Updating the UI from a Secondary Thread

I am probably missing something here, but I suspect that the problem is
your background thread is calling a delegate in your splash screen that
attempts to update the label in the same thread as the delegate was
called on.

You cannot (well, I guess technically you can, but it can get ugly)
update any forms elements except on the thread that they were created
on. Instead, in the recipient of the call to update the label (in your
splash screen) check to see if InvokeRequired is true, and then use
invoke on that control to have it execute on the proper thread.

John

BG wrote:

We're having trouble writing the code to update a UI control (label.Text)
from a secondary thread. We're using C# with Windows Forms.

We have a main form named MainForm, a splash screen form named SplashScreen,
and a C# class library named BackgroundProce ss.

On application start, we simply want to show the splash screen form, kick
off the background processing on a separate thread, and, at different points
in the background processing, update a label on the splash screen to inform
the user of what the application is doing. When the processing is done, the
splash screen form goes away and the main form appears.

In the main form's form load event we have:

private void MainForm_Load(o bject sender, System.EventArg s e)

{

// Create and show the splash screen

SplashScreen frmSS = new SplashScreen();

frmSS.Show();

// Create the object for background processing and set an initial
property

BackgroundProce ss bp = new BackgroundProce ss();

bp.StartingDire ctory = @"C:\";

// Create a new thread for the background processing and start it

System.Threadin g.Thread activeThread = new System.Threadin g.Thread(new
System.Threadin g.ThreadStart(b p.StartProcess) );

activeThread.St art();

while (activeThread.I sAlive)

{

Application.DoE vents();

}

// Clean up

activeThread = null;

bp = null;

frmSS.Close();

frmSS = null;

}

This code works, and the code in our class library (i.e., the background
processing code) works, but we still can't get the splash screen's label to
update.

We've trying using delegates/events, etc. and have not been successful in
getting the label updated from the secondary thread that's doing all the
"behind-the-scenes" processing.

Any suggestions?

Thanks.

Here is one way that loads the spash form and kicks the worker and waits
till close, then starts the main form using Application.Run (new MyForm());
as normal. The worker calls private methods in the form that make sure the
calls are done on the forms thread. Using this pattern, you can update any
controls on your form.

using System;
using System.Drawing;
using System.Collecti ons;
using System.Componen tModel;
using System.Windows. Forms;
using System.Threadin g;

namespace CSharpExample.M yApp
{
/// <summary>
/// Summary description for SpashForm.
/// </summary>
public class SplashForm : System.Windows. Forms.Form
{
private System.Windows. Forms.Label label1;
private System.Windows. Forms.Label lblText;
private System.Windows. Forms.Button btnClose;
private System.Windows. Forms.ProgressB ar progressBar1;

/// <summary>
/// Required designer variable.
/// </summary>
private System.Componen tModel.Containe r components = null;

public SplashForm()
{
//
// Required for Windows Form Designer support
//
InitializeCompo nent();

//
// TODO: Add any constructor code after InitializeCompo nent call
//
}

[STAThread]
static void Main()
{
SplashForm splashForm = new SplashForm();
splashForm.Show Dialog();
splashForm.Dial ogResult = DialogResult.OK ;
if ( splashForm.Dial ogResult == DialogResult.Ca ncel )
return;
Application.Run (new MyForm());
}

/// <summary>
/// Clean up any resources being used.
/// </summary>
protected override void Dispose( bool disposing )
{
if( disposing )
{
if(components != null)
{
components.Disp ose();
}
}
base.Dispose( disposing );
}

#region Windows Form Designer generated code
/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
private void InitializeCompo nent()
{
this.label1 = new System.Windows. Forms.Label();
this.lblText = new System.Windows. Forms.Label();
this.btnClose = new System.Windows. Forms.Button();
this.progressBa r1 = new System.Windows. Forms.ProgressB ar();
this.SuspendLay out();
//
// label1
//
this.label1.Fon t = new System.Drawing. Font("Microsoft Sans Serif", 9.75F,
System.Drawing. FontStyle.Bold, System.Drawing. GraphicsUnit.Po int,
((System.Byte)( 0)));
this.label1.Loc ation = new System.Drawing. Point(16, 16);
this.label1.Nam e = "label1";
this.label1.Siz e = new System.Drawing. Size(264, 23);
this.label1.Tab Index = 0;
this.label1.Tex t = "Welcome";
//
// lblText
//
this.lblText.Lo cation = new System.Drawing. Point(16, 56);
this.lblText.Na me = "lblText";
this.lblText.Si ze = new System.Drawing. Size(264, 23);
this.lblText.Ta bIndex = 1;
this.lblText.Te xt = "Loading XYZ...";
//
// btnClose
//
this.btnClose.L ocation = new System.Drawing. Point(208, 128);
this.btnClose.N ame = "btnClose";
this.btnClose.T abIndex = 2;
this.btnClose.T ext = "Cancel";
this.btnClose.C lick += new System.EventHan dler(this.btnCl ose_Click);
//
// progressBar1
//
this.progressBa r1.Location = new System.Drawing. Point(16, 96);
this.progressBa r1.Name = "progressBa r1";
this.progressBa r1.Size = new System.Drawing. Size(264, 23);
this.progressBa r1.Step = 1;
this.progressBa r1.TabIndex = 3;
//
// SplashForm
//
this.AutoScaleB aseSize = new System.Drawing. Size(5, 13);
this.ClientSize = new System.Drawing. Size(292, 159);
this.Controls.A dd(this.progres sBar1);
this.Controls.A dd(this.btnClos e);
this.Controls.A dd(this.lblText );
this.Controls.A dd(this.label1) ;
this.FormBorder Style = System.Windows. Forms.FormBorde rStyle.FixedDia log;
this.Name = "SplashForm ";
this.Text = "Loading MyApp...";
this.Load += new System.EventHan dler(this.Spash Form_Load);
this.ResumeLayo ut(false);

}
#endregion

private void SpashForm_Load( object sender, System.EventArg s e)
{
Thread t = new Thread(new ThreadStart(Wor kerStart));
t.IsBackground = true;
t.Start();
}

private delegate void UpdatePBarDeleg ate(int step);
private void UpdatePBar(int step)
{
if ( InvokeRequired )
{
UpdatePBarDeleg ate ud = new UpdatePBarDeleg ate(UpdatePBar) ;
Invoke(ud, new object[]{step});
}
else
{
this.progressBa r1.Value = this.progressBa r1.Value + step;
}
}

private void Done()
{
if ( InvokeRequired )
{
Invoke(new MethodInvoker(D one));
}
else
{
this.lblText.Te xt = "Done";
this.Close();
}
}

private void WorkerStart()
{
for(int i = 0; i < 100; i++)
{
this.UpdatePBar (1); // Update progress bar 1 step.
Thread.Sleep(50 ); // Fake some delay - your processing goes here.
}
this.Done(); // Do any other updates on the splash form and load main and
quit.
}

private void btnClose_Click( object sender, System.EventArg s e)
{
this.DialogResu lt = DialogResult.Ca ncel;
//this.Close();
}
}
}

--
William Stacey
MVP Directory Services
http://mvp.support.microsoft.com