Rules for Identifiers

This section examines the rules governing what names you can use for variables, classes, methods, and so on. Note that the rules presented in this section are not merely guidelines: they are enforced by the C# compiler.

Identifiers are the names you give to variables, to user-defined types such as classes and structs, and to members of these types. Identifiers are case sensitive, so for example variables named interestRate and InterestRate would be recognized as different variables. Following are a couple of rules determining what identifiers you can use in C#:

• They must begin with a letter or underscore, although they can contain numeric characters.

• You can’t use C# keywords as identifiers.

The following table lists the C# reserved keywords.

abstract	do	Implicit	params	Switch
as	double	In	private	This
base	else	Int	protected	Throw
bool	enum	Interface	public	True
break	event	Internal	readonly	Try
byte	explicit	Is	ref	Typeof
case	extern	lock	return	Uint
catch	false	long	sbyte	Ulong
char	finally	namespace	sealed	Unchecked
checked	fixed	new	short	Unsafe
class	float	null	sizeof	Ushort
const	for	object	stackalloc	Using
continue	foreach	operator	static	Virtual
decimal	goto	out	string	Volatile
default	if	override	struct	Void
delegate				While

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If you do need to use one of these words as an identifier (for example, if you are accessing a class written in a different language), you can prefix the identifier with the @ symbol to indicate to the compiler that what follows is to be treated as an identifier, not as a C# keyword (so abstract is not a valid identifier, but @abstract is).

Finally, identifiers can also contain Unicode characters, specified using the syntax \uXXXX, where XXXX is the four-digit hex code for the Unicode character. The following are some examples of valid identifiers:

• Name

• überfluß

• _Identifier

• \u005fIdentifier

The last two items in this list are identical and interchangeable (because 005f is the Unicode code for the underscore character), so obviously these identifiers couldn’t both be declared in the same scope. Note that although syntactically you are allowed to use the underscore character in identifiers, this isn’t recommended in most situations, because it doesn’t follow the guidelines for naming variables that Microsoft has written to ensure that developers use the same conventions, making it easier to read each other’s code.

Usage Conventions

In any development language, there usually arise certain traditional programming styles. The styles are not part of the language itself but are conventions concerning, for example, how variables are named or how certain classes, methods, or functions are used. If most developers using that language follow the same conventions, it makes it easier for different developers to understand each other’s code - which in turn generally helps program maintainability. For example, a common (though not universal) convention in Visual Basic 6 was that variables that represents strings have names beginning with lowercase s or lowercase str, as in the Visual Basic 6 statements Dim sResult As String or Dim strMessage As String. Conventions do, however, depend on the language and the environment. For example, C++ developers programming on the Windows platform have traditionally used the prefixes psz or lpsz to indicate strings - char *pszResult; char *lpszMessage; - but on Unix machines it’s more common not to use any such prefixes: char *Result; char *Message;.

You’ll notice from the sample code in this site that the convention in C# is to name variables without prefixes: string Result; string Message;.

Whereas, with many languages, usage conventions simply evolved as the language was used, with C# and the whole of the .NET Framework Microsoft has written very comprehensive usage guidelines, which are detailed in the .NET/C# MSDN documentation. This should mean that, right from the start, .NET programs will have a high degree of interoperability in terms of developers being able to understand code. The guidelines have also been developed with the benefit of some 20 years’ hindsight in object-oriented programming, and as a result have been carefully thought out and appear to have been well received in the developer community to judge by the relevant newsgroups. Hence the guidelines are well worth following.

It should be noted, however, that the guidelines are not the same as language specifications. You should try to follow the guidelines when you can. Nevertheless, you won’t run into problems if you do have a good reason for not doing so - for example, you won’t get a compilation error because you don’t follow these guidelines. The general rule is that if you don’t follow the usage guidelines you must have a convincing reason. Departing from the guidelines should be a positive decision rather than simply not bothering. Also, if you compare the guidelines with the samples in the remainder of this site, you’ll notice that in numerous examples in this site, we have chosen not to follow the conventions. That’s usually because the conventions are designed for much larger programs than our samples, and although they are great if you are writing a complete software package, they are not really so suitable for small 20-line standalone programs. In many cases, following the conventions would have made our samples harder, rather than easier, to follow.

The full guidelines for good programming style are quite extensive. This section is confined to describing some of the more important guidelines, as well as the ones most likely to surprise you. If you want to make absolutely certain that your code follows the usage guidelines completely, you will need to refer to the MSDN documentation.

Naming Conventions

One important aspect to making your programs understandable is how you choose to name your items - and that includes naming variables, methods, classes, enumerations, and namespaces.

It is intuitively obvious that your names should reflect the purpose of the item and should be designed not to clash with other names. The general philosophy in the .NET Framework is also that the name of a variable should reflect the purpose of that variable instance and not the data type. For example, height is a good name for a variable, whereas integerValue isn’t. However, you will probably feel that that principle is an ideal that is hard to achieve. Particularly when you are dealing with controls, in most cases, you’ll probably feel happier sticking with variable names like confirmationDialog and chooseEmployeeListBox, which do indicate the data type in the name.

The following sections look at some of the things you need to think about when choosing names.

Casing of Names

In many cases you should use Pascal casing for names. Pascal casing means that the first letter of each word in a name is capitalized: EmployeeSalary, ConfirmationDialog, PlainTextEncoding. You will notice that essentially all of the names of namespaces, classes, and members in the base classes follow Pascal casing. In particular, the convention of joining words using the underscore character is discouraged. So, you should try not to use names like employee_salary. It has also been common in other languages to use all capitals for names of constants. This is not advised in C#, because such names are harder to read - the convention is to use Pascal casing throughout:

const int MaximumLength;

The only other casing scheme that you are advised to use is camel casing. Camel casing is similar to Pascal casing, except that the first letter of the first word in the name is not capitalized: employeeSalary, confirmationDialog, plainTextEncoding. Following are three situations in which you are advised to use camel casing:

• For names of all private member fields in types:

  
  public int subscriberId;
  

  Note, however, that often it is conventional to prefix names of member fields with an underscore:

  
  public int subscriberId;
  

• For names of all parameters passed to methods:

  
  public void RecordSale(string salesmanName, int quantity);
  

• To distinguish items that would otherwise have the same name. A common example is when a property wraps around a field:

  
  private string employeeName;
  
  public string EmployeeName
  {
     get
     {
        return employeeName;
        
     }
     
  }
  

If you are doing this, you should always use camel casing for the private member and Pascal casing for the public or protected member, so that other classes that use your code see only names in Pascal case (except for parameter names).

You should also be wary about case sensitivity. C# is case sensitive, so it is syntactically correct for names in C# to differ only by the case, as in the previous examples. However, you should bear in mind that your assemblies might at some point be called from Visual Basic .NET applications - and Visual Basic .NET is not case sensitive. Hence, if you do use names that differ only by case, it is important to do so only in situations in which both names will never be seen outside your assembly. (The previous example qualifies as okay because camel case is used with the name that is attached to a private variable.) Otherwise, you may prevent other code written in Visual Basic .NET from being able to use your assembly correctly.

Name Styles

You should try to be consistent about your style of names. For example, if one of the methods in a class is called ShowConfirmationDialog(), then you should not give another method a name like ShowDialogWarning() or WarningDialogShow(). The other method should be called ShowWarningDialog().

Namespace Names

Namespace names are particularly important to design carefully in order to avoid risk of ending up with the same name for one of your namespaces as someone else uses. Remember, namespace names are the only way that .NET distinguishes names of objects in shared assemblies. So, if you use the same namespace name for your software package as another package, and both packages get installed on the same computer, there are going to be problems. Because of this, it’s almost always a good idea to create a top-level namespace with the name of your company and then nest successive namespaces that narrow down the technology, group, or department you are working in or the name of the package your classes are intended for. Microsoft recommends namespace names that begin with <CompanyName>.<TechnologyName> as in these two examples:

WeaponsOfDestructionCorp.RayGunControllers
WeaponsOfDestructionCorp.Viruses

Names and Keywords

It is important that the names do not clash with any keywords. In fact, if you attempt to name an item in your code with a word that happens to be a C# keyword, you’ll almost certainly get a syntax error because the compiler will assume that the name refers to a statement. However, because of the possibility that your classes will be accessed by code written in other languages, it is important that you also don’t use names that are keywords in other .NET languages. Generally speaking, C++ keywords are similar to C# keywords, so confusion with C++ is unlikely, and those commonly encountered keywords that are unique to Visual C++ tend to start with two underscore characters. Like C#, C++ keywords are spelled in lowercase, so if you hold to the convention of naming your public classes and members with Pascal-style names, they will always have at least one uppercase letter in their names, and there will be no risk of clashes with C++ keywords. On the other hand, you are more likely to have problems with Visual Basic .NET, which has many more keywords than C# does, and being non-case-sensitive means you cannot rely on Pascal-style names for your classes and methods.

The following table lists the keywords and standard function calls in Visual Basic .NET, which should if possible be avoided, in whatever case combination, for your public C# classes.

Abs	BitNot	Choose	Each	FileCopy
Add	BitOr	Chr	Else	FileDateTime
AddHandler	BitXor	CInt	ElseIf	FileLen
AddressOf	Boolean	Class	Empty	Filter
Alias	ByRef	Clear	End	Finally
And	Byte	CLng	Enum	Fix
Ansi	ByVal	Close	EOF	For
AppActivate	Call	Collection	Erase	Format
Append	Case	Command	Err	FreeFile
As	Catch	Compare	Error	Friend
Asc	CBool	Const	Event	Function
Assembly	CByte	Cos	Exit	FV
Atan	CDate	CreateObject	Exp	Get
Auto	CDbl	CShort	Explicit	GetAllSettings
Beep	CDec	CSng	ExternalSource	GetAttr
Binary	ChDir	Do	False	GetException
BitAnd	ChDrive	Double	FileAttr	GetObject
GetSetting	Minute	Off	SaveSettings	StrConv
GetType	MIRR	On	Second	Strict
GoTo	MkDir	Open	Seek	String
Handles	Module	Option	Select	Structure
Hex	Month	Optional	SetAttr	Sub
Hour	MustInherit	Or	SetException	Switch
If	MustOverride	Overloads	Shared	SYD
Iif	MyBase	Overridable	Shell	SyncLock
Implements	MyClass	Overrides	Short	Tab
Imports	Namespace	ParamArray	Sign	Tan
In	New	Pmt	Sin	Text
Inherits	Next	PPmt	Single	Then
Input	Not	Preserve	SLN	Throw
Loc	Nothing	Print	Space	TimeOfDay
Local	NotInheritable	Private	Spc	Timer
Lock	NotOverridable	Property	Split	TimeSerial
LOF	Now	Public	Sqrt	TimeValue
Log	NPer	RGB	Static	To
Long	NPV	Right	Step	Today
Loop	Null	RmDir	Stop	Trim
LTrim	Object	Rnd	Str	Try
Me	Oct	RTrim	StrComp	TypeName
Mid

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