Using List.Find or LINQ in Listing Lists in .NET 3.5

Question

Why does List.Find (and LINQ queries in a list) always return the first element of an enumeration when the list does not contain the element I'm looking for?

Scenario:

My listing:

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 return TestEnum.EnumOne. If I add TestEnum3 to the list, it will return correctly.

Answer 1

TestEnum - , , null ( , ), default(TestEnum), EnumOne . , ?

Answer 2

, , . :

• :

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

• TryXXX:

  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;
          }
      }
  }
  

Answer 3

, , , .

Answer 4

The default value TestEnumwill be the first item, so when you do .FirstOrDefault(), it TestEnum.EnumOnewill be returned as the default when you cannot find the "first" that matches your criteria.

Answer 5

this will help you get a list of values ​​in Enum for a general list. No need to add a static extension.

var _allEnumVals = ((MyEnum[])Enum.GetValues(typeof(MyEnum))).ToList();