Map all urls with IApplicationBuilder.Run

Daily Knowledge Drop

The Run method on the WebApplication (which implements IApplicationBuilder) can be used to map any and all urls to the specific delegate.

This can be leveraged in the case where the api doesn't expose any functional endpoints (such as with a Background Service) to always display a consistent message to the caller.

Root endpoint

Usually when a web application doesn't expose any endpoints, the root endpoint might be mapped so when browsing to the url, a message is returned to at least inform the caller that the service is up and running:

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddTransient<RunMiddleware>();

WebApplication? app = builder.Build();

app.MapGet("/", async context =>
{
    await context.Response.WriteAsync("Sample application - no endpoints exposed");
});

app.Run();

Browsing to / will display:

Sample application - no endpoints exposed

The downside of this approach, is that if the user browses to any URL other than /, nothing will be returned. For example, browsing to /api just returns a blank page.

Run method

The Run method can be leveraged to map a delegate to all endpoints:

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddTransient<RunMiddleware>();

WebApplication? app = builder.Build();

app.Run(async context =>
{
   await context.Response.WriteAsync("Sample application - no endpoints exposed");
});

app.Run();

With this approach, browsing to / or /api or any other url, will return the following:

Sample application - no endpoints exposed

Now, no matter the endpoint called - the same message is returned.

Endpoint override

A word of warning - using the Run method will cause the specific delegate to be called on all endpoints - even if another endpoint has specifically been defined:

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddTransient<RunMiddleware>();

WebApplication? app = builder.Build();

app.MapGet("hello", async context =>
{
    await context.Response.WriteAsync("World");
});

app.Run(async context =>
{
    await context.Response.WriteAsync("Sample application - no endpoints exposed");
});

app.Run();

In the above, browsing to /hello will return Sample application - no endpoints exposed and NOT World - the Run method delegate overrides all over endpoint delegates.

Notes

This is not a technique to be used for every application, but in the cases where the application is running as a background service (for example) which doesn't expose any specific endpoints, this technique can be leveraged to always display something to the user.

References

Application Environment

Daily Drop 174: 04-10-2022

At the start of 2022 I set myself the goal of learning one new coding related piece of knowledge a day.
It could be anything - some.NET / C# functionality I wasn't aware of, a design practice, a cool new coding technique, or just something I find interesting. It could be something I knew at one point but had forgotten, or something completely new, which I may or may never actually use.

The Daily Drop is a record of these pieces of knowledge - writing about and summarizing them helps re-enforce the information for myself, as well as potentially helps others learn something new as well.