Why is this an issue?

Creating a new Exception without actually throwing does not achieve the intended purpose.

if (x < 0)
{
    new ArgumentException("x must be nonnegative");
}

Ensure to throw the Exception with a throw statement.

if (x < 0)
{
    throw new ArgumentException("x must be nonnegative");
}

Resources

Documentation

• Microsoft Learn - Exception Class
• Microsoft Learn - Exception-handling statements - throw, try-catch, try-finally, and try-catch-finally