generic type.

Then, you get the generic type definition for those interface types, and see if it is the same as IList<>.

From there, it's a simple matter of getting the generic arguments for the original interface.

Remember, a type can have multiple IList<T> implementations, which is why the IEnumerable<Type> is returned.

    public static bool Implements<I>(this Type type) where I : class
    {
         if (!typeof(I).IsInterface)
         {
             throw new ArgumentException("Only interfaces can be 'implemented'.");
         }

         return typeof(I).IsAssignableFrom(type);
    }

Using Anton Tykhyy's proposal, here is a small Extension Method to check if some type implements a generic interface with one given generic type parameters:

public static class ExtensionMethods
{
    /// <summary>
    /// Checks if a type has a generic interface. 
    /// For example 
    ///     mytype.HasGenericInterface(typeof(IList<>), typeof(int)) 
    /// will return TRUE if mytype implements IList<int>
    /// </summary>
    public static bool HasGenericInterface(this Type type, Type interf, Type typeparameter)
    {
        foreach (Type i in type.GetInterfaces())
            if (i.IsGenericType && i.GetGenericTypeDefinition() == interf)
                if (i.GetGenericArguments()[0] == typeparameter)
                    return true;

        return false;
    }
}

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!