Re: Passing Variable to Parent form - C# Discussion Boards

11-May-08 23:14

Hi, i have a login form as a child form.
I want to pass the value txtUsername to the parent form. How do i do it?

Christian Graus11-May-08 23:17

11-May-08 23:17

There's a great article on this here on CP. Short answer - a delegate.

Christian Graus

Please read this if you don't understand the answer I've given you

"also I don't think "TranslateOneToTwoBillion OneHundredAndFortySevenMillion FourHundredAndEightyThreeThousand SixHundredAndFortySeven()" is a very good choice for a function name" - SpacixOne ( offering help to someone who really needed it ) ( spaces added for the benefit of people running at < 1280x1024 )

benjamin yap11-May-08 23:33

11-May-08 23:33

i seen the article on delegate here http://www.codeproject.com/KB/cs/event_fundamentals.aspx#3.Delegates2[^

But i dont quite understand how it works. can u give me a simple example?

Christian Graus11-May-08 23:54

11-May-08 23:54

If the child form contains this code

public delegate void SendString(string s);
public SendString OnSendString = null;

Then your main form can do this

public void GetString(string theString)
{

}

and in your code that creates the child form:

MyChild dlg = new MyChild();
dlg.OnSendString += new MyChild.SendString(this.GetString);

Then when the child form does this:

if (OnSendString != null) OnSendString("test");

the value "test" is called in GetString in the parent form.

All of this assumes your child form is modeless. If it's modal, then you may as well just assign properties like the other guy said. I assumed that it was modeless from your initial question, you said you want to send something back to the main form. But, if the child form has been closed, then you're not sending anything, properties make more sense in that case.

I can't find the article on communication between forms Frown | :(

benjamin yap12-May-08 0:02

12-May-08 0:02

How do i know if my file is modeless or modal?
I did not close my child form. I use .Hide().

I dont know whether i should close the form or use Hide.

probably u might wanna take a look at my file
here is my project file http://opencube.com.sg/EBMS.zip

Christian Graus12-May-08 0:04

12-May-08 0:04

If you call ShowDialog, it's modal. IF you call Show, so the parent form stays active, it's modeless, and that's why you need a delegate. If your main form code stops when you show the child, your main form code knows when the otehr form ends it's life, so you can examine it's state. If you called Show, you don't know when the child form has something to tell you, a delegate lets the form tell you.

If you call Hide, then you're calling Show, I guess. Otherwise, your modal form would freeze your application.

A login form has to be modal, or there's no point. A properties form, would be modeless, you change properties and see the results in the main form. Just some examples.

benjamin yap12-May-08 0:10

12-May-08 0:10

Oh, so inside my FrmMain_Load(), i tried set my childLogin to ShowDialog

i get this error
"Form that is not a top-level form cannot be displayed as a modal dialog box. Remove the form from any parent form before calling showDialog."

private void FrmMain_Load(object sender, EventArgs e)
{
this.toolStripStaff.Visible = false;
this.toolStripCatalogue.Visible = false;
this.toolStripProduct.Visible = false;
this.toolStripOrder.Visible = false;
this.toolStripInventory.Visible = false;

FrmLogin childLogin = new FrmLogin();
childLogin.MdiParent = this;
childLogin.ShowDialog();
stripLblLoginAs.Text = "Login As :";
}

natsuyaki12-May-08 0:13

12-May-08 0:13

childForm.ShowDialog(parentForm)

Christian Graus12-May-08 0:14

12-May-08 0:14

Yes, you can pass it on the constructor quite often, instead of setting it first. I know you can for a messagebox, for example.

Christian Graus12-May-08 0:13

12-May-08 0:13

benjamin yap wrote:
childLogin.MdiParent = this;

This is why you can't do it. Get rid of this line, it makes no sense to have it. Set te Owner property instead ( or Parent, I forget which, but I think it's Owner you want ). This stops your modal form from ever being hidden by it's parent.

benjamin yap wrote:
stripLblLoginAs.Text = "Login As :";

This is going to occur *after* your login form has shown.

Now the easy easy way to make this work is to set the DIalogResult to DialogResult.OK only if the login succeeds. Then you can do

if (childLogin.ShowDialog() == DialogResult.OK)
{
// logged in
}
else
{
// failed, close the program or whatever
}

benjamin yap12-May-08 0:25

12-May-08 0:25

Christian, i am very sorry for asking so many question, I just started c# few days ago. Please bear with me

childLogin.MdiParent = this; --> i though this is to link to the parent form?

How do i set to owner property?

Christian Graus12-May-08 0:44

12-May-08 0:44

This is a link in an MDI form, I've never used it and it apparently only works to create child forms within an MDI app. childLogin.Owner = this; will do what you want.

natsuyaki12-May-08 0:51

12-May-08 0:51

A dialog is not a MdiChild, so <ode>childLogin.MdiParent = this; can be ignored.
According to my experience:

<br />
childForm child=new childForm();<br />
child.ShowDialog(parent);<br />

works.
My code and I is not professional. take careful advantage.

benjamin yap12-May-08 1:57

12-May-08 1:57

hmm after i remove that line, and set to owner property, i get this error when i press the login button

else if ((txtUsername.Text == username) && (txtPassword.Text == password))
{
FrmMain frmMain = (FrmMain)this.MdiParent;
frmMain.toolStripStaff.Visible = true; <----ERROR HERE

this.Hide();
}

Object reference not set to an instance of an object.

natsuyaki12-May-08 2:09

12-May-08 2:09

FrmMain frmMain = (FrmMain)this.MdiParent
your login form doesn't have a parent, so the frmMain will be null when running.

try this:

in main form:

LoginForm login=new LoginForm();
login.ShowDialog(this);
if (login.DialogResult==DialogResult.OK)
{
this.toolStripStaff.Visible = true;
........

in login form:

else if ((txtUsername.Text == username) && (txtPassword.Text == password))
{
this.DialogResult=DialogResult.OK;
........

benjamin yap12-May-08 3:02

Re: Passing Variable to Parent form [modified]

12-May-08 3:02

Great! Thanks alot!

So how do i pass the value of the txtUsername inside my Login to my statusstrip at my mainform

stripLblLoginAs.Text = "Login As :" + login.txtUsername.Text;

natsuyaki12-May-08 3:54

12-May-08 3:54

Err, I see, txtUsername is private huh?
define a property:

<br />
public string Username<br />
{<br />
get{return txtUsername.Text;}<br />
}<br />

So, in main form:

<br />
stripLblLoginAs.Text = "Login As :" + login.Username;<br />

natsuyaki11-May-08 23:18

11-May-08 23:18

set the txtUsername as public, or just define a public variable in child form.

Christian Graus11-May-08 23:20