Developing ASP.NET Core applications using dotnet watch

By Victor Hurdugaci

Introduction

dotnet watch is a development time tool that runs a dotnet command when source files change. It can be used to compile, run tests, or publish when code changes.

In this tutorial we'll use an existing WebApi application that calculates the sum and product of two numbers to demonstrate the use cases of dotnet watch. The sample application contains an intentional bug that we'll fix as part of this tutorial.

Getting started

Start by downloading the sample application. It contains two projects, WebApp (a web application) and WebAppTests (unit tests for the web application)

In a console, open the folder where you downloaded the sample application and run:

  1. dotnet restore
  2. cd WebApp
  3. dotnet run

The console output will show messages similar to the ones below, indicating that the application is now running and waiting for requests:

$ dotnet run
Project WebApp (.NETCoreApp,Version=v1.0) will be compiled because inputs were modified
Compiling WebApp for .NETCoreApp,Version=v1.0

Compilation succeeded.
  0 Warning(s)
  0 Error(s)

Time elapsed 00:00:02.6049991

Hosting environment: Production
Content root path: /Users/user/dev/aspnet/Docs/aspnet/tutorials/dotnet-watch/sample/WebApp
Now listening on: http://localhost:5000
Application started. Press Ctrl+C to shut down.

In a web browser, navigate to http://localhost:5000/api/math/sum?a=4&b=5 and you should see the result 9.

If you navigate to http://localhost:5000/api/math/product?a=4&b=5 instead, you'd expect to get the result 20. Instead, you get 9 again.

We'll fix that.

Adding dotnet watch to a project

  1. Add Microsoft.DotNet.Watcher.Tools to the tools section of the WebApp/project.json file.
  2. Run dotnet restore.

The console output will show messages similar to the ones below:

log  : Restoring packages for /Users/user/dev/aspnet/Docs/aspnet/tutorials/dotnet-watch/sample/WebApp/project.json...
log  : Restoring packages for tool 'Microsoft.DotNet.Watcher.Tools' in /Users/user/dev/aspnet/Docs/aspnet/tutorials/dotnet-watch/sample/WebApp/project.json...
log  : Installing Microsoft.DotNet.Watcher.Core ...
log  : Installing Microsoft.DotNet.Watcher.Tools ...

Running dotnet commands using dotnet watch

Any dotnet command can be run with dotnet watch: For example:

Command Command with watch
dotnet run dotnet watch run
dotnet run -f net451 dotnet watch run -f net451
dotnet run -f net451 -- --arg1 dotnet watch run -f net451 -- --arg1
dotnet test dotnet watch test

To run WebApp using the watcher, run dotnet watch run in the WebApp folder. The console output will show messages similar to the ones below, indicating that dotnet watch is now watching code files:

user$ dotnet watch run
[DotNetWatcher] info: Running dotnet with the following arguments: run
[DotNetWatcher] info: dotnet process id: 39746
Project WebApp (.NETCoreApp,Version=v1.0) was previously compiled. Skipping compilation.
Hosting environment: Production
Content root path: /Users/user/dev/aspnet/Docs/aspnet/tutorials/dotnet-watch/sample/WebApp
Now listening on: http://localhost:5000
Application started. Press Ctrl+C to shut down.

Making changes with dotnet watch

Make sure dotnet watch is running.

Let's fix the bug in WebApp/Controllers/MathController.cs that we discovered when we tried to compute the product of two number:

public static int Product(int a, int b)
{
    // We have an intentional bug here
    // + should be *
    return a + b; 
}

Fix the code by replacing a + b with a * b.

Save the file. The console output will show messages similar to the ones below, indicating that dotnet watch detected a file change and restarted the application.

[DotNetWatcher] info: File changed: /Users/user/dev/aspnet/Docs/aspnet/tutorials/dotnet-watch/sample/WebApp/Controllers/MathController.cs
[DotNetWatcher] info: Running dotnet with the following arguments: run
[DotNetWatcher] info: dotnet process id: 39940
Project WebApp (.NETCoreApp,Version=v1.0) will be compiled because inputs were modified
Compiling WebApp for .NETCoreApp,Version=v1.0
Compilation succeeded.
  0 Warning(s)
  0 Error(s)
Time elapsed 00:00:03.3312829

Hosting environment: Production
Content root path: /Users/user/dev/aspnet/Docs/aspnet/tutorials/dotnet-watch/sample/WebApp
Now listening on: http://localhost:5000
Application started. Press Ctrl+C to shut down.

Verify http://localhost:5000/api/math/product?a=4&b=5 returns the correct result.

Running tests using dotnet watch

The file watcher can run other dotnet commands like test or publish.

  1. Open the WebAppTests folder that already has dotnet watch in project.json.

  2. Run dotnet watch test.

    If you previously fixed the bug in the MathController then you'll see an output similar to the one below, otherwise you'll see a test failure:

     WebAppTests user$ dotnet watch test
     [DotNetWatcher] info: Running dotnet with the following arguments: test
     [DotNetWatcher] info: dotnet process id: 40193
     Project WebApp (.NETCoreApp,Version=v1.0) was previously compiled. Skipping compilation.
     Project WebAppTests (.NETCoreApp,Version=v1.0) was previously compiled. Skipping compilation.
     xUnit.net .NET CLI test runner (64-bit .NET Core osx.10.11-x64)
       Discovering: WebAppTests
       Discovered:  WebAppTests
       Starting:    WebAppTests
       Finished:    WebAppTests
     === TEST EXECUTION SUMMARY ===
        WebAppTests  Total: 2, Errors: 0, Failed: 0, Skipped: 0, Time: 0.259s
     SUMMARY: Total: 1 targets, Passed: 1, Failed: 0.
     [DotNetWatcher] info: dotnet exit code: 0
     [DotNetWatcher] info: Waiting for a file to change before restarting dotnet...
    

    Once all the tests run, the watcher will indicate that it's waiting for a file to change before restarting dotnet test.

  3. Open the controller file in WebApp/Controllers/MathController.cs and change some code. If you haven't fixed the product bug, do it now. Save the file.

dotnet watch detects the file change and reruns the tests. The console output will show messages similar to the one below:

[DotNetWatcher] info: File changed: /Users/user/dev/aspnet/Docs/aspnet/tutorials/dotnet-watch/sample/WebApp/Controllers/MathController.cs
[DotNetWatcher] info: Running dotnet with the following arguments: test
[DotNetWatcher] info: dotnet process id: 40233
Project WebApp (.NETCoreApp,Version=v1.0) will be compiled because inputs were modified
Compiling WebApp for .NETCoreApp,Version=v1.0
Compilation succeeded.
  0 Warning(s)
  0 Error(s)
Time elapsed 00:00:03.2127590
Project WebAppTests (.NETCoreApp,Version=v1.0) will be compiled because dependencies changed
Compiling WebAppTests for .NETCoreApp,Version=v1.0
Compilation succeeded.
  0 Warning(s)
  0 Error(s)
Time elapsed 00:00:02.1204052

xUnit.net .NET CLI test runner (64-bit .NET Core osx.10.11-x64)
  Discovering: WebAppTests
  Discovered:  WebAppTests
  Starting:    WebAppTests
  Finished:    WebAppTests
=== TEST EXECUTION SUMMARY ===
   WebAppTests  Total: 2, Errors: 0, Failed: 0, Skipped: 0, Time: 0.260s
SUMMARY: Total: 1 targets, Passed: 1, Failed: 0.
[DotNetWatcher] info: dotnet exit code: 0

[DotNetWatcher] info: Waiting for a file to change before restarting dotnet...