Debug live ASP.NET Azure apps using the Snapshot Debugger

The Snapshot Debugger takes a snapshot of your in-production apps when code that you're interested in executes. To instruct the debugger to take a snapshot, you set snappoints and logpoints in your code. The debugger lets you see exactly what went wrong, without impacting traffic of your production application. The Snapshot Debugger can help you dramatically reduce the time it takes to resolve issues that occur in production environments.

Snappoints and logpoints are similar to breakpoints, but unlike breakpoints, snappoints don't halt the application when hit. Typically, capturing a snapshot at a snappoint takes 10-20 milliseconds.

In this tutorial, you will:

  • Start the Snapshot Debugger
  • Set a snappoint and view a snapshot
  • Set a logpoint

Prerequisites

  • Snapshot Debugger is only available for Visual Studio 2017 Enterprise version 15.5 or higher with the ASP.NET and web development workload. For ASP.NET Core, you also need the .NET Core development workload installed.

    If it's not already installed, install Visual Studio 2017 Enterprise version 15.5 or later. If you are updating from a previous Visual Studio 2017 installation, run the Visual Studio Installer and check the Snapshot Debugger component in the ASP.NET and web development workload.

  • Basic or higher Azure App Service plan.

  • Snapshot collection is available for the following web apps running in Azure App Service:

    • ASP.NET applications running on .NET Framework 4.6.1 or later.
    • ASP.NET Core applications running on .NET Core 2.0 or later on Windows.

Open your project and start the Snapshot Debugger

  1. Open the project you would like to snapshot debug.

    Important

    To snapshot debug, you need to open the same version of source code that is published to your Azure App Service.

  2. In the Cloud Explorer (View > Cloud Explorer), right-click the Azure App Service your project is deployed to and select Attach Snapshot Debugger.

    Launch the snapshot debugger

    The first time you select Attach Snapshot Debugger, you're prompted to install the Snapshot Debugger site extension on your Azure App Service. This installation requires a restart of your Azure App Service.

    Visual Studio is now in snapshot debugging mode.

    Note

    The Application Insights site extension also supports Snapshot Debugging. If you encounter a "site extension out of date" error message, see troubleshooting tips and known issues for snapshot debugging for upgrading details.

    Snapshot debugging mode

    The Modules window shows you when all the modules have loaded for the Azure App Service (choose Debug / Windows / Modules to open this window).

    Check the Modules window

Set a snappoint

  1. In the code editor, click the left gutter next to a line of code you are interested in to set a snappoint. Make sure it is code that you know will execute.

    Set a snappoint

  2. Click Start Collection to turn on the snappoint.

    Turn on the snappoint

    Tip

    You can't step when viewing a snapshot, but you can place multiple snappoints in your code to follow execution at different lines of code. If you have multiple snappoints in your code, the Snapshot Debugger makes sure that the corresponding snapshots are from the same end-user session. The Snapshot Debugger does this even if there are many users hitting your app.

Take a snapshot

When a snappoint is turned on, it will capture a snapshot whenever the line of code where the snappoint is placed executes. This execution can be caused by a real request on your server. To force your snappoint to hit, go to the browser view of your web site and take any actions required that cause your snappoint to be hit.

Inspect snapshot data

  1. When the snappoint is hit, a snapshot appears in the Diagnostic Tools window. To open this window, choose Debug / Windows / Show Diagnostic Tools.

    Open a snappoint

  2. Double-click the snappoint to open the snapshot in the code editor.

    Inspect snapshot data

    From this view, you can hover over variables to view DataTips, use the Locals, Watches, and Call Stack windows, and also evaluate expressions.

    The website itself is still live and end users aren't impacted. Only one snapshot is captured per snappoint by default: after a snapshot is captured the snappoint turns off. If you want to capture another snapshot at the snappoint, you can turn the snappoint back on by clicking Update Collection.

You can also add more snappoints to your app and turn them on with the Update Collection button.

Need help? See the Troubleshooting and known issues and FAQ for snapshot debugging pages.

Set a conditional snappoint

If it is difficult to recreate a particular state in your app, consider whether the use of a conditional snappoint can help. Conditional snappoints help you avoid taking a snapshot until the app enters a desired state, such as when a variable has a particular value that you want to inspect. You can set conditions using expressions, filters, or hit counts.

To create a conditional snappoint

  1. Right-click a snappoint icon (the hollow ball) and choose Settings.

    Choose Settings

  2. In the snappoint settings window, type an expression.

    Type an expression

    In the preceding illustration, the snapshot is only taken for the snappoint when visitor.FirstName == "Dan".

Set a logpoint

In addition to taking a snapshot when a snappoint is hit, you can also configure a snappoint to log a message (that is, create a logpoint). You can set logpoints without having to redeploy your app. Logpoints are executed virtually and cause no impact or side effects to your running application.

To create a logpoint

  1. Right-click a snappoint icon (the blue hexagon) and choose Settings.

  2. In the snappoint settings window, select Actions.

    Create a logpoint

  3. In the Message field, you can enter the new log message you want to log. You can also evaluate variables in your log message by placing them inside curly braces.

    If you choose Send to Output Window, when the logpoint is hit, the message appears in the Diagnostic Tools window.

    Logpoint data in the diagsession window

    If you choose Send to application log, when the logpoint is hit, the message appears anywhere that you can see messages from System.Diagnostics.Trace (or ILogger in .NET Core), such as App Insights.

Next steps

In this tutorial, you've learned how to use the Snapshot Debugger. You may want to read more details about this feature.