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 co

Answer 1

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

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

Answer 2

Personally I tend to use a method that I wrote myself, called TryGetInterfaceGenericParameters, which I posted below. Here is how to use it in your case:

Example of use

object currentObj = ...;  // get the object
Type[] typeArguments;
if (currentObj.GetType().TryGetInterfaceGenericParameters(typeof(IEnumerable<>), out typeArguments))
{
    var innerType = typeArguments[0];
    // currentObj implements IEnumerable<innerType>
}
else
{
    // The type does not implement IEnumerable<T> for any T
}

It is important to note here that you pass in typeof(IEnumerable<>), not typeof(IEnumerable) (which is an entirely different type) and also not typeof(IEnumerable<T>) for any T (if you already know the T, you don’t need this method). Of course this works with any generic interface, e.g. you can use typeof(IDictionary<,>) as well (but not typeof(IDictionary)).

Method source

/// <summary>
///     Determines whether the current type is or implements the specified generic interface, and determines that
///     interface's generic type parameters.</summary>
/// <param name="type">
///     The current type.</param>
/// <param name="interface">
///     A generic type definition for an interface, e.g. typeof(ICollection&lt;&gt;) or typeof(IDictionary&lt;,&gt;).</param>
/// <param name="typeParameters">
///     Will receive an array containing the generic type parameters of the interface.</param>
/// <returns>
///     True if the current type is or implements the specified generic interface.</returns>
public static bool TryGetInterfaceGenericParameters(this Type type, Type @interface, out Type[] typeParameters)
{
    typeParameters = null;

    if (type.IsGenericType && type.GetGenericTypeDefinition() == @interface)
    {
        typeParameters = type.GetGenericArguments();
        return true;
    }

    var implements = type.FindInterfaces((ty, obj) => ty.IsGenericType && ty.GetGenericTypeDefinition() == @interface, null).FirstOrDefault();
    if (implements == null)
        return false;

    typeParameters = implements.GetGenericArguments();
    return true;
}

Answer 3

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 4

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 5

If you want to do a check and get true for any list/collection/IEnumerable, but get false for type of string, then

   private static bool IsIEnumerable(Type requestType)
   {
      var isIEnumerable = typeof(IEnumerable).IsAssignableFrom(requestType);
      var notString = !typeof(string).IsAssignableFrom(requestType);
      return isIEnumerable && notString;
   }

Answer 6

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

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