CLS, or the Common Language Specification is a set of rules which make up the most basic features of all .NET languages. Many .NET languages exceed these basic requirements and allow for features above and beyond the basics. For instance, C# allows for case sensitive public members, meaning that object.Property is different to object.property.

When developing .NET libraries for use by other developers it is a very good idea to have your library work in a CLS Compliant way, so that you are enabling all .NET languages to leverage your library. It also proves to be a good idea as many of the CLS Rules reflect good design practices.

In order to indicate that your library is CLS Complaint the whole assembly can be marked as such using the System.CLSCompliantAttribute, so in Visual Studio projects using C# this is often placed in AssemblyInfo.cs and looks something like this:

[assembly:CLSCompliantAttribute(true)]

This attribute tells the C# compiler to check the public aspects of whole assembly against the CLS Specification and will report any breaches of these rules as compile errors.

It is also possible to mark individual types, methods, properties, etc, as CLS Compliant by decorating the definition with the same attribute, like so:

[CLSCompliantAttribute(true)]
public class MyType{ ....

[CLSCompliantAttribute(true)]
public string MyFunction() {....

Finally, if you find that you need to exclude something from CLS Compliance, it can be marked as non-compliant by passing a false into the attribute:

//This method can't be CLS compliant as UInt32 is not a CLS type
[CLSCompliantAttribute(false)]
public UInt32 MyFunction() {....

CLS Compliance may seem like an extra complication to development, but it really is a good idea as it forces you to consider the needs of other developers (in other languages) and does enforce a number of good practices via compile time checks.

Further Information:
Cross-Language Interoperability on MSDN

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