C#中静态变量有什么用什么时候使用它为什么我不能在方法内声明静态变量

静态变量的作用及使用时机

csharp
using System;
class Car
{
    public static int numberOfCars = 0;
    public string brand;
    public Car(string brand)
    {
        this.brand = brand;
        numberOfCars++;
    }
    public static void DisplayNumberOfCars()
    {
        Console.WriteLine("Total number of cars: " + numberOfCars);
    }
}
class Program
{
    static void Main(string[] args)
    {
        Car car1 = new Car("Toyota");
        Car car2 = new Car("Honda");
        Car car3 = new Car("Ford");
        Car.DisplayNumberOfCars();
    }
}

Total number of cars: 3