C# BackgroundWorkerUse BackgroundWorker from Windows Forms and call the RunWorkerAsync method.
BackgroundWorker. This makes threads easy to implement in Windows Forms. Intensive tasks need to be done on another thread so the UI does not freeze. It is necessary to post messages and update the user interface when the task is done.
DoWork. The DoWork method is like any other event handler. Here we must look at the C# view of your file, where we will see the DoWork method. You should see that the backgroundWorker1_DoWork event is generated when you double-click on DoWork.
First Click on BackgroundWorker. Look at the gray bar near the bottom of your window. A BackgroundWorker will appear there.
Second Highlight backgroundWorker1. Click on the backgroundWorker1 item in the gray bar on the bottom. Now, look at the Properties panel.
Third Look for the lightning bolt. You will see a lightning bolt icon in the Properties pane. Then, double-click on DoWork.
Note The Thread.Sleep method will pause the execution of the BackgroundWorker. It will show us threading is functioning.
Properties. I want to emphasize the Argument, Result, and the RunWorkerAsync methods. These are the properties of BackgroundWorker required to accomplish anything. I show the properties as you would reference them in your code.
Example that shows DoWork event handler, C#:
using System; using System.ComponentModel; using System.Windows.Forms; using System.Threading; namespace WindowsFormsApplication1 { public partial class Form1 : Form { public Form1() { InitializeComponent(); } private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e) { Thread.Sleep(1000); // One second. } } }
RunWorkerAsync. We can add instructions to the BackgroundWorker by adding arguments and return values. We need to add arguments, invoke the BackgroundWorker, and then receive the results of the thread.
Tip Changing variables from multiple threads at once often leads to bugs. A variable can become invalid.
TestObject An example argument is created. You will have something more important and complex in your program.
RunWorkerAsync This can be called anywhere in your code. We use a constructor, but that isn't important.
BackgroundWorker property list:
DoWorkEventArgs e Contains e.Argument and e.Result. It is used to access those properties. e.Argument Used to get the parameter reference received by RunWorkerAsync. e.Result Check to see what the BackgroundWorker processing did. backgroundWorker1.RunWorkerAsync Called to start a process on the worker thread.
DoWork 2. You can implement DoWork by putting your expensive code in it. Then, use the DoWorkEventArgs in its body and as its result. Here we hook up the arguments and results from the RunWorkerAsync call.
Note The TestObject was passed to RunWorkerAsync, and that is received as e.Argument. We also have to cast.
Example that shows RunWorkerAsync call, C#:
public partial class Form1 : Form { public Form1() { InitializeComponent(); // // Example argument object // TestObject test = new TestObject { OneValue = 5, TwoValue = 4 }; // // Send argument to our worker thread // backgroundWorker1.RunWorkerAsync(test); } } /// <summary> /// The test class for our example. /// </summary> class TestObject { public int OneValue { get; set; } public int TwoValue { get; set; } }
RunWorkerCompleted. You can use the RunWorkerCompleted event handler by going to the lightning bolt in the designer and clicking on the backgroundWorker1 icon in the tray. Now double-click in RunWorkerCompleted.
Next You will get some autogenerated code that looks just like this. Put the argument receiving code in this method.
Example that implements DoWork, C#:
/// <summary> /// Where we do the work in the program (the expensive slow stuff). /// </summary> private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e) { // // e.Argument always contains whatever was sent to the background worker // in RunWorkerAsync. We can simply cast it to its original type. // TestObject argumentTest = e.Argument as TestObject; // // Boring.... // Thread.Sleep(10000); argumentTest.OneValue = 6; argumentTest.TwoValue = 3; // // Now, return the values we generated in this method. // ... Use e.Result. // e.Result = argumentTest; }
Discussion. You probably know more than you think about the BackgroundWorker class. BackgroundWorker has a name that might indicate it is more complex than it really is. There are many more details about threading and abort calls.
However Once you understand that BackgroundWorker is just a structural overlay to threads in Windows Forms, it is intuitive.
Example that shows RunWorkerCompleted event handler, C#:
/// <summary> /// This is on the main thread, so we can update a TextBox or anything. /// </summary> private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e) { // // Receive the result from DoWork, and display it. // TestObject test = e.Result as TestObject; this.Text = test.OneValue.ToString() + " " + test.TwoValue.ToString(); // // Will display "6 3" in title Text (in this example) // }
Review, steps. Call RunWorkerAsync with an argument. You can pass any argument to this method on BackgroundWorker, including null. It simply must inherit from object, which everything does.
Second Custom processing is run. Your expensive code is executed in the DoWork method. Insert pause here as your program works.
Third It finishes. When your processing is done, RunWorkerCompleted is called. In this method, you receive the result.
ProgressBar. A ProgressBar visually displays progress. And the ProgressBar control in Windows Forms can be used with a BackgroundWorker. This site has a detailed tutorial on how to use these controls together.
Also The WPF ProgressBar is covered. It too can be used with a BackgroundWorker control.
Summary. We implemented the code for a BackgroundWorker control. We generally instead prefer ThreadPool when we need many threads. And threads are not always useful. IO operations (for example) cannot always be multithreaded well.
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