Recipe1.19.Testing for a Valid Enumeration of Flags

Recipe 1.19. Testing for a Valid Enumeration of Flags

Problem

You need to determine if a given value is a valid enumeration value or a valid combination of enumeration values (i.e., bit flags ORed together in an enumeration marked with the Flags attribute).

Solution

To make it possible to test whether a value is a valid enumeration value or some combination of valid enumeration values, add an All member to the existing enumeration equal to all the members of the enumeration ORed together. Then use the HandleFlagsEnum method to do the test.

There is a problem with using Enum.IsDefined with an enumeration marked with the Flags attribute. Consider if the Language enumeration was written as follows:

 [Flags] enum Language  {      CSharp = 1, VBNET = 2, VB6 = 4 }

Valid values for Language are the set of numbers {1, 2, 3, 4, 5, 6, 7}. However, the values 3, 5, 6, and 7 are not explicitly represented in this enumeration. The value 3 is equal to the CSharp and VBNET enumeration members ORed together, and the value 7 is equal to all of the enumeration members ORed together. For the values 3, 5, 6, and 7, the Enum.IsDefined method will return false, indicating that these are not valid values, when in fact they are. You need a way to determine if a correct set of flags has been passed into a method.

To fix this problem you can add a new member to the Language enumeration to define all values for which the Language enumeration is valid. In this case, the Language enumeration would be rewritten as:

 [Flags] enum Language {     CSharp = 1, VBNET = 2, VB6 = 4,     All = (CSharp | VBNET | VB6) }

The new All enumeration member is equal to all other Language members ORed together. Now, when you want to validate a Language flag, all you have to do is the following:

 public bool HandleFlagsEnum(Language language) {     if ((language != 0) && ((language & Language.All) == language))     {         return (true);     }     else     {         return (false);     } }

Discussion

If you want to use the HandleFlagsEnum method with existing code, all that is required is to add an All member to the existing enumeration. The All member should be equal to all the members of the enumeration ORed together.

The HandleFlagsEnum method then uses this All member to determine if an enumeration value is valid. This is accomplished by ANDing the language value with Language.All, then verifying that the result equals the original language parameter.

This method can also be overloaded to handle the underlying type of the enumeration as well (in this case, the underlying type of the Language enumeration is an integer). The following code determines if an integer variable contains a valid Language enumeration value:

 public static bool HandleFlagsEnum(int language) {     if ((language != 0) && ((language & (int)Language.All) == language))     {         return (true);     }     else     {         return (false);     } }

The overloaded HandleFlagsEnum methods return true if the language parameter is valid and false otherwise.