internal const int SC_CLOSE = 0xF060; 
          internal const int MF_ENABLED = 0x00000000; 
          internal const int MF_GRAYED = 0x1; 
          internal const int MF_DISABLED = 0x00000002; 
          [DllImport("user32.dll")] 
          private static extern IntPtr GetSystemMenu(IntPtr HWNDValue, bool isRevert); 
   
          [DllImport("user32.dll")] 
          private static extern int EnableMenuItem(IntPtr tMenu, int targetItem, int targetStatus); 
          public void EnableCloseButton() 
          { 
              EnableMenuItem(GetSystemMenu(this.Handle, false), SC_CLOSE, MF_ENABLED); 
          } 
   
          public void DisableCloseButton() 
          { 
              EnableMenuItem(GetSystemMenu(this.Handle, false), SC_CLOSE, MF_GRAYED); 
          } 
   
          private void button1_Click(object sender, EventArgs e) 
          { 
   
              EnableCloseButton(); 
          } 
   
          private void button2_Click(object sender, EventArgs e) 
          { 
              DisableCloseButton(); 
          }