Enumerations

An enumeration is a value type containing a set of named constants. An example of an enumeration is DialogResult (see "MessageBoxes" and “File Dialogs”). The DialogResult type contains the following members:

  • DialogResult.Abort
  • DialogResult.Cancel
  • DialogResult.Ignore
  • DialogResult.No
  • DialogResult.None
  • DialogResult.OK
  • DialogResult.Retry
  • DialogResult.Yes

Each of the above members has a different constant value. In many cases, we are not interested in the specific value of a given member. Instead, we are often only interested in whether two expressions of this type have the same value. For example, the following code fragment is given in the "MessageBoxes" section:

DialogResult result = MessageBox.Show("The file is not saved. Really quit?", "Confirm Quit", MessageBoxButtons.YesNo);
if (result == DialogResult.Yes)
{
    Application.Exit();
}

In the if-statement above, we are only interested in whether the user closed the MessageBox with the “Yes” button; i.e., we want to know whether the Show method returned the same value as DialogResult.Yes. For this purpose, we don’t need to know anything about the value of DialogResult.Yes or any of the other DialogResult members.

However, there are times when it is useful to know that the values in an enumeration are always integers. Using a cast, we can assign a member of an enumeration to an int variable or otherwise use it as we would an int; for example, after the code fragment above, we can write:

int i = (int)result;

As a more involved example, we can loop through the values of an enumeration:

for (DialogResult r = 0; (int)r < 8; r++)
{
    MessageBox.Show(r.ToString());
}

The above loop will display 8 MessageBoxes in sequence, each displaying the name of a member of the enumeration (i.e., “None”, “OK”, etc.).

Variables of an enumeration type may be assigned any value of the enumeration’s underlying type (usually int, as we will discuss below). For example, if we had used the condition (int)r < 10 in the above for statement, the loop would continue two more iterations, showing 8 and 9 in the last two MessageBoxes.

An enumeration is defined using an enum statement, which is similar to a class statement except that in the simplest case, the body of an enum is simply a listing of the members of the enumeration. For example, the DialogResult enumeration is defined as follows:

public enum DialogResult
{
    None, OK, Cancel, Abort, Retry, Ignore, Yes, No
}

This definition defines DialogResult.None as having the value 0, DialogResult.OK as having the value 1, etc.

As mentioned above, each enumeration has underlying type. By default, this type is int, but an enum statement may specify another underlying type, as follows:

public enum Beatles : byte
{
    John, Paul, George, Ringo
}

The above construct defines the underlying type for the enumeration Beatles to be byte; thus, a variable of type Beatles may be assigned any byte value. The following integer types may be used as underlying types for enumerations:

  • byte (0 through 255)
  • sbyte (-128 through 127)
  • short (-32,768 through 32,767)
  • ushort (0 through 65,535)
  • int (-2,147,483,648 through 2,147,483,647)
  • uint (0 through 4,294,967,295)
  • long (-9,223,372,036,854,775,808 through 9,223,372,036,854,775,807)
  • ulong (0 through 18,446,744,073,709,551,615)

It is also possible to define members of an enumeration so that they are not simply the values 0, 1, etc. For example, we might alter the Beatles enumeration as follows:

public enum Beatles : byte
{
    John = 1, Paul, George = 5, Ringo
}

This defines the following values for the members:

  • Beatles.John: 1
  • Beatles.Paul: 2
  • Beatles.George: 5
  • Beatles.Ringo: 6

Thus, if a value is explicitly assigned to a member, that member takes on that value; otherwise, that member takes on the next value greater than the previous member listed, or 0 if that member is the first listed. Note that using this technique, it is possible to define two members with the same value, although this is usually undesirable. If assigning values in this way would lead to a value outside the range of the underlying type, a syntax error results (for example, if George were assigned 255 in the above definition, thus causing Ringo to have a value outside the range of a byte).

One reason we might want to define explicit values for members of an enumeration is if we want to use the members as flags. For example, one of the MessageBox.Show methods takes as one of its parameters a MessageBoxOptions, which is an enumeration containing the following members:

  • MessageBoxOptions.DefaultDesktopOnly
  • MessageBoxOptions.RightAlign
  • MessageBoxOptions.RtlReading
  • MessageBoxOptions.ServiceNotification

The meaning of each of these members is unimportant for the purposes of this discussion. The point is that the values of these members are chosen in such a way that more than one of them can be combined into a single value. The way this is done is to define each member as a different power of 2. The binary representation of a power of 2 contains exactly one bit with a value of 1. Thus, these values can be combined using a logical OR operator, and the original values can be retrieved using a logical AND operator.

For example, suppose the MessageBoxOptions enumeration is defined as follows:

public enum MessageBoxOptions
{
    DefaultDesktopOnly = 1,
    RightAlign = 2,
    RtlReading = 4,
    ServiceNotification = 8
}
Note

The definition in .NET 6 uses different powers of 2, but the priciple is the same.

Now suppose we want to create a MessageBox that will be displayed on the default desktop with right-aligned text. We can combine these options using the expression

MessageBoxOptions.DefaultDesktopOnly | MessageBoxOptions.RightAlign

This expression combines corresponding bits of the two operands using a logical OR. Recall that the logical OR of two bits is 1 if at least one of the two bits is 1. If both operands are 0, the result is 0. In this example, the operands have a 1 in different bit locations. When we combine them using logical OR, both of these bit positions will contain a 1:

0000 0000 0000 0000 0000 0000 0000 0001
0000 0000 0000 0000 0000 0000 0000 0010
---------------------------------------
0000 0000 0000 0000 0000 0000 0000 0011

We can therefore specify both of these options to the Show method as folows:

MessageBox.Show("Hello\nworld!", "Hello", MessageBoxButtons.OK,
    MessageBoxIcon.Information, MessageBoxDefaultButton.Button1, 
    MessageBoxOptions.DefaultDesktopOnly |
    MessageBoxOptions.RightAlign);

The \n in the above example specifies the end of a line; hence, “Hello” and “world!” will be displayed on separate lines, aligned on the right:

A picture of a dialog should appear here.

The Show method determines which bits are 1 in the MessageBoxOptions parameter using a logical AND. Recall that a logical AND of two bits is 1 only if both bits are 1. In all othercases, the result is 0. Suppose, then, that options is a MessageBoxOptions variable with an unknown value. Because each named member of the MessageBoxOptions enumeration (e.g., MessageBoxOptions.RightAlign) has exactly one bit with a value of 1, an expression like

options & MessageBoxOptions.RightAlign

can have only two possible values:

  • If the bit position containing the 1 in MessageBoxOptions.RightAlign also contains a 1 in options, then the expression’s value is MessageBoxOptions.RightAlign.
  • Otherwise, the expression’s value is 0.

Thus, the Show method can use code like:

if ((options & MessageBoxOptions.RightAlign) == MessageBoxOptions.RightAlign)
{
    // Code to right-align the text
}
else
{
    // Code to left-align the text
}

Defining enumerations to be used as flags in this way can be made easier by writing the powers of 2 in hexadecimal, or base 16. Each hex digit contains one of 16 possible values: the ten digits 0-9 or the six letters a-f (in either lower or upper case). A hex digit is exactly four bits; hence, the hex values containing one occurrence of either 1, 2, 4, or 8, with all other digits 0, are exactly the powers of 2. To write a number in hex in a C# program, start with 0x, then give the hex digits. For example, we can define the following enumeration to represent the positions a baseball player is capable of playing:

public enum Positions
{
    Pitcher = 0x1,
    Catcher = 0x2,
    FirstBase = 0x4,
    SecondBase = 0x8,
    ThirdBase = 0x10,
    Shortstop = 0x20,
    LeftField = 0x40,
    CenterField = 0x80,
    RightField = 0x100
}

We can then encode that a player is capable of playing 1st base, left field, center field, or right field with the expression:

Positions.FirstBase | Positions.LeftField | Positions.CenterField | Positions.RightField

This expression would give a value having four bit positions containing 1:

0000 0000 0000 0000 0000 0001 1100 0100

For more information on enumerations, see the section, Enumeration Types in the C# Reference.

6.0.0

Last modified by: Rod Howell May 29, 2023