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It’s always nice to be able to debug through your code when something is not working as expected.

This is especially true with Lambda functions because once they are deployed you have to rely on logs to see what your code is doing.

Luckily, there is an easy way to debug your Lambda functions locally - you can step through code, hit breakpoints, add watches, look at variables, everything you would expect.

1. Install the tools

Install the .NET Lambda function templates.

dotnet new --install Amazon.Lambda.Templates

Install the Lambda Test Tool

dotnet tool install -g Amazon.Lambda.TestTool-6.0

2. Create a Lambda function

dotnet new lambda.EmptyFunction -n SimpleFunctionVSCode

Open Function.cs and replace the FunctionHandler(..) code with the following -

public string FunctionHandler(string input, ILambdaContext context)
{
    Console.WriteLine("I am able to debug!");
    return input.ToUpper();
}

3. Update the launch configuration

This is where things differ for Windows, Linux, and Mac.

Windows

Open the launch.json file inside the .vscode directory, and replace the contents with the below -

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": ".NET Core Launch (console)",
            "type": "coreclr",
            "request": "launch",
            "preLaunchTask": "build",
            "program": "${env:USERPROFILE}/.dotnet/tools/dotnet-lambda-test-tool-6.0.exe",
            "args": [],
            "cwd": "${workspaceFolder}",
            "console": "internalConsole",
            "stopAtEntry": false,
            "internalConsoleOptions": "openOnSessionStart"
        }
    ]
}

Note on line 9, I use ${env:userprofile}, this works for Windows, but won’t for Linux or Mac. Also, note on line 9, that the program ends with .exe, that won’t be the case for Linux or Mac.

Linux and Mac

Open the launch.json file inside the .vscode directory, and replace the contents with the below -

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": ".NET Core Launch (console)",
            "type": "coreclr",
            "request": "launch",
            "preLaunchTask": "build",
            "program": "${env:HOME}/.dotnet/tools/dotnet-lambda-test-tool-6.0",
            "args": [],
            "cwd": "${workspaceFolder}",
            "console": "internalConsole",
            "stopAtEntry": false,
            "internalConsoleOptions": "openOnSessionStart"
        }
    ]
}

Note on line 9, I use ${env:HOME}, and program does not have a .exe at the end.

4. Debugging

Back in the Function.cs file, put a break point on the line -

Console.WriteLine("I am able to debug!");

Hit F5 to start debugging. Somewhere in the output of the debug console it will say -

Environment running at http://localhost:5050

A browser window will open (if it doesn’t open http://localhost:5050/), and you should see something like this -

Put some text in quotes into the Function input, and click Execute.

The debugger will kick in and hit the breakpoint you set.

You can step through the code now.

After it has returned, you will see something like this in the browser with the response and log -

Download full source code.