Reserved and contextual keywords

Many programming languages, C# included, treat certain sequences of letters as “special”.

Some sequences are so special that they cannot be used as identifiers. Let’s call those the “reserved keywords” and the remaining special sequences we’ll call the “contextual keywords”. They are “contextual” because the character sequence might one meaning in a context where the keyword is expected and another in a context where an identifier is expected.[1. An unfortunate consequence of this definition is that using is said to be a reserved keyword even though its meaning depends on its context; whether the using begins a directive or a statement determines its meaning. And similarly for other keywords that have multiple usages, like fixed.]

The C# specification defines the following reserved keywords:

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