Why is this an issue?

Although unnecessary using won’t change anything to the produced application, removing them:

• Will help readability and maintenance.
• Will help reduce the number of items in the IDE auto-completion list when coding.
• May avoid some name collisions.
• May improve compilation time because the compiler has fewer namespaces to look-up when it resolves types.
• The build will fail if this namespace is removed from the project.

Noncompliant code example

using System.Collections.Generic; // Noncompliant - unnecessary using

namespace Foo
{
    public class Bar
    {
        public Bar(string path)
        {
            File.ReadAllLines(path);
        }
    }
}

Compliant solution

using System.IO;

namespace Foo
{
    public class Bar
    {
        public Bar(string path)
        {
            File.ReadAllLines(path);
        }
    }
}