c# - Can't Set variable value in Static Field -

in simple example why stra in sample() function can't set "hi world" string ?

string stra { get; set; }  private void button1(object sender, eventargs e) {     stra = "hi world";   //=======>   stra value }   public static string sample() {     myclass myclass1 = new myclass();      string = myclass1.stra;  //==========> can't set stra value "hi world" string ???      return (messagebox.show(a).tostring()); } 

make stra static field, right it's local property of instance, when create new instance myclass1 = new myclass();

the stra property empty (null) , if set value, after create new instance new instance have srta empty...