How do I tell whether a Type implements IList<>?

In fact, you cannot have an instance of a generic type definition. Therefore, the IsAssignableFrom() method works as expected. To achieve what you want, do the following:

public bool IsGenericList(Type type)
{
    if (type == null) {
        throw new ArgumentNullException("type");
    }
    foreach (Type @interface in type.GetInterfaces()) {
        if (@interface.IsGenericType) {
            if (@interface.GetGenericTypeDefinition() == typeof(ICollection<>)) {
                // if needed, you can also return the type used as generic argument
                return true;
            }
        }
    }
    return false;
}

Just out of curiosity, what do you need this for?

I too want to test if a type implements IList<T> for some T. I made the obvious change to Lucero's answer but it caused a subtle bug not present in the original answer. Here's my final edit:

    /// <summary>
    /// Test if a type derives from IList of T, for any T.
    /// </summary>
    public bool TestIfGenericList(Type type)
    {
        if (type == null)
        {
            throw new ArgumentNullException("type");
        }

        var interfaceTest = new Predicate<Type>(i => i.IsGenericType && i.GetGenericTypeDefinition() == typeof(IList<>));

        return interfaceTest(type) || type.GetInterfaces().Any(i => interfaceTest(i));
    }