C# Language Specification

Static classes

The static modifier is used to mark the class being declared as a static class.A static class cannot be instantiated, cannot be used as a type and can contain only static members. Only a static class can contain declarations of extension methods (§10.6.9).

A static class declaration is subject to the following restrictions:

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  A static class may not include a sealed or abstract modifier. Note, however, that since a static class cannot be instantiated or derived from, it behaves as if it was both sealed and abstract.
  
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  A static class may not include a class-base specification (§10.1.4) and cannot explicitly specify a base class or a list of implemented interfaces. A static class implicitly inherits from type object.
  
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  A static class can only contain static members (§10.3.7). Note that constants and nested types are classified as static members.
  
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  A static class cannot have members with protected or protected internal declared accessibility.
  

It is a compile-time error to violate any of these restrictions.

A static class has no instance constructors. It is not possible to declare an instance constructor in a static class, and no default instance constructor (§10.11.4) is provided for a static class.

The members of a static class are not automatically static, and the member declarations must explicitly include a static modifier (except for constants and nested types). When a class is nested within a static outer class, the nested class is not a static class unless it explicitly includes a static modifier.

1. Referencing static class types

A namespace-or-type-name (§3.8) is permitted to reference a static class if

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  The namespace-or-type-name is the T in a namespace-or-type-name of the form T.I, or
  
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  The namespace-or-type-name is the T in a typeof-expression (§7.5.11) of the form typeof(T).
  

A primary-expression (§7.5) is permitted to reference a static class if

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  The primary-expression is the E in a member-access (§7.5.4) of the form E.I.
  

In any other context it is a compile-time error to reference a static class. For example, it is an error for a static class to be used as a base class, a constituent type (§10.3.8) of a member, a generic type argument, or a type parameter constraint. Likewise, a static class cannot be used in an array type, a pointer type, a new expression, a cast expression, an is expression, an as expression, a sizeof expression, or a default value expression.

2.