[c#] How to return a value from a Form in C#?

I have a main form (let's call it frmHireQuote) that is a child of a main MDI form (frmMainMDI), that shows another form (frmImportContact) via ShowDialog() when a button is clicked.

When the user clicks the 'OK' on frmImportContact, I want to pass a few string variables back to some text boxes on frmHireQuote.

Note that there could be multiple instances of frmHireQuote, it's obviously important that I get back to the instance that called this instance of frmImportContact.

What's the best method of doing this?

delegates are the best option for sending data from one form to another.

public partial class frmImportContact : Form
{
     public delegate void callback_data(string someData);
    public event callback_data getData_CallBack;

    private void button_Click(object sender, EventArgs e)
    {
      string myData = "Top Secret Data To Share";
      getData_CallBack(myData);
    }

}

public partial class frmHireQuote : Form
{
     private void Button_Click(object sender, EventArgs e)
    {

      frmImportContact obj = new frmImportContact();
      obj.getData_CallBack += getData;
    }

    private void getData(string someData)
    {
         MessageBox.Show("someData");
    }
}

Found another small problem with this code... or at least it was problematic when I tried to implement it.

The buttons in frmMain do not return a compatible value, using VS2010 I added the following and everything started working fine.

public static ResultFromFrmMain Execute() {
     using (var f = new frmMain()) {

          f.buttonOK.DialogResult = DialogResult.OK;
          f.buttonCancel.DialogResult = DialogResult.Cancel;

          var result = new ResultFromFrmMain();
          result.Result = f.ShowDialog();

          if (result.Result == DialogResult.OK) {
             // fill other values
          }
          return result;
     }
}

After adding the two button values, the dialog worked great! Thanks for the example, it really helped.

I just put into constructor something by reference, so the subform can change its value and main form can get new or modified object from subform.

I use MDI quite a lot, I like it much more (where it can be used) than multiple floating forms.

But to get the best from it you need to get to grips with your own events. It makes life so much easier for you.

A skeletal example.

Have your own interupt types,

//Clock, Stock and Accoubts represent the actual forms in
//the MDI application. When I have multiple copies of a form
//I also give them an ID, at the time they are created, then
//include that ID in the Args class.
public enum InteruptSource
{
    IS_CLOCK = 0, IS_STOCKS, IS_ACCOUNTS
}
//This particular event type is time based,
//but you can add others to it, such as document
//based.
public enum EVInterupts
{
    CI_NEWDAY = 0, CI_NEWMONTH, CI_NEWYEAR, CI_PAYDAY, CI_STOCKPAYOUT, 
   CI_STOCKIN, DO_NEWEMAIL, DO_SAVETOARCHIVE
}

Then your own Args type

public class ControlArgs
{
    //MDI form source
    public InteruptSource source { get; set; }
    //Interrupt type
    public EVInterupts clockInt { get; set; }
    //in this case only a date is needed
    //but normally I include optional data (as if a C UNION type)
    //the form that responds to the event decides if
    //the data is for it.
    public DateTime date { get; set; }
    //CI_STOCKIN
    public StockClass inStock { get; set; }

}

Then use the delegate within your namespace, but outside of a class

namespace MyApplication
{
public delegate void StoreHandler(object sender, ControlArgs e);
  public partial class Form1 : Form
{
  //your main form
}

Now either manually or using the GUI, have the MDIparent respond to the events of the child forms.

But with your owr Args, you can reduce this to a single function. and you can have provision to interupt the interupts, good for debugging, but can be usefull in other ways too.

Just have al of your mdiparent event codes point to the one function,

        calendar.Friday += new StoreHandler(MyEvents);
        calendar.Saturday += new StoreHandler(MyEvents);
        calendar.Sunday += new StoreHandler(MyEvents);
        calendar.PayDay += new StoreHandler(MyEvents);
        calendar.NewYear += new StoreHandler(MyEvents);

A simple switch mechanism is usually enough to pass events on to appropriate forms.

If you want to pass data to form2 from form1 without passing like new form(sting "data");

Do like that in form 1

using (Form2 form2= new Form2())
{
   form2.ReturnValue1 = "lalala";
   form2.ShowDialog();
}

in form 2 add

public string ReturnValue1 { get; set; }

private void form2_Load(object sender, EventArgs e)
{
   MessageBox.Show(ReturnValue1);
}

Also you can use value in form1 like this if you want to swap something in form1

just in form1

textbox.Text =form2.ReturnValue1

I raise an event in the the form setting the value and subscribe to that event in the form(s) that need to deal with the value change.

First you have to define attribute in form2(child) you will update this attribute in form2 and also from form1(parent) :

 public string Response { get; set; }

 private void OkButton_Click(object sender, EventArgs e)
 {
    Response = "ok";
 }

 private void CancelButton_Click(object sender, EventArgs e)
 {
    Response = "Cancel";
 }

Calling of form2(child) from form1(parent):

  using (Form2 formObject= new Form2() )
  {
     formObject.ShowDialog();

      string result = formObject.Response; 
      //to update response of form2 after saving in result
      formObject.Response="";

      // do what ever with result...
      MessageBox.Show("Response from form2: "+result); 
  }

I normally create a static method on form/dialog, that I can call. This returns the success (OK-button) or failure, along with the values that needs to be filled in.

 public class ResultFromFrmMain {
     public DialogResult Result { get; set; }
     public string Field1 { get; set; }


 }

And on the form:

public static ResultFromFrmMain Execute() {
     using (var f = new frmMain()) {
          var result = new ResultFromFrmMain();
          result.Result = f.ShowDialog();
          if (result.Result == DialogResult.OK) {
             // fill other values
          }
          return result;
     }
}

To call your form;

public void MyEventToCallForm() {
   var result = frmMain.Execute();
   if (result.Result == DialogResult.OK) {
       myTextBox.Text = result.Field1; // or something like that
   }
}