Writing to text file gives strange output

I am trying to write to a text file but I have a problem when I try write a lot to the file. The code is as follows:

         gridZones=new byte[400,400];
         // populate byte array...
         //...
         
         System.IO.StreamWriter Cool =new System.IO.StreamWriter("COOL.txt");        
         
         for(int i=0;i<400;i++)
         {
             Cool.Write("{");
             for(int j=0;j<400;j++)
             {
                 Cool.Write(gridZones[i,j].ToString());
                 Cool.Write(",");
             }
             Cool.WriteLine("},");
         }

This is an example of what I find in the text file when I open it: ほ紬ബ笊ⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⱽ਍ほ紬ബ笊ⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰⰰ

I have also tried the System.IO.WriteAllLines command, inputting a string array with the same result. If I use the print function as such:

         string[]win=new string[400];
         for(int i=0;i<400;i++)
         {
             win[i]="{";
             for(int j=0;j<400;j++)
             {
                 win[i]=win[i]+gridZones[i,j].ToString()+",";
             }
             win[i]=win[i]+"},";
             print (win[i]);
         }    

The output is as it should be: {0,0,0,0,0,0,0,0, ... 0,0,0,0,}, etc

Any reason why this happens and how to fix it?

Thanks in advance for any help!!