Carter is an excellent library that makes writing ASP.NET Web Applications and APIs that much easier.

Today I ran into a problem that manifested as follows:

startupError

The application did not start at all, complaining about an issue during service registration.

I spent an entire day chasing this problem and finally discovered the issue.

After several hours of searching (including commenting out all the service registrations), I finally had the idea to look across the project for any endpoints with a parameter named request, as indicated by the error message.

And I found it:

Do you see the problem?

problem2

The problem is that the ASP.NET pipeline has no idea where to find the SearchRequest.

The solution is as folllows:

solveProblem

Here I have decorated the request with the FromBody attribute, to tell the pipeline to look for the parameter in the body of the HttpRequest.

Now, why are we getting this error at startup instead of when we actually hit this endpoint?

Carter scans the application for all classes implementing ICarterModule, and each module is used to configure endpoints via IEndpointRouteBuilder, which Carter uses internally to register routes with ASP.NET Core’s minimal API pipeline.

You can see the difference in a normal Minimal API project.

var builder = WebApplication.CreateBuilder(args);

var app = builder.Build();

app.UseHttpsRedirection();

app.MapGet("/Spy", (Spy spy) =>
{
    // Return the spy
    return Results.Ok(spy);
});

app.Run();

record Spy(string FirstName, string LastName);

This will start successfully.

spyStart

It is only when you actually hit the endpoint that ASP.NET complains it doesn’t know where to find the Spy.

spyProblem

In other words, you only get problems when you hit the endpoint.

If you use Carter, it scans them all at startup.

This is actually a good thing, as you know in advance if you have any problems with your routes, rather than having lesser-used routes throw errors after periods of normal operation.

If only it helped you point towards the culprit!

TLDR

Carter will scan all modules for endpoints at startup and throw an exception if it cannot map all the parameters.

Happy hacking!