A using directive which declares a member of the System namespace appears after a using directive which declares a member of a different namespace, within a C# code file.
A violation of this rule occurs when a using directive for the System namespace is placed after a non-System using directive. Placing all System using directives at the top of the using directives can make the code cleaner and easier to read, and can help make it easier to identify the namespaces that are being used by the code.
How to Fix Violations
To fix an instance of this violation, place the System using directive above all using directives for other namespaces.