Development tools practice

This optional appendix uses the same project to practice the edit, compile, and rerun cycle after you complete First steps. Skipping it will not affect the later chapters.

The complete code used in this practice is below. Run the commands on this page from the FirstSteps project directory you created in Chapter 02. If you are using the repository example, go to samples/02-first-steps.

02-first-steps/Program.cs

using Scalar.AspNetCore;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddOpenApi();

var app = builder.Build();

if (app.Environment.IsDevelopment())
{
    app.MapOpenApi();
    app.MapScalarApiReference();
}

app.MapGet("/", () => new { Message = "Hello, ASP.NET Core!" });

app.Run();

Let the compiler check your code

C# is a statically typed language, so many errors are found before the program runs. Deliberately change MapGet on line 15 to MapGt; the editor will underline it in red right away. Running dotnet build will show:

Program.cs(15,5): error CS1061: 'WebApplication' does not contain a definition for 'MapGt' and no accessible extension method 'MapGt' accepting a first argument of type 'WebApplication' could be found (are you missing a using directive or an assembly reference?)

The error tells you that WebApplication has no member named MapGt at line 15, column 5. The program never starts, so it cannot wait until a request arrives before crashing.

After the experiment, change MapGt back to MapGet, confirm dotnet build succeeds, then continue below. The diagnostic language depends on the SDK language setting.

Likewise, type Map after app. and the editor will list the available methods (MapGet, MapPost, MapGroup, and more) and show their parameter descriptions. Type information helps the editor complete your code and provides response shapes for OpenAPI, reducing duplicated maintenance.

Use dotnet watch for automatic reloads

If the service you started earlier is still running, press Ctrl+C in its terminal to stop it. Then, from the same project directory, run:

dotnet watch

It runs the project and watches for file changes. After it starts, change the text on line 15 to "Hello, Hot Reload!" and save. The terminal will show:

dotnet watch ⌚ File updated: .\Program.cs
dotnet watch 🔥 C# and Razor changes applied in 1029ms.

Make another request and the response will have changed, while the service has not restarted:

curl http://localhost:5080/

{"message":"Hello, Hot Reload!"}

This is called Hot Reload: supported changes are applied directly to the running program. The elapsed time in the log varies by machine. Changes that cannot be applied live may prompt a restart. After changing service registrations or other startup configuration, press Ctrl+R in the dotnet watch terminal to restart and run the startup code again.

When you are done, change the greeting back to "Hello, ASP.NET Core!", then press Ctrl+C to stop the service so it does not affect the output or ports used in later chapters.

FastAPI comparison

dotnet watch is similar to fastapi dev or uvicorn --reload. For changes that support Hot Reload, .NET can preserve the process and its in-memory state; a restart loses that state.

Summary

• Compiler errors identify the file and location. After fixing an error, use dotnet build to confirm the project builds.
• dotnet watch monitors code changes and applies supported changes without restarting the process.
• After changing startup configuration, restart with Ctrl+R; a restart loses in-memory state.

Back to First steps. Continue with Route parameters.