In computer programming languages, a reserved word is a word which has a special grammatical meaning to a language and cannot be used as an identifier in that language. For instance, in SQL, a user cannot be called "group" because the word group is used to indicate that an identifier refers to a group, not a user. Such a word is a keyword; it is because its use is restricted that it is also a reserved word. Sometimes the specification for a programming language will have reserved words that are intended for possible use in future versions. In Java, const and goto are reserved words — they have no meaning in Java but they also cannot be used as identifiers. By "reserving" the terms, they can be implemented in future versions of Java, if desired, without "breaking" older Java source code.
Reserved words and language independence
(See also: Language-independent_specification) In .NET's Common Language Specification, all languages must provide a mechanism for using public identifiers that are reserved words in that language. To see why this is necessary, suppose we defined a class in VB.NET as follows:
Public Class this
End Class
Then, we compile this class into an assembly and distribute it as part of a toolbox. A C# programmer, wishing to define a variable of type “this” would encounter a problem: “this” is a reserved word in C#. The following will not compile in C#:
this x = new this();
A similar issue arises when accessing members, overriding virtual methods, and identifying namespaces. In C#, placing the at-sign before the identifier, will force it to be considered an identifier rather than a reserved word by the compiler. The at-sign is not considered part of the identifier.
@this x = new @this();
For consistency, this usage is also permitted in non-public settings such as local variables, parameter names, and private members.
