C#

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Enum

Enums. In a program, special values are needed. The key is 10. The size is 20. In an enum, we have a type that lists these values. This makes the program easier to modify.


Method

With an enum, these magic constants are separate. This modular design makes things easier to understand. Fewer bugs will be introduced.

Tip:Enums can be used with IntelliSense in Visual Studio. This feature will guess the value you are typing.

Press tab:We can simply press tab and select the enum type we want. This is an advantage to using enum types.


Important

First, we see an enum that expresses importance. An enum type internally contains an enumerator list. The values, like Trivial and Critical, are actually ints like 1 and 4.

Int:The underlying value of this enum is the default, which is int. When you use an Importance variable, it is actually an int.

Syntax:An enum is a form of syntactic sugar. It makes programs cleaner and easier to read. It also causes no slowdown at runtime.

Based on:

.NET 4.5

C# program that uses enums

using System;

class Program
{
    enum Importance
    {
	None,
	Trivial,
	Regular,
	Important,
	Critical
    };

    static void Main()
    {
	// ... An enum local variable.
	Importance value = Importance.Critical;

	// ... Test against known Importance values.
	if (value == Importance.Trivial)
	{
	    Console.WriteLine("Not true");
	}
	else if (value == Importance.Critical)
	{
	    Console.WriteLine("True");
	}
    }
}

Output

True
Enum locals, Visual Studio debugger

Debugger. Next, we examine what enums look like in the Visual Studio debugger. We see that enums are strongly-typed. You cannot assign them to just any value.

Note:The debugger shows that en1 and en2 are of type Program.E. But internally, these two variables are stored as integers.

C# program that specifies tag types with enums

using System;

class Program
{
    enum E
    {
	None,
	BoldTag,
	ItalicsTag,
	HyperlinkTag,
    };

    static void Main()
    {
	// ... These values are enum E types.
	E en1 = E.BoldTag;
	E en2 = E.ItalicsTag;

	if (en1 == E.BoldTag)
	{
	    // Will be printed.
	    Console.WriteLine("Bold");
	}
	if (en1 == E.HyperlinkTag)
	{
	    // Won't be printed.
	    Console.WriteLine("Not true");
	}
    }
}

Output

Bold
Console

Strings. We convert enums to strings for display on the Console. Enum values always have a name, such as E.None, E.BoldTag or E.ItalicsTag.

Tip:To print out the enum values, you can call ToString on the enum variable in a program.

Also:Another method such as Console.WriteLine can automatically call the ToString method.

Console.WriteLine
C# program that writes enums

using System;

class Program
{
    static void Main()
    {
	// ... Two enum variables.
	B b1 = B.Dog;
	V v1 = V.Hidden;

	// ... Print out their values.
	Console.WriteLine(b1);
	Console.WriteLine(v1);
    }

    enum V
    {
	None,
	Hidden = 2,
	Visible = 4
    };

    enum B
    {
	None,
	Cat = 1,
	Dog = 2
    };
}

Output

Dog
Hidden
String face

ToString. Console.WriteLine will call the ToString method on all types passed to it. Internally, ToString invokes methods that use reflection to acquire the string representation.

Enum ToString
Exclamation mark

A warning. Some examples here use short, letter-based identifiers (b1, v2) for variables. These are not ideal. It is better to use more descriptive words, such as "animal" or "visibility."


Parse

Parse. Sometimes we have a string value that we want to convert to an equivalent enum. This could happen when accepting user input.

Tip:When using the .NET Framework, calling a built-in method to do conversions (where one exists) is usually best.

Enum.Parse:The tricky part of using this method involves typeof and casting. It is best to avoid this if possible.

Enum.ParseTypeof
Get

GetName, Getnames. Built-in methods get strings that represent enums. With GetName, we can get the name for an enum value. With GetNames we get all the string representations at once.

Enum.GetName
Format

Format enums. It is possible to format the values stored in enums in different ways. We can display an integer representation, or a hex representation.

Enum.Format
Switch

Switch. The above samples show if-statements used with enums. But switch in the C# language is sometimes compiled to more efficient IL.

Switch Enum

Here:In this example, the IsFormat method works as a filter that tells us something about sets of enum values.

Logic:We can separate the logic here instead of repeating ourselves. This helps clarify the program logic.

C# program that uses switch, enums

using System;

class Program
{
    static void Main()
    {
	// ... Test enum with switch method.
	G e1 = G.None;
	if (IsFormat(e1))
	{
	    // Won't succeed.
	    // ... G.None is not a format value.
	    Console.WriteLine("Error");
	}

	// ... Test another enum with switch.
	G e2 = G.ItalicsFormat;
	if (IsFormat(e2))
	{
	    // Will succeed.
	    // ... G.ItalicsFormat is a format value.
	    Console.WriteLine("True");
	}
    }

    enum G
    {
	None,
	BoldFormat,    // Is a format value.
	ItalicsFormat, // Is a format value.
	Hyperlink      // Not a format value.
    };

    /// <summary>
    /// Returns true if the G enum value is a format value.
    /// </summary>
    public static bool IsFormat(G e)
    {
	switch (e)
	{
	    case G.BoldFormat:
	    case G.ItalicsFormat:
		{
		    // These two values are format values.
		    return true;
		}
	    default:
		{
		    // The argument is not a format value.
		    return false;
		}
	}
    }
}

Output

True
Default operator

Default. Values are always initialized to zero when they are fields of a class. Upon class creation, an enum field will also be initialized to zero (and the equivalent value).

Tip:To make enums valid, always use the default value of zero. This way, we can test for the default value of fields.

Sometimes:This issue is not worth fixing. But it is often useful for verifying correctness.

Enum with default value of None

enum E
{
    None,
    A,
    B,
    C
};
FxCop

FxCop. This is a code analysis tool released by Microsoft. It helps us improve certain issues in code. It will tell us that "enums should have zero value."

A non-flags attributed enumeration should define a member with the value of zero so that the default value is a valid value of the enumeration. If appropriate, name the member None.

Enums should have zero value: MSDN
Net

Collections. Here we apply the Stack collection in the .NET Framework. With Stack, we can develop a parser that keeps the most recently encountered enum value on the top.

Stack:The Stack here can only have E-type values added to it. This is an example of type checking and validation.

Stack

Pop:With the Pop method we get the top element from the stack. This is of type E.ItalicsTag.

Result:In the execution of this program, the stack has two enums added and one removed.

C# program that uses Stack with enums

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
	M();
    }

    enum E
    {
	None,         // integer value = 0
	BoldTag,      // 1
	ItalicsTag,   // 2
	HyperlinkTag, // 3
    };

    static public void M()
    {
	// ... Stack of enums.
	Stack<E> stack = new Stack<E>();

	// ... Add enum values to the Stack.
	stack.Push(E.BoldTag); // Add bold
	stack.Push(E.ItalicsTag); // Add italics

	// ... Get the top enum value.
	E thisTag = stack.Pop(); // Get top tag.
	Console.WriteLine(thisTag);
    }
}

Output

ItalicsTag
Byte type

Type. An enum has an underlying type. Each time you use the enum, you are actually using the underlying type with syntactic sugar on top.

Int:Enums are by default an int type, but we can adjust this to a different numeric type.

Byte:Here we create an enum with a type of byte. This is sometimes useful on small enums. A byte can only contain 256 different values.

Byte

Memory:The CoffeeSize enum will use memory equivalent to a byte. This can make classes more efficient and smaller.

C# program that uses underlying type

using System;

class Program
{
    enum CoffeeSize : byte
    {
	None,
	Tall,
	Venti,
	Grande
    };

    static void Main()
    {
	// ... Create a coffee size local.
	CoffeeSize size = CoffeeSize.Venti;
	Console.WriteLine(size);
    }
}

Output

Venti
Method

GetUnderlyingType. We can determine an enum's type (like int) at runtime. Enum.GetUnderlyingType, a static method, determines the underlying type.

Static Method

Next:We declare an enum Importance. For this example it uses an underlying type of byte.

Then:When the GetUnderlyingType method is called, the System.Byte type is returned.

C# that uses GetUnderlyingType

using System;

class Program
{
    enum Importance : byte
    {
	Low,
	Medium,
	High
    };

    static void Main()
    {
	// Determine the underlying type of the enum.
	Type type = Enum.GetUnderlyingType(typeof(Importance));
	Console.WriteLine(type);
    }
}

Output

System.Byte
Attribute Flags syntax

Flags. The language allows us to specify a Flags attribute on an enum. This enables the enum to be used as a bitfield. We can use combinations of enum values this way.

Flags
Array

Arrays. Enums are values. We can use enums to index arrays. This approach is useful for some kinds of tables or data structures in programs.

Enum Array
Performance fast-forward

Performance. Enums are fast. They are almost never a performance concern. To study enum performance, we look at the intermediate language of enum types.

Enum Performance
Byte

Memory usage. Suppose we develop a class that uses a field that is an enum type. The underlying type of this enum contributes to how much memory the class will use.

So:Using a smaller type, such as byte, can make programs that create many instances of our class more efficient.

Note:Generally thousands, or hundreds of thousands, of class instances are required before this optimization is important.


C# language

To review: enums enhance clarity and reduce the probability of invalid constants. We use them to represent constant values such as integers in a consistent way.


For code quality, enums are checked by the compiler. It is a good idea to avoid magic constants and numbers. Instead, prefer a self-documenting enum type.