Determine if object derives from collection type

Question

I want to determine if a generic object type ("T") method type parameter is a collection type. I would typically be sending T through as a Generic.List but it could be any collection type as this is used in a helper function.

Would I be best to test if it implements IEnumerable<T>?

If so, what would the code look like?

Update 14:17 GMT+10 Possibly extending on a solution here (however only works for List<T>'s not IEnumerable<T>'s when it should if List derives ?)

T currentObj;    
// works if currentObj is List<T>
currentObj.GetType().GetGenericTypeDefinition() == typeof(List<>)
// does not work if currentObj is List<T>
currentObj.GetType().GetGenericTypeDefinition() == typeof(IEnumerable<>)

Answer 1

I would test IEnumerable instead, since collection type could only implement IEnumerable , it doesn't have to implement IEnumerable.

It also depends what do you mean collection type? You could have an collection without implementing any of those interfaces.

Answer 2

You can use Type.GetInterface() with the mangled name.

private bool IsTAnEnumerable<T>(T x)
{
    return null != typeof(T).GetInterface("IEnumerable`1");
}

Answer 3

In order to get the actual type of T at runtime, you can use the typeof(T) expression. From there the normal type comparison operators will do the trick

bool isEnumerable = typeof(IEnumerable<int>).IsAssignableFrom(typeof(T));

Full Code Sample:

static bool Foo<T>()
{
  return typeof(IEnumerable<int>).IsAssignableFrom(typeof(T));
}

Foo<List<T>>();  // true
Foo<int>(); // false

Answer 4

Also, remember just because you are using generics, don't forget other basic techniques, in this case, like overloading. I suspect the you are planning something like this:

void SomeFunc<T>(T t)
{
    if (IsCollectionCase(t))
       DoSomethingForCollections()
    else
       DoSOmethingElse();
}

This would be far better handled as:

void SomeFunc(IEnumerable t)
{
       DoSomethingForCollections()
}
void SomeFunc<T>(T t)
{
       DoSomethingElse()
}

Answer 5

This will be the simplest check..

if(Obj is ICollection)
{
    //Derived from ICollection
}
else
{
    //Not Derived from ICollection
}