You need to make sure your C# code will interoperate with all other managed languages that are CLS-compliant consumers, such as VB.NET.
Mark the assembly with the
By default, your C# assemblies created with VS.NET are not marked
CLSCompliantAttribute. This does not mean
that the assembly will not work in the managed environment. It means
that this assembly may use elements that are not recognized by other
Common Language Specification (CLS)-compliant languages. For example,
unsigned numeric types are not recognized by all managed languages,
but they can be used in the C# language. The problem occurs when C#
returns an unsigned data type, such as
either through a return value or a parameter to a calling component
in another language that does not recognize unsigned data
types—VB.NET is one example.
CLS compliance is enforced only on types/members marked public or protected. This makes sense because components written in other languages will only be able to use the public or protected types/members of components written in C#.
Marking your assembly as CLS-compliant means that any CLS-compliant language will be able to seamlessly interoperate with your code; that is, it enables CLS-compliance checking. It should also be noted that if you have types and/or members within those types that are not CLS-compliant, ...