Why is this an issue?

A method using the VarArgs calling convention is not Common Language Specification (CLS) compliant and might not be accessible across programming languages, while the params keyword works the same way and is CLS compliant.

This rule raises an issue when a public or protected type contains a public or protected method that uses the VarArgs calling convention.

Noncompliant code example

using System;

namespace MyLibrary
{
    public class Foo
    {
        public void Bar(__arglist) // Noncompliant
        {
            ArgIterator argumentIterator = new ArgIterator(__arglist);
            for(int i = 0; i < argumentIterator.GetRemainingCount(); i++)
            {
                Console.WriteLine(
                    __refvalue(argumentIterator.GetNextArg(), string));
            }
        }
    }
}

Compliant solution

using System;

[assembly: CLSCompliant(true)]
namespace MyLibrary
{
    public class Foo
    {
        public void Bar(params string[] wordList)
        {
            for(int i = 0; i < wordList.Length; i++)
            {
                Console.WriteLine(wordList[i]);
            }
        }
    }
}

Exceptions

Interop methods using VarArgs calling convention do not raise an issue.

[DllImport("msvcrt40.dll")]
public static extern int printf(string format, __arglist); // Compliant