# Debug a .NET for Apache Spark application

This how-to provides the steps to debug your .NET for Apache Spark application on Windows.

Warning

.NET for Apache Spark targets an out of support version of .NET (.NET Core 3.1). For more details see the .NET Support Policy.

Open a new command prompt window and run the following command:

spark-submit \
--class org.apache.spark.deploy.dotnet.DotnetRunner \
--master local \
<path-to-microsoft-spark-jar> \
debug


When you run the command, you see the following output:

***********************************************************************
* .NET Backend running debug mode. Press enter to exit *
***********************************************************************


In debug mode, DotnetRunner does not launch the .NET application, but instead waits for you to start the .NET app. Leave this command prompt window open and start your .NET application through C# debugger to debug your application. Start your .NET application with a C# debugger (Visual Studio Debugger for Windows/macOS or C# Debugger Extension in Visual Studio Code) to debug your application.

## Debug a user-defined function (UDF)

Note

User-defined functions are supported only on Windows with Visual Studio Debugger.

Before running spark-submit, set the following environment variable:

set DOTNET_WORKER_DEBUG=1


When you run your Spark application, a Choose Just-In-Time Debugger window will pop up. Choose a Visual Studio debugger.

The debugger will break at the following location in TaskRunner.cs:

if (EnvironmentUtils.GetEnvironmentVariableAsBool("DOTNET_WORKER_DEBUG"))
{
Debugger.Launch(); // <-- The debugger will break here.
}


Navigate to the .cs file that contains the UDF that you plan to debug, and set a breakpoint. The breakpoint will say The breakpoint will not currently be hit because the worker hasn't loaded the assembly that contains UDF yet.

Hit F5 to continue your application and the breakpoint will eventually be hit.

Note

The Choose Just-In-Time Debugger window pops up for each task. To avoid excessive pop-ups, set the number of executors to a low number. For example, you can use the --master local[1] option for spark-submit to set the number of tasks to 1, which launches a single debugger instance.

## Debug Scala code

If you need to debug the Scala-side code (DotnetRunner, DotnetBackendHandler, etc.), you can use the following command and attach a debugger to the running process using IntelliJ:

spark-submit \