do.. while checkbox is checked??

Question

Hello first off im a real novice when it comes down to programming so dont wonder if my approach is totally wrong. I have a CheckBox and want to implement a do while loop that will run as long as the checkbox stays checked. Once I uncheck the checkbox it should jump out of the loop and stop. Since I'm a beginner I really dont know how to approach this. I've tried the following but once in the loop it will hang and stay in the loop forever even if I uncheck the box and I'll have to shut down the process via task manager heres the snippet: private void checkBox1_CheckedChanged(object sender EventArgs e) { if (checkBox1.Checked) { do { textBox1.AppendText(On); } while (checkBox1.Checked); } else if (!checkBox1.Checked) { textBox1.AppendText(Off); } } The AppendText thing makes no sense but its just for testing purposes at the moment to see if I can get it working as I want. I would be glad if anyone could help me to solve this issue. Thanks.

Vulpes · Answer

Try this: private void checkBox1_CheckedChanged(object sender EventArgs e) { if (checkBox1.Checked) { do { textBox1.AppendText(On); Application.DoEvents(); } while (checkBox1.Checked); } else if (!checkBox1.Checked) { textBox1.AppendText(Off); } }

Sam Hobbs · Answer

Sure John; we know there is so much to learn it is easy to miss a lot. You and Vulpes have a lot to offer and have provided plenty of support to others.

John Penn · Answer

I already forgot about BackgroundWorker! Thank you Sam for re-educating me. :)

Sam Hobbs · Answer

The BackgroundWorker class is designed for the exact purpose you need it for here. There are already excellent articles in this web site explaining how to use it. It is used much like System.Threading.Thread as John shows but BackgroundWorker is customized for use with a UI as you are doing. It is definitely worth learning.

Vulpes · Answer

Application.DoEvents() works by suspending the UI thread and allowing any pending windows messages to be processed before resuming the thread from where it was called. So in this case it will process the message that the CheckBox has been unchecked and will call the CheckBox_CheckedChanged eventhandler again appending off to the textbox. It will then resume the original call to CheckBox_CheckedChanged and terminate the while loop because the checkbox is no longer checked. In contrast John's code creates a new thread and appends to the textbox from there so that it doesn't interfere with message processing on the UI thread. However because you can only change controls from the thread on which they were created John needs to marshal the calls to textBox1.AppendText back to the UI thread. As John's figures indicate the threading solution will be slower but Application.DoEvents() is more CPU intensive because it uses some very fancy code under the hood to do what it does.

John Penn · Answer

In my dev environment the DoEvents() solution ran at about 12% CPU usage whereas the threading solution ran at 3% but it's not updating the textbox as quickly so there is a tradeoff there. YMMV.

Dan Ho · Answer

Interesting I'll try these out. Do these solutions have any impact on CPU usage? Will it require more usage? It looks indeed more complicated at least code-wise.

Pradeep Chandraker · Answer

Nice command. Thanks @Vulpes.

John Penn · Answer

Here's a different implementation of TextAppender that's cleaner: private void TextAppender() { MethodInvoker methodInvoker = new MethodInvoker(delegate { textBox1.AppendText(On); }); do { if (textBox1.InvokeRequired) { textBox1.Invoke(methodInvoker); } System.Threading.Thread.Sleep(1); // sleep so that UI can respond to clicks } while (checkBox1.Checked); }

John Penn · Answer

This works but it's a little more complicated. public partial class Form1 : Form { private System.Threading.Thread t = null; public Form1() { InitializeComponent(); } private void checkBox1_CheckedChanged(object sender EventArgs e) { if (checkBox1.Checked) { if (t == null) { t = new System.Threading.Thread(TextAppender); t.Start(); } } else { t.Join(); // wait for TextAppender to finish its current iteration t = null; textBox1.AppendText(Off); } } private void TextAppender() { do { if (textBox1.InvokeRequired) { textBox1.Invoke(new MethodInvoker(delegate { textBox1.AppendText(On); })); } System.Threading.Thread.Sleep(1); // sleep so that UI can respond to clicks } while (checkBox1.Checked); } }

Dan Ho · Answer

@ Vulpes thanks a lot it worked. Amazing what a simple command can achieve. @ John Penn I thought about this aswell and this is most likely an alternative solution opposed to the one from Vulpes. I'll give it a try aswell. Thanks for your replies guys.

John Penn · Answer

Sorry i should have read your code more carefully. You have to move the loop outside of the event handler. The reason is that when you check the box for the 1st time you enter the event handler and start the loop but you never exit the handler. This means that the checkbox can't respond to the next click to uncheck the box.

Dan Ho · Answer

thanks for your reply. Unfortunately it didnt change anything apart from the fact that the text output is slower now. I am still unable to uncheck the box even during the period of Thread Sleep.

John Penn · Answer

Your loop is taking up all of the CPU time and is making the UI unresponsive. Add the following line at the end of the while loop: System . Threading . Thread . Sleep (10);