Static vars not seen too often in C #

I am new to C # - this is almost my first program. I am trying to create some public static variables and constants for use anywhere in the program. The wrong way I've tried is to declare them in a separate class in the same namespace, but they don't match the context for the main program. This is a WPF application. The code looks like this:

namespace testXyz
{
    class PublicVars
    {
        public const int BuffOneLength = 10000;
        public static int[] Buff1 = new int[BuffOneLength];

        public const int BuffTwoLength = 2500;
        public static int[] Buff2 = new int[BuffTwoLength];

        private void fillBuff1()
        {
            Buff1[0] = 8;
            Buff1[1] = 3;           
            //etc
        }

        private void fillBuff2()
        {
            Buff2[0] = 5;
            Buff2[1] = 7;           
            //etc
        }   
    }
}

      

        
        
        
      

    

Second file:

namespace testXyz
{
    public partial class MainWindow : Window
    {
        public MainWindow() 
        {
            InitializeComponent();
        }

        public static int isInContext = 0;
        int jjj = 0, mmm = 0;

        private void doSomething()
        {
            isInContext = 5;    // this compiles
            if (jjj < BuffOneLength)    // "the name 'BuffOneLength' does not exist in the current context"
            {
                mmm = Buff2[0]; // "the name 'Buff2' does not exist in the current context"
            }
        }
    }
}

      

        
        
        
      

    

My actual program is much longer, of course. I created the above WPF application exactly as shown in order to test this issue and I got these errors also encountered in a real program. I really don't want to fill the arrays in the main program as they are very long and that would mean a lot of scrolling. I also want to have one place where I can declare certain public static variables. What is the correct way to do this?