When is a C# List not an IEnumerable?

Question

I want to be able to determine if a passed argument is an IEnumerable.

static bool TestForEnumerable(dynamic toTest)
{
    return toTest is IEnumerable<Object>;
}

I wrote the following console app to investigate a problem I as having ...

static void Main(string[] args)
{
    Program _program = new Program();
    Console.WriteLine("");

    List<TestObject> toObj = new List<TestObject>
        {
            new TestObject { FirstName = "James", MiddleName = "Maitland", LastName = "Stewart", DOB = new DateTime(1908, 05, 20), IsFemale = false },
            new TestObject { FirstName = "Gary", MiddleName = "James", LastName = "Cooper", DOB = new DateTime(1901, 05, 07), IsFemale = false },
            new TestObject { FirstName = "Archibald", MiddleName = "Alec", LastName = "Leach", AKA="Cary Grant", DOB = new DateTime(1904, 01, 18), IsFemale = false },
            new TestObject { FirstName = "Erol", MiddleName = "Leslie Thompson", LastName = "Flynn", DOB = new DateTime(1909, 06, 20), IsFemale = false },
            new TestObject { FirstName = "Humphrey", MiddleName = "DeForest", LastName = "Bogart", DOB = new DateTime(1899, 12, 25), IsFemale = false },
            new TestObject { FirstName = "Marion", MiddleName = "Robert", LastName = "Morrison", AKA="John Wayne", DOB = new DateTime(1907, 05, 26), IsFemale = false },
        };

    var kvpObj = new List<KeyValuePair<string, string>>
        {
            new KeyValuePair<string, string>("GPC", "3"),
            new KeyValuePair<string, string>("CENTRAL CONTRACT", "4"),
            new KeyValuePair<string, string>("N/A", "2"),
            new KeyValuePair<string, string>("--Select Item--", "-1"),
        };

    Console.WriteLine(String.Format("List<TestObject> is IEnumerable : {0}", TestForEnumerable(toObj)));
    Console.WriteLine(String.Format("List<KeyValuePair<string, string>> is IEnumerable : {0}", TestForEnumerable(kvpObj)));

}

The out put of this test was ...

List<TestObject> is IEnumerable : True
List<KeyValuePair<string, string>> is IEnumerable : False

So the question is ... These are both Lists which implement IEnumerable. Why is the List of KeyValuePairs not considered an IEnumerable?

Note: there is no need to use `dynamic` here, and using `dynamic` can do nothing but add problems. — Marc Gravell, Jun 11 '21 at 15:32
keep in mind that `IEnumerable` is not the same as `IEnumerable — Franz Gleichmann, Jun 11 '21 at 15:36
@MarcGravell What would you replace **dynamic** with? In the production method the argument could be almost anything. — Gary O. Stenstrom, Jun 11 '21 at 15:44
@GaryO.Stenstrom With a generic. That would even allow you to use reflection on the actual type to apply your checks — Irwene, Jun 11 '21 at 15:47

score 10 · Accepted Answer · answered Jun 11 '21 at 15:31

10

It isn't IEnumerable<object>, because KeyValuePair<TKey, TValue> is a value-type (struct), and generic variance rules doesn't allow for value-types and object to be interchangeable during type tests (as a conversion - boxing - is required).

So: while it isn't IEnumerable<object>, it is, however, IEnumerable in the non-generic sense.

answered Jun 11 '21 at 15:31

Marc Gravell

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How would I interrogate the argument for IEnumerable? The following won't work ... `toTest as IEnumerable` and `toTest is IEnumerable` – Gary O. Stenstrom Jun 11 '21 at 15:36
1

@GaryO.Stenstrom `toTest is System.Collections.IEnumerable` will do the trick – Heslacher Jun 11 '21 at 15:36
@Heslacher Thank you! I don't understand why it had to be fully qualified, but ... it worked!! – Gary O. Stenstrom Jun 11 '21 at 15:46
1

@GaryO.Stenstrom because `List` is using `System.Collections.Generic` namespace and as a bet you don't have `using System.Collections`in your code. – Heslacher Jun 11 '21 at 15:47

When is a C# List not an IEnumerable?

1 Answers1