int mark = 50;
switch (mark)
{
    case int n when n >= 80:
        Console.WriteLine("Grade is A");
        break;

    case int n when n >= 60:
        Console.WriteLine("Grade is B");
        break;

    case int n when n >= 40:
        Console.WriteLine("Grade is C");
        break;

    default:
        Console.WriteLine("Grade is D");
        break;
}