Is my generic type a list, or just an item?

Question

I have a method that takes a generic parameter T. Internally, to decide what other methods to call, I need to know (without constraining it) if that parameter is a List or just something.

How do I do that?

I've been using

var isList = typeof(T).Name.ToLower().Contains("list`1");

but that feels like a dirty approach. What's cleaner?

Answer 1

var type = typeof(T);
bool isList = type.IsGenericType && type.GetGenericTypeDefinition() == typeof(List<>);

If you don't need an exact match for List, you might also just check if typeof(IList).IsAssignableFrom(typeof(T)), which many list-like collections implement.

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If you want to support T being equal to IList<T2> directly (so the type parameter T is an interface), then you need to check for that separately (GetGenericTypeDefinition() could also return typeof(IList<>)).

If you want to support any type T that inherits from any IList<>, then you have to get a bit more creative. You have to enumerate all interfaces, check if they are generic (as above) and then check if the generic type of the interface is IList<>. Also, because the GetInterfaces() call on Type only returns the top-level interfaces, you need to navigate through all of the interfaces implemented by each interface (recursively), to check those as well. Ugly, I know.

Answer 2

Well, you can always just use typeof(T) and the is keyword, but I would take another look at your design.

Your approach is really defeating the purpose of a generic method. If you have to check the type of the generic argument, well, your method isn't generic. Generics are great for scenarios where you don't care what the underlying type is. If you care then the method shouldn't be generic, it should be overloaded (probably). How abut telling us what you are actually trying to accomplish here so that we can give more helpful responses?