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.

using System;
[assembly: CLSCompliant(true)]
namespace CLSExamples
{
public class CaseSensitiveExample
{
static void Main(string[] args)
{
}
public void XYZ()
{
Console.WriteLine("Inside XYZ");
}
public void Xyz()
{
Console.WriteLine("Inside Xyz");
}
}
}

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

using System;
[assembly: CLSCompliant(true)]
namespace CLSExamples
{
public class OptionalParameterExample
{
static void Main(string[] args)
{
}
public UInt32 XYZ(int a)
{
Console.WriteLine("Inside XYZ");
return (UInt32)(a * a);
}
}
}

Upon compilation the above code will give a warning as: Return type of 'OptionalParameterExample.XYZ(int)' is not CLS-compliant.

Note

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