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Common Language Specification In .NET

Introduction

Sometimes there is a need for code written in one language to access another language, but programming languages that target CLR are different from one other, for example C# is case sensitive but VB is not. This can cause a problem when we access code written in one language from another language. So .Net has come up with CLS; i.e., Common Language Specification.

Definition

CLS defines a minimum set of features that must be supported by all languages that target CLR.

Figure - The below Venn diagram will be helpful to understand CLS.
 
 
 
CLR/CTS supports lots of feature but that does not mean all the languages that target CLR supports all these feature.
 
As per the above Venn diagram:
  • CLR is a superset of all languages that target CLR.
  • C# and VB are subsets of CLR
  • CLS is a minimal subset that is a must-have for all languages targeting CLR.
  • Also some of the features are common in C# and VB other than CLS features.
CLSCompliant Attribute

Before starting with any concrete program/example on CLS, I will explain a useful attrubute that needs to be included while writing CLS Compliant Code.

Syntax : [assembly:CLSCompliant(true)]

If you set CLSCompliant attribute to true, it gives a compile time warning if you write any public or protected member which is not CLSCompliant. It will give a warning for only public and protected members, not for private, as private members can be accessed only within that class.

Example 1

C# is case sensitive but VB is not; that's why public members that differ by case will give you a warning.
  1. using System;  
  2.   
  3. [assembly: CLSCompliant(true)]  
  4. namespace CLSExamples  
  5. {  
  6.   
  7.     public class CaseSensitiveExample  
  8.     {  
  9.         static void Main(string[] args)  
  10.         {  
  11.   
  12.         }  
  13.   
  14.         public void XYZ()  
  15.         {  
  16.             Console.WriteLine("Inside XYZ");  
  17.         }  
  18.         public void Xyz()  
  19.         {  
  20.             Console.WriteLine("Inside Xyz");  
  21.         }  
  22.     }  
  23. }                
Upon execution of the above code you will get a warning as :Identifier 'CaseSensitiveExample.Xyz()' differing only in case is not CLS-compliant.

Example 2

C# supports Unsigned Integer but VB does not support Unsigned Integer
  1. using System;  
  2.   
  3. [assembly: CLSCompliant(true)]  
  4. namespace CLSExamples  
  5. {  
  6.   
  7.     public class OptionalParameterExample  
  8.     {  
  9.         static void Main(string[] args)  
  10.         {  
  11.   
  12.         }  
  13.   
  14.         public UInt32 XYZ(int a)  
  15.         {  
  16.             Console.WriteLine("Inside XYZ");  
  17.             return (UInt32)(a * a);  
  18.         }  
  19.     }  
  20. }  
Upon compilation the above code will give a warning as: Return type of 'OptionalParameterExample.XYZ(int)' is not CLS-compliant.

Note
  1. There  can be many language specific features that are not CLSCompliant. To help you understand,  I have used the above examples.
  2. Above code snippets are tested and working.
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