Develop ASP.NET Core apps using a file watcher

By Rick Anderson and Victor Hurdugaci

dotnet watch is a tool that runs a .NET CLI command when source files change. For example, a file change can trigger compilation, test execution, or deployment.

This tutorial uses an existing web API with two endpoints: one that returns a sum and one that returns a product. The product method has a bug, which is fixed in this tutorial.

Download the sample app. It consists of two projects: WebApp (an ASP.NET Core web API) and WebAppTests (unit tests for the web API).

In a command shell, navigate to the WebApp folder. Run the following command:

dotnet run

Note

You can use dotnet run --project <PROJECT> to specify a project to run. For example, running dotnet run --project WebApp from the root of the sample app will also run the WebApp project.

The console output shows messages similar to the following (indicating that the app is running and awaiting requests):

$ dotnet run
Hosting environment: Development
Content root path: C:/Docs/aspnetcore/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:<port number>/api/math/sum?a=4&b=5. You should see the result of 9.

Navigate to the product API (http://localhost:<port number>/api/math/product?a=4&b=5). It returns 9, not 20 as you'd expect. That problem is fixed later in the tutorial.

Run .NET CLI commands using dotnet watch

Any .NET CLI command can be run with dotnet watch. For example:

Command	Command with watch
dotnet run	dotnet watch run
dotnet run -f netcoreapp3.1	dotnet watch run -f netcoreapp3.1
dotnet run -f netcoreapp3.1 -- --arg1	dotnet watch run -f netcoreapp3.1 -- --arg1
dotnet test	dotnet watch test

Run dotnet watch run in the WebApp folder. The console output indicates watch has started.

Running dotnet watch run on a web app launches a browser that navigates to the app's URL once ready. dotnet watch does this by reading the app's console output and waiting for the ready message displayed by WebHost.

dotnet watch refreshes the browser when it detects changes to watched files. To do this, the watch command injects a middleware to the app that modifies HTML responses created by the app. The middleware adds a JavaScript script block to the page that allows dotnet watch to instruct the browser to refresh. Currently, changes to all watched files, including static content such as .html and .css files cause the app to be rebuilt.

dotnet watch:

• Only watches files that impact builds by default.
• Any additionally watched files (via configuration) still results in a build taking place.

For more information on configuration, see dotnet-watch configuration in this document.

Note

You can use dotnet watch --project <PROJECT> to specify a project to watch. For example, running dotnet watch --project WebApp run from the root of the sample app will also run and watch the WebApp project.

Make changes with dotnet watch

Make sure dotnet watch is running.

Fix the bug in the Product method of MathController.cs so it returns the product and not the sum:

public static int Product(int a, int b)
{
    return a * b;
}

Save the file. The console output indicates that dotnet watch detected a file change and restarted the app.

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

Run tests using dotnet watch

1. Change the Product method of MathController.cs back to returning the sum. Save the file.

2. In a command shell, navigate to the WebAppTests folder.

3. Run dotnet restore.

4. Run dotnet watch test. Its output indicates that a test failed and that the watcher is awaiting file changes:

   Total tests: 2. Passed: 1. Failed: 1. Skipped: 0.
   Test Run Failed.
   

5. Fix the Product method code so it returns the product. Save the file.

dotnet watch detects the file change and reruns the tests. The console output indicates the tests passed.

Customize files list to watch

By default, dotnet-watch tracks all files matching the following glob patterns:

• **/*.cs
• *.csproj
• **/*.resx
• Content files: wwwroot/**, **/*.config, **/*.json

More items can be added to the watch list by editing the .csproj file. Items can be specified individually or by using glob patterns.

<ItemGroup>
    <!-- extends watching group to include *.js files -->
    <Watch Include="**\*.js" Exclude="node_modules\**\*;**\*.js.map;obj\**\*;bin\**\*" />
</ItemGroup>

Opt-out of files to be watched

dotnet-watch can be configured to ignore its default settings. To ignore specific files, add the Watch="false" attribute to an item's definition in the .csproj file:

<ItemGroup>
    <!-- exclude Generated.cs from dotnet-watch -->
    <Compile Include="Generated.cs" Watch="false" />

    <!-- exclude Strings.resx from dotnet-watch -->
    <EmbeddedResource Include="Strings.resx" Watch="false" />

    <!-- exclude changes in this referenced project -->
    <ProjectReference Include="..\ClassLibrary1\ClassLibrary1.csproj" Watch="false" />
</ItemGroup>

<ItemGroup>
     <!-- Exclude all Content items from being watched. -->
    <Content Update="@(Content)" Watch="false" />
</ItemGroup>

Custom watch projects

dotnet-watch isn't restricted to C# projects. Custom watch projects can be created to handle different scenarios. Consider the following project layout:

• test/
  • UnitTests/UnitTests.csproj
  • IntegrationTests/IntegrationTests.csproj

If the goal is to watch both projects, create a custom project file configured to watch both projects:

<Project>
    <ItemGroup>
        <TestProjects Include="**\*.csproj" />
        <Watch Include="**\*.cs" />
    </ItemGroup>

    <Target Name="Test">
        <MSBuild Targets="VSTest" Projects="@(TestProjects)" />
    </Target>

    <Import Project="$(MSBuildExtensionsPath)\Microsoft.Common.targets" />
</Project>

To start file watching on both projects, change to the test folder. Execute the following command:

dotnet watch msbuild /t:Test

VSTest executes when any file changes in either test project.

dotnet-watch configuration

Some configuration options can be passed to dotnet watch through environment variables. The available variables are:

Setting	Description
DOTNET_USE_POLLING_FILE_WATCHER	If set to "1" or "true", dotnet watch uses a polling file watcher instead of CoreFx's FileSystemWatcher. Used when watching files on network shares or Docker mounted volumes.
DOTNET_WATCH_SUPPRESS_MSBUILD_INCREMENTALISM	By default, dotnet watch optimizes the build by avoiding certain operations such as running restore or re-evaluating the set of watched files on every file change. If set to "1" or "true", these optimizations are disabled.
DOTNET_WATCH_SUPPRESS_LAUNCH_BROWSER	dotnet watch run attempts to launch browsers for web apps with launchBrowser configured in launchSettings.json. If set to "1" or "true", this behavior is suppressed.
DOTNET_WATCH_SUPPRESS_BROWSER_REFRESH	dotnet watch run attempts to refresh browsers when it detects file changes. If set to "1" or "true", this behavior is suppressed. This behavior is also suppressed if DOTNET_WATCH_SUPPRESS_LAUNCH_BROWSER is set.

Browser refresh

dotnet watch injects a script into the app that allows it to refresh the browser when the content changes. In some scenarios, such as when the app enables response compression, dotnet watch might not be able to inject the script. For such cases in development, manually inject the script into the app. For example, to configure the web app to manually inject the script, update the layout file to include _framework/aspnet-browser-refresh.js:

@* _Layout.cshtml *@
<environment names="Development">
    <script src="/_framework/aspnetcore-browser-refresh.js"></script>
</environment>

Non-ASCII characters

Visual Studio 17.2 and later includes the .NET SDK 6.0.300 and later. With the .NET SDK and 6.0.300 later, dotnet-watch emits non-ASCII characters to the console during a hot reload session. On certain console hosts, such as the Windows conhost, these characters may appear garbled. To avoid garbled characters, consider one of the following approaches:

• Configure the DOTNET_WATCH_SUPPRESS_EMOJIS=1 environment variable to suppress emitting these values.
• Switch to a different terminal, such as https://github.com/microsoft/terminal, that supports rendering non-ASCII characters.