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The access modifier is a keyword that specifies the accessibility of a declared member or type. This section describes the four access modifiers:
Use these access modifiers to specify the following five levels of accessibility:
Public : Access is not restricted.
Protected: Access is limited to types that contain or derive from the containing class.
Internal: Access is limited to the current assembly.
Protected internal: Access is restricted to the current assembly or from a category that contains derived types.
Private: Access is limited to the containing type.
This section also describes the following:
Accessibility level: Five levels of accessibility are declared with four access modifiers.
Accessible domains: Specifies where in a program section A member can be referenced.
Usage restrictions for accessibility levels: Provides an overview of the usage limits for the declared accessibility level.
Access modifiers (C # Reference)