Winforms databinding and validation, why is datasource updated when validation fails? -

the code below illustrates unexpected (for me!) behaviour when combining data binding , validation in winforms. can tell me how can prevent datasource being updated when validation fails?

many thanks.

using system; using system.collections.generic; using system.componentmodel; using system.data; using system.drawing; using system.linq; using system.text; using system.windows.forms;  namespace validationbug {     /// <summary>     /// illustrates unexpected behaviour winforms validation , binding     ///      /// reproduce: run program, enter value textbox, click x close form.     ///      /// expected behaviour: validation of textbox fails data source not updated.     ///      /// observed behaviour: data source updated.     /// </summary>     public class form1 : form     {         private class data         {             private string _field;             public string field             {                 { return _field; }                 set                  {                      // should never called, is.                     _field = value;                  }             }         }          private system.componentmodel.icontainer components = null;          public form1()         {             this.load += new system.eventhandler(this.form1_load);         }          private void form1_load(object sender, eventargs e)         {             autovalidate = system.windows.forms.autovalidate.enablepreventfocuschange;              var txt = new textbox();              // validation fails.             txt.validating += new canceleventhandler((s, ev) => ev.cancel = true);             controls.add(txt);              var data = new data();              this.components = new system.componentmodel.container();             bindingsource bs = new bindingsource(this.components);             bs.datasource = typeof(data);              // update datasource on succesful validation.             txt.databindings.add(new binding("text", data, "field", false, datasourceupdatemode.onvalidation));         }     } } 

i tend "brute force" code - set initial value private string _field, perhaps in constructor?

also, sure setting canceleventhandler's cancel property true marks data invalid?

you may want add private bool _valid field data class returns values if valid.

    private class data     {         private bool _valid;         private string _field;          public data()         {             _field = null;             _valid = false;         }          public string field         {                         {                 if (_valid)                 {                   return _field;                 } else {                   return null;                 }             set              {                  // should never called, is.                 _field = value;                  _valid = !string.isnullorempty(_field);             }         }     } 

just ideas into.