Walkthrough: Debug Custom Windows Forms Controls at Design Time

When you create a custom control, you will often find it necessary to debug its design-time behavior. This is especially true if you are authoring a custom designer for your custom control. For details, see Walkthrough: Creating a Windows Forms Control That Takes Advantage of Visual Studio Design-Time Features.

You can debug your custom controls using Visual Studio, just as you would debug any other .NET Framework classes. The difference is that you will debug a separate instance of Visual Studio that is running your custom control's code.

Create the project

The first step is to create the application project. You will use this project to build the application that hosts the custom control.

In Visual Studio, create a Windows Application project, and name it DebuggingExample.

Create the control library project

  1. Add a Windows Control Library project to the solution.

  2. Add a new UserControl item to the DebugControlLibrary project. Name it DebugControl.

  3. In Solution Explorer, delete the project's default control by deleting the code file with a base name of UserControl1.

  4. Build the solution.


At this point, you will be able to see your custom control in the Toolbox.

To check your progress, find the new tab called DebugControlLibrary Components and click to select it. When it opens, you will see your control listed as DebugControl with the default icon beside it.

Add a property to your custom control

To demonstrate that your custom control's code is running at design-time, you will add a property and set a breakpoint in the code that implements the property.

  1. Open DebugControl in the Code Editor. Add the following code to the class definition:

    Private demoStringValue As String = Nothing
    Public Property DemoString() As String
            Return Me.demoStringValue
        End Get
        Set(ByVal value As String)
            Me.demoStringValue = value
        End Set
    End Property
    private string demoStringValue = null;
    public string DemoString
            return this.demoStringValue;
            demoStringValue = value;
  2. Build the solution.

Add your custom control to the host form

To debug the design-time behavior of your custom control, you will place an instance of the custom control class on a host form.

  1. In the "DebuggingExample" project, open Form1 in the Windows Forms Designer.

  2. In the Toolbox, open the DebugControlLibrary Components tab and drag a DebugControl instance onto the form.

  3. Find the DemoString custom property in the Properties window. Note that you can change its value as you would any other property. Also note that when the DemoString property is selected, the property's description string appears at the bottom of the Properties window.

Set up the project for design-time debugging

To debug your custom control's design-time behavior, you will debug a separate instance of Visual Studio that is running your custom control's code.

  1. Right-click on the DebugControlLibrary project in the Solution Explorer and select Properties.

  2. In the DebugControlLibrary property sheet, select the Debug tab.

    In the Start Action section, select Start external program. You will be debugging a separate instance of Visual Studio, so click the ellipsis (The Ellipsis button (...) in the Properties window of Visual Studio) button to browse for the Visual Studio IDE. The name of the executable file is devenv.exe, and if you installed to the default location, its path is %ProgramFiles(x86)%\Microsoft Visual Studio\2019\<edition>\Common7\IDE.

  3. Select OK to close the dialog box.

  4. Right-click the DebugControlLibrary project and select Set as StartUp Project to enable this debugging configuration.

Debug your custom control at design time

Now you are ready to debug your custom control as it runs in design mode. When you start the debugging session, a new instance of Visual Studio will be created, and you will use it to load the "DebuggingExample" solution. When you open Form1 in the Forms Designer, an instance of your custom control will be created and will start running.

  1. Open the DebugControl source file in the Code Editor and place a breakpoint on the Set accessor of the DemoString property.

  2. Press F5 to start the debugging session. A new instance of Visual Studio is created. You can distinguish between the instances in two ways:

    • The debugging instance has the word Running in its title bar

    • The debugging instance has the Start button on its Debug toolbar disabled

    Your breakpoint is set in the debugging instance.

  3. In the new instance of Visual Studio, open the "DebuggingExample" solution. You can easily find the solution by selecting Recent Projects from the File menu. The "DebuggingExample.sln" solution file will be listed as the most recently used file.

  4. Open Form1 in the Forms Designer and select the DebugControl control.

  5. Change the value of the DemoString property. When you commit the change, the debugging instance of Visual Studio acquires focus and execution stops at your breakpoint. You can single-step through the property accessor just as your would any other code.

  6. To stop debugging, exit the hosted instance of Visual Studio or select the Stop Debugging button in the debugging instance.

Next steps

Now that you can debug your custom controls at design time, there are many possibilities for expanding your control's interaction with the Visual Studio IDE.

See also