public class frmEinleser : System.Windows.Forms.Form
   {
      private System.Windows.Forms.ColumnHeader columnHeader1;
      private System.Windows.Forms.ColumnHeader columnHeader2;
      private System.Windows.Forms.ColumnHeader columnHeader3;
      private System.Windows.Forms.Button btnSchliessen;
      private System.Windows.Forms.ListView lvDaten;
 
      public class lvDatenSort: System.Windows.Forms.Form
      {
         private lvDatenColumnSorter lvColumnSorter = null;
         /// <summary>
         /// Erforderliche Designervariable.
         /// </summary>
         private System.ComponentModel.Container components = null;
 
         public frmEinleser() // << müsste in diesen/deinem Fall _
           "lvDatenSort" stehen.
         // Also:
         // public lvDatenSort()