Defining an Enum in Visual C#.NET

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Listing 810 Defining an Enum
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enum ConnectionState { Disconnected, Connecting, Connected, Disconnecting }
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An enum is helpful even for Boolean parameters For example, a method call such as SetState(true) is less readable than SetState(DeviceStateOn)
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You refer to an enum value by prefixing it with the enum name; to refer to the Connected value, for example, you use ConnectionStateConnected You should not use the enum names within the enum value name, to avoid the redundancy of something like ConnectionStateConnectionStateConnected By convention, the enum name itself should be singular, unless the enums are bit flags (discussed shortly) By default, the first enum value is 0 (technically, it is 0 explicitly cast to the underlying enum type), and each subsequent entry increases by one However, you can assign explicit values to enums, as shown in Listing 811
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Listing 811 Defining an Enum Type
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enum ConnectionState : short { Disconnected, Connecting = 10, Connected, Joined = Connected, Disconnecting }
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Disconnected has a default value of 0, Connecting has been explicitly assigned 10, and consequently, Connected will be assigned 11 Joined is assigned 11, the value referred to by Connected (In this case, you do not need to prefix Connected with the enum name, since it appears within its scope) Disconnecting is 12
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An enum always has an underlying type, which may be any of the integral types, except for char In fact, the enum type's performance is equivalent to that of the underlying type By default, the underlying value type is int , but you can specify a different type using inheritance type syntax Instead of int , for example, Listing 811 uses a short For consistency, the syntax emulates that of inheritance, but this doesn't actually make an inheritance relationship The base class for all enums is SystemEnum Furthermore, these classes are sealed; you can't derive from an existing enum type to add additional members Successful conversion doesn't work just for valid enum values It is possible to cast 42 into a ConnectionState, even though there is no corresponding ConnectionState enum value If the value
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successfully casts to the underlying type, the conversion will be successful The advantage to allowing casting, even without a corresponding enum value, is that enums can have new values added in later API releases, without breaking earlier versions Additionally, the enum values provide names for the known values while still allowing unknown values to be assigned at runtime The burden is that developers must code defensively for the possibility of unnamed values It would be unwise, for example, to replace case ConnectionStateDisconnecting with default and expect that the only possible value for the default case was ConnectionStateDisconnecting Instead, you should handle the Disconnecting case explicitly and the Default case should report an error or behave innocuously As indicated before, however, conversion between the enum and the underlying type, and vice versa, involves an explicit cast, not an implicit one 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 cast 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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