Using List.Find or LINQ on lists of enums in .NET 3.5

Question

How come List.Find (and LINQ-queries on the list as well) always return the first enum element when the list does not contain the element I am searching for?

Scenario:

My enum:

public enum TestEnum
{
    EnumOne,
    EnumTwo,
    EnumThree
}

My test:

var TestEnum1 = TestEnum.EnumOne;
var TestEnum2 = TestEnum.EnumTwo;
var TestEnum3 = TestEnum.EnumThree;

List<TestEnum> testEnumList = new List<TestEnum>();//{ TestEnum1, TestEnum2 };
var selectedWithLinq = (from c in testEnumList where c.Equals(TestEnum3) select c).FirstOrDefault();
var selectedWithListFind = testEnumList.Find(myEnum => TestEnum3.Equals(myEnum)));

Both selectedWithLinq and selectedWithListFind in this case returns TestEnum.EnumOne. If I add TestEnum3 to the list, it will return correctly.

Answer 1

Because the first member of an enum is, unless specified otherwise, zero, which is the default value for any enum.

Answer 2

The default value of TestEnum will be its first item, so when you do .FirstOrDefault(), TestEnum.EnumOne will be returned as the default when a "first" matching your criteria can't be found.

Answer 3

As TestEnum is a value type, when no element is found in the list it cannot simply return null (as it would do if you had an array of reference types) but it will return default(TestEnum) which equals to EnumOne in this case. But anyway what are you trying to achieve?

Answer 4

As others have said, this is due to the default value of the enum coinciding with your first enum value. Options to work around this:

1. Use a nullable type:

   var selectedWithLinq = testEnumList.Where(x => x == TestEnum3)
                                      .Select(x => (TestEnum?) x)
                                      .FirstOrDefault();
   if (selectedWithLinq != null)
   {
      var realValue = selectedWithLinq.Value;
       // etc
   }
   

2. Write your own extension method based on the TryXXX pattern:

   public static bool TryFirst<T>(this IEnumerable<T> source,
                                  out T found)
   {
       using (IEnumerator<T> iterator = source.GetEnumerator())
       {
           if (iterator.MoveNext())
           {
               found = iterator.Current;
               return true;
           }
           else
           {
               found = default(T);
               return false;
           }
       }
   }