Enums in C#

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before, however, conversion between the enum and the underlying type, and vice versa, involves an explicit cast, not an implicit conversion For example, code cannot call ReportState(10) where the signature is void ReportState(ConnectionState state) (The only exception is passing 0 because there is an implicit conversion from 0 to any enum) The compiler will perform a type check and require an explicit cast if the type is not identical Although you can add additional values to an enum in a later version of your code, you should do this with care Inserting an enum value in the middle of an enum will bump the values of all later enums (adding Flooded or Locked before Connected will change the Connected value, for example) This will affect the versions of all code that is recompiled against the new version However, any code compiled against the old version will continue to use the old values, making the intended values entirely different Besides inserting an enum value at the end of the list, one way to avoid changing enum values is to assign values explicitly Enums are slightly different from other value types because enums derive from SystemEnum before deriving from SystemValueType
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Type Compatibility between Enums C# also does not support a direct cast between arrays of two different enums However, there is a way to coerce the conversion by casting first to an array and then to the second enum The requirement is that both enums share the same underlying type, and the trick is to cast first to SystemArray, as shown at the end of Listing 812
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Listing 812: Casting between Arrays of Enums
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enum ConnectionState1 { Disconnected, Connecting, Connected, Disconnecting }
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enum ConnectionState2 { Disconnected, Connecting,
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Connected, Disconnecting }
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class Program { static void Main() { ConnectionState1[] states = (ConnectionState1[])(Array)new ConnectionState2[42]; } }
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This exploits the fact that the CLR s notion of assignment compatibility is more lenient than C# s (The same trick is possible for illegal conversions, such as int[] to uint[]) However, use this approach cautiously because there is no C# specification detailing that this should work across different CLR implementations
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Converting between Enums and Strings One of the conveniences associated with enums is the fact that the ToString() method, which is called by methods such as SystemConsoleWriteLine(), writes out the enum value identifier:
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SystemDiagnosticsTraceWriteLine(stringFormat( "The Connection is currently {0}", ConnectionStateDisconnecting));
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The preceding code will write the text in Output 83 to the trace buffer
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OUTPUT 83:
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The Connection is currently Disconnecting
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Conversion from a string to an enum is a little harder to find because it involves a static method on the SystemEnum base class Listing 813 provides an example of how to do it without generics (see 11), and Output 84 shows the results
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Listing 813: Converting a String to an Enum Using EnumParse()
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ThreadPriorityLevel priority = (ThreadPriorityLevel)EnumParse( typeof(ThreadPriorityLevel), "Idle"); ConsoleWriteLine(priority);
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OUTPUT 84:
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Idle
The first parameter to EnumParse() is the type, which you specify using the keyword typeof() This is a compile-time way of identifying the type, like a literal for the type value (see 17) Until NET Framework 4, there was no TryParse() method, so code prior to then should include appropriate exception handling if there is a chance the string will not correspond to an enum value identifier NET Framework 4 s TryParse<T>() method uses generics, but the type parameters can be implied, resulting in the to-enum conversion example shown in Listing 814
Listing 814: Converting a String to an Enum Using EnumTryParse<T>()
SystemThreadingThreadPriorityLevel priority; if(EnumTryParse("Idle", out priority)) { ConsoleWriteLine(priority); }
This conversion offers the advantage that there is no need to use exception handling if the string doesn t convert Instead, code can check the Boolean result returned from the call to TryParse<T>() Regardless of whether code uses the Parse or TryParse approach, the key caution about converting from a string to an enum is that such a cast is not localizable Therefore, developers should use this type of cast only for messages that are not exposed to users (assuming localization is a requirement)
Enums as Flags Many times, developers not only want enum values to be unique, but they also want to be able to combine them to represent a combinatorial value