The Role of the Common Language Specification  (CLS)
The Common Language Specification (CLS) is a subset of the CTS.  The CLS defines the minimal features a
language must support to be considered a .NET language.  If a complier abides by the CLS rules, all other .NET
languages can use the resulting binaries in a uniform manner.  By and large, only developers creating a new .NET
language need to engross themselves in the full details of the CLS.  Nevertheless, it is enlightening to examine
some of these rules firsthand.  

CLS rule: All classes ultimately derive from System.Object:   

For the time being, don’t concern yourself with the exact syntax seen below.   We will see the details of System.
Object later in this class.

    ' The top-most class in the .NET world: System.Object
    Public Class Object      
       ' Reference equality by default. Override for value semantics.
       Public Overridable Function Equals(obj As Object) As Boolean
       Public Shared Function Equals(objA As Object, objB As Object) _
          As Boolean

       ' Get hash value for object. Override for efficient Hash table use.
       Public Overridable Function GetHashCode() As Integer
             
       ' RTTI for this object.     
       Public Function GetType() As Type
       
       ' Override to produce state data in string format.                      
       Public Overridable Function ToString() As String
       
       ' Override to cleanup resources (non-deterministic destruction)        
       Protected Overridable Sub Finalize()                      
    End Class

CLS rule: Every class has exactly one base class but may implement multiple interfaces.

Like Java, the CLS does not support multiple inheritance for class types. C++ developers take note.  Like Java,
the CLS allows a class to implement multiple interfaces.  It is possible for an interface to have multiple base
interfaces. In other words, multiple interface inheritance is supported.  CLS rule: Every type in an assembly
must specify its visibility. Although a given language may have unique keywords, the basic visibility settings
are:

  • Public: visible to anyone, anywhere.
  • Private: only visible to members of the defining type.
  • Protected: only visible to members of the defining type or members of derived types.
  • Internal: visible to any type defined within the assembly.  In VB, internal visibility is defined using the
    Friend keyword.

CLS rule: All data points must map to the types defined by the CTS.

The CTS defines a set of core data types, each of which has a corresponding object in the System namespace.  
As far as the type system itself is concerned, a type can be from the set {class, interface, structure,
enumeration, delegate}.  Since the release of .NET 2.0, all types but enumerations may be created generically.
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Common Language Specification (CLS)
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