Enums
Enums:
Enums are strongly typed constants. They are essentially unique types that allow you to assign symbolic names to integral values. In the C# tradition, they are strongly typed, meaning that an enum of one type may not be implicitly assigned to an enum of another type even though the underlying value of their members are the same. Along the same lines, integral types and enums are not implicitly interchangable. All assignments between different enum types and integral types require an explicit cast.
Enums lend themselves to more maintainable code because they are symbolic, allowing you to work with integral values, but using a meaningful name to do so. For example, what type of code would you rather work with - a set of values named North, South, East, and West or the set of integers 0, 1, 2, and 3 that mapped to the same values, respectively? Enums make working with strongly typed constants via symbolic names easy.
Enums are value types, which means they contain their own value, can't inherit or be inherited from, and assignment copies the value of one enum to another. You will see in this lesson and elsewhere that enums are used and referred to with both lower case, enum, and upper case,Enum. The relationship between the two is that the C# type, enum, inherits the Base Class Library (BCL) type, Enum. Use the C# type, enum, to define new enums and use the BCL type, Enum, to implement static enum methods.
Creating an Enum
The .NET Framework Class Library contains many enums and examples of how they are used. For example, every time you put an icon on aMessageBox, you use the MessageBoxIcon enum. For a list of available enums in the .NET Framework Class Library, look at the documentation for the Enum class and click on the Derived Classes link.
Whenever there are situations where you are using a set of related numbers in a program, consider replacing those numbers with enums. It will make a program more readable and type safe. Listing 17-1 contains an enum definition and code that uses that enum in a switchstatement. Instead of using the numbers 0, 1, and 2 in the switch statement, the code is more meaningful through the use of the Volumeenum.
Listing 17-1. Creating and Using an Enum: EnumSwitch.cs
using System;// declares the enumpublic enum Volume
{ Low, Medium,
High
}
// demonstrates how to use the enum
class EnumSwitch
{ static void Main()
{ // create and initialize // instance of enum type
Volume myVolume = Volume.Medium;
// make decision based // on enum value switch (myVolume)
{ case Volume.Low:
Console.WriteLine("The volume has been turned Down.");
break; case Volume.Medium:
Console.WriteLine("The volume is in the middle.");
break; case Volume.High:
Console.WriteLine("The volume has been turned up.");
break;
}
Console.ReadLine();
}
}
Listing 17-1 contains a definition for an enum. Notice that it is declared with the enum keyword, has a type identifier (Volume), and contains a comma separated list of values enclosed within curly braces.
This enum is of type Volume and we use it to declare the myVolume variable in the Main method. Since an enum is a value type, we can assign a value (Volume.Medium) to it directly, similar to the simple types such as int or double. Once the myVolume variable is declared and initialized, it is used in the switch statement.Each of the case statements represent a unique member of the Volume enum.
Any time a member of the Volume enum is used, it is fully qualified with the "Volume" identifier to guarantee type safety. For example, if there were a Meat enum in scope, then Meat.Medium would definitely have different semantics than Volume.Medium. With both enums in scope, it would be ambiguous to just use the Medium identifier without type qualification. Using the type identifier ensures such mistakes are not made.
Using Enums
An enum is typically specified as shown in Listing 17-1, but may be customized by changing its base type and member values. By default, the underlying type of an enum is int. This default may be changed by specifying a specific base when declaring the enum. You would specify a different base if the enum was used extensively and there was an opportunity for space savings by selecting a smaller type. Another reason may be if you wanted the underlying type of the enum to correspond to another type in your program and you wanted to explicitly cast between the two without loss of precision. Valid base types include byte, sbyte, short, ushort, int, uint, long, and ulong.
Another modification you can make to an enum is to set the value of any enum member. By default, the first member of an enum takes the value of zero. If this value doesn't make sense for your enum, you can change it to one or some other number. Additionally, you can change any of the members of an enum to any value that is valid for its base type. Unassigned enum members have a value that is one more than their predecessor. Listing 17-2 shows how to modify the base type and member values of an enum.
Listing 17-2. Setting the Enum Base and Initializing Members: EnumBaseAndMembers.cs
using System;// declares the enumpublic enum Volume : byte{
Low = 1,
Medium,
High
}class EnumBaseAndMembers
{ static void Main()
{ // create and initialize
// instance of enum type Volume myVolume = Volume.Low;
// make decision based
// on enum value switch (myVolume)
{ case Volume.Low:
Console.WriteLine("The volume has been turned Down."); break; case Volume.Medium:
Console.WriteLine("The volume is in the middle."); break; case Volume.High:
Console.WriteLine("The volume has been turned up."); break;
}
Console.ReadLine();
}
}
The Volume enum in Listing 17-2 shows how to modify the base type and members of an enum.Its base type is changed to byte with the : <type> syntax following the enum identifier, Volume.This ensures that the Volume enum may only have members with values that are valid for type byte.
The first member of the Volume enum, Low, has its value changed to 1. The same syntax, <member> = <value>, may be applied to any member of the enum. You are restricted from creating forward references, circular references, and duplicate references in enum members.
The default values of the Volume enum are Low=0, Medium=1, and High=2 because the first member of an enum defaults to 0 and the following members default to one more than their predecessor. However, the Volume enum in Listing 17-2 has its Low member set to 1, which means that Medium=2 and High=3.
Enum tricks
Enum types implicitly inherit the System.Enum type in the Base Class Library (BCL). This also means that you can use the members of System.Enum to operate on enum types. This section does just that, showing some useful tips and tricks to use with enums in your programs.
A common requirement with enums is to convert between the enum and a variable of its base type. For example, if you are getting input in the form of an int from a user or a file stream, then you can cast it to an enum and use it in a meaningful way in your program. You can also get a complete list of enum member names or enum values, which is useful if you have logic that needs to iterate through every enum member. Listing 17-3 shows how to perform conversions between enums and their base types and how to use some of the System.Enum type members.
Listing 17-3. Enum Conversions and using the System.Enum Type: Enumtricks.cs
using System;// declares the enumpublic enum Volume : byte{
Low = 1,
Medium,
High
}
// shows different ways
// to work with enumsclass Enumtricks
{ static void Main(string[] args)
{ // instantiate type Enumtricks enumtricks = new Enumtricks(); // demonstrates explicit cast
// of int to Volume enumtricks.GetEnumFromUser();
// iterate through Volume enum by name enumtricks.ListEnumMembersByName(); // iterate through Volume enum by value enumtricks.ListEnumMembersByValue();
Console.ReadLine();
} // demonstrates explicit cast
// of int to Volume public void GetEnumFromUser()
{
Console.WriteLine("\n----------------");
Console.WriteLine("Volume Settings:");
Console.WriteLine("----------------\n");
Console.Write(@"
1 - Low
2 - Medium
3 - High
Please select one (1, 2, or 3): "); // get value user provided string volString = Console.ReadLine(); int volInt = Int32.Parse(volString); // perform explicit cast from
// int to Volume enum type Volume myVolume = (Volume)volInt;
Console.WriteLine();
// make decision based
// on enum value switch (myVolume)
{ case Volume.Low:
Console.WriteLine("The volume has been turned Down."); break; case Volume.Medium:
Console.WriteLine("The volume is in the middle."); break; case Volume.High:
Console.WriteLine("The volume has been turned up."); break;
}
Console.WriteLine();
}
// iterate through Volume enum by name public void ListEnumMembersByName()
{
Console.WriteLine("\n---------------------------- ");
Console.WriteLine("Volume Enum Members by Name:");
Console.WriteLine("----------------------------\n");
// get a list of member names from Volume enum,
// figure out the numeric value, and display foreach (string volume in Enum.GetNames(typeof(Volume)))
{
Console.WriteLine("Volume Member: {0}\n Value: {1}",
volume, (byte)Enum.Parse(typeof(Volume), volume));
}
}
// iterate through Volume enum by value public void ListEnumMembersByValue()
{
Console.WriteLine("\n----------------------------- ");
Console.WriteLine("Volume Enum Members by Value:");
Console.WriteLine("-----------------------------\n"); // get all values (numeric values) from the Volume
// enum type, figure out member name, and display foreach (byte val in Enum.GetValues(typeof(Volume)))
{
Console.WriteLine("Volume Value: {0}\n Member: {1}",
val, Enum.GetName(typeof(Volume), val));
}
}
}
The code in Listing 17-3 includes three method calls to GetEnumFromUser, ListEnumMembersByName, and ListEnumMembersByValue. Each of these methods demonstrate a different aspect of using System.Enum to work with enums.
The GetEnumFromUser method shows how to obtain int input and translate it to an appropriate enum type. Converting an int to an enum makes the code more readable and type safe. The following is an excerpt from Listing 17-3 that shows the pertinent part of the code that performs the conversion:
// get value user provided string volString = Console.ReadLine(); int volInt = Int32.Parse(volString); // perform explicit cast from
// int to Volume enum type Volume myVolume = (Volume)volInt;
After the program displays a menu, it prompts the user for a selection in the form of a number (1, 2, or 3). When the user makes a selection and presses the Enter key, the code reads the value with Console.ReadLine, which returns the value as a string type. Since you can only cast an int to a Volume enum type, the user's input must be converted from a string to an int with the Int32.Parse method. Converting the int to aVolume enum type is simply a matter of applying a cast operation during assignment.
To get all the members of an enum at the same time, you can use the GetNames method of the System.Enum type, which returns a stringarray of the names of all an enum's members. An excerpt from the ListEnumMembersByName method in Listing 17.3 that shows this appears below:
// get a list of member names from Volume enum,
// figure out the numeric value, and display foreach (string volume in Enum.GetNames(typeof(Volume)))
{
Console.WriteLine("Volume Member: {0}\n Value: {1}",
volume, (byte)Enum.Parse(typeof(Volume), volume));
}
Because GetNames returns an array of strings, it is easy to use in a loop statement such as foreach. Something you may be curious about in the code above is the second parameter to the WriteLine method's format string. Given the enum type and a string representation of the member name, you can use the Enum.Parse method to get the underlying value of that member. Because the Volume enum's base type is byte, the return value from Enum.Parse must be cast to a byte before assignment, forcing the numeric representation of the enum value to appear. If we would have omitted the byte cast, the output would be the Volume enum member, which would then be converted to a string representation of the member name, which is not what the code intended to show.
Instead of getting names of all the members of an enum, you may have a reason to get all the values of the enum at one time. The code below, from the ListEnumMembersByValue method in Listing 17.3, shows how to accomplish this:
// get all values (numeric values) from the Volume
// enum type, figure out member name, and display foreach (byte val in Enum.GetValues(typeof(Volume)))
{
Console.WriteLine("Volume Value: {0}\n Member: {1}",
val, Enum.GetName(typeof(Volume), val));
}
Given the type of the enum, the GetValues method of System.Enum will return an array of the given enum's base type, which in this case isbyte. While iterating through this list, each member is printed to the console showing its value and name. The name is obtained by using theGetName method of System.Enum, which accepts an enum type and value for which to get the corresponding name of.
Enums are lists of strongly typed constants with members that are symbolic names, corresponding to an underlying integral type. Enum base types can be changed and member values can be specified. The System.Enum .NET Framework Class Library type is the base class of enum types and contains methods that allow you to work with enums in different ways, such as working with a list of names or values, converting from value to name, and converting from name to value. For more information on the System.Enum type, see the .NET Framework SDK
