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Debug at design time in Visual Studio (C#, C++/CLI, Visual Basic, F#)

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To debug code at design time instead of while an app is running, you can use the Immediate window.

To debug XAML code behind an app from the XAML designer, such as declarative data binding scenarios, you can use Debug > Attach to Process.

Use the Immediate window

You can use the Visual Studio Immediate window to execute a function or subroutine without running your app. If the function or subroutine contains a breakpoint, Visual Studio will break at the breakpoint. You can then use the debugger windows to examine your program state. This feature is called debugging at design time.

The following example is in Visual Basic. You can also use the Immediate window at design time in C#, F#, and C++/CLI apps. For C++/CLI, compile without the /clr option to use the Immediate window.

  1. Paste the following code into a blank Visual Basic console app:

    Module Module1

    Sub Main()
        MySub()
    End Sub

    Function MyFunction() As Decimal
        Static i As Integer
        i = i + 1
        Return i
    End Function

    Sub MySub()
        MyFunction()

    End Sub
End Module

  2. Set a breakpoint on the line End Function.

  3. Open the Immediate window by selecting Debug > Windows > Immediate. Type ?MyFunction in the window, and then press Enter.

    The breakpoint is hit, and the value of MyFunction in the Locals window is 1. You can examine the call stack and other debugging windows while the app is in break mode.

  4. Select Continue on the Visual Studio toolbar. The app ends, and 1 is returned in the Immediate window. Make sure you are still in design mode.

  5. Type ?MyFunction in the Immediate window again, and press Enter. The breakpoint is hit, and the value of MyFunction in the Locals window is 2.

  6. Without selecting Continue, type ?MySub() in the Immediate window, and then press Enter. The breakpoint is hit, and the value of MyFunction in the Locals window is 3. You can examine the app state while the app is in break mode.

  7. Select Continue. The breakpoint is hit again, and the value of MyFunction in the Locals window is now 2. The Immediate window returns Expression has been evaluated and has no value.

  8. Select Continue again. The app ends, and 2 is returned in the Immediate window. Make sure that you are still in design mode.

  9. To clear the contents of the Immediate window, right-click in the window and select Clear All.

Debug a custom XAML control at design time by attaching to XAML designer

  1. Open your solution or project in Visual Studio.

  2. Build the solution/project.

  3. Open the XAML page containing the custom control that you want to debug.

    For UWP projects targeting Windows build 16299 or above, this step will start the UwpSurface.exe process. For WPF projects targeting Windows build 16299 or above, this step will start the WpfSurface.exe process. For WPF or UWP versions prior to Windows build 16299, this step will start the XDesProc.exe process.

  4. Open a second instance of Visual Studio. Do not open a solution or project in the second instance.

  5. In the second instance of Visual Studio, open the Debug menu and choose Attach to Process….

  6. Depending on your project type (see preceding steps), select the UwpSurface.exe, WpfSurface.exe, or the XDesProc.exe process from the list of available processes.

  7. In the Attach to field of the Attach to Process dialog, choose the correct code type for the custom control you want to debug.

    If your custom control has been written in a .NET language, choose the appropriate .NET code type such as Managed (CoreCLR). If your custom control has been written in C++, choose Native.

  8. Attach the second instance of Visual Studio by clicking the Attach button.

  9. In the second instance of Visual Studio, open the code files associated with the custom control you want to debug. Make sure to just open the files, not the entire solution or project.

  10. Place the necessary breakpoints in the previously opened files.

  11. In the first instance of Visual Studio, close the XAML page containing the custom control you want to debug (the same page you opened in earlier steps).

  12. In the first instance of Visual Studio, open the XAML page you closed in the previous step. This will cause the debugger to stop at the first breakpoint you set in the second instance of Visual Studio.

  13. Debug the code in the second instance of Visual Studio.

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