C# identifiers and keywords

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Section 1. Introduction. 1.7 C# identifiers and keywords

C# identifiers

In programming languages, identifiers are used for identification purposes. Or in other words, identifiers are the user-defined name of the program components. In C#, an identifier can be a class name, method name, variable name or label. Identifiers can be short names (like x and y) or more descriptive names (firstName, volume). It is recommended to use descriptive names in order to create understandable and maintainable code.

There are certain valid rules for defining a valid C# identifier.

The only allowed characters for identifiers are all alphanumeric characters ([A-Z], [a-z], [0-9]), ‘_’ (underscore).
Identifiers should not start with digits ([0-9]). For example “12moons” is a not a valid in C# identifier.
Identifiers should not contain white spaces.
Identifiers should start with a lowercase letter.
Keywords are reserved words predefined to the C# compiler. These keywords cannot be used as identifiers. However, if you want to use these keywords as identifiers, you may prefix the keyword with the @ character.
C# identifiers allow Unicode Characters.
C# identifiers are case-sensitive – “MyName” and “myname” has different meaning.
C# identifiers cannot contain more than 512 characters.
Identifiers does not contain two consecutive underscores in its name because such types of identifiers are used for the implementation.

C# keywords

Keywords are reserved words predefined to the C# compiler. These keywords cannot be used as identifiers. Doing this will result in a compile-time error. However, if you want to use these keywords as identifiers, you may prefix the keyword with the @ character.

In C#, some identifiers have special meaning in context of code, such as get and set are called contextual keywords.

Reserved keywords

There are total 78 reserved keywords in C# as follows:

Keywords in C# is mainly divided into 10 categories as follows:

Value Type Keywords: There are 15 keywords in value types which are used to define various data types:
bool, byte, char, decimal, double, enum, float, int, long, sbyte, short, struct, unit, ulong, ushort.
Reference Type Keywords: There are 6 keywords in reference types which are used to store references of the data or objects. The keywords in this category are: class, delegate, interface, object, string, void.
Modifiers Keywords: There are 17 keywords in modifiers which are used to modify the declarations of type member: public, private, internal, protected, abstract, const, event, extern, new, override, partial, readonly, sealed, static, unsafe, virtual, volatile.
Statements Keywords: There are total 18 keywords which are used in program instructions: if, else, switch, do, for, foreach, in, while, break, continue, goto, return, throw, try, catch, finally, checked, unchecked, fixed, case, lock.
Method Parameters Keywords: There are total 4 keywords which are used to change the behavior of the parameters that passed to a method. The keyword includes in this category are: params, in, ref, out.
Namespace Keywords: There are total 3 keywords in this category which are used in namespaces. The keywords are: namespace, using, extern.
Operator Keywords: There are total 8 keywords which are used for different purposes like creating objects, getting a size of object etc. The keywords are: as, is, new, sizeof, typeof, true, false, stackalloc.
Conversion Keywords: There are 3 keywords which are used in type conversions. The keywords are: explicit, implicit, operator.
Access Keywords: There are 2 keywords which are used in accessing and referencing the class or instance of the class. The keywords are base, this.
Literal Keywords: There are 2 keywords which are used as literal or constant. The keywords are null, default.

Contextual Keywords

These are used to give a specific meaning in the program. Whenever a new keyword comes in C#, it is added to the contextual keywords, not in the keyword category. This helps to avoid the crashing of programs which are written in earlier versions.

These are not reserved words.
It can be used as identifiers outside the context that’s why it named contextual keywords.
These can have different meanings in two or more contexts.
There are total 30 contextual keywords in C#: add, alias, ascending, async, await, by, descending, dynamic, equals, from, get, global, group, into, join, let, nameof, on, orderby, partial (type), partial (method), remove, select, set, value, var, when, where, where, yield.

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