ProblemDetailsService Can Simplify Writers Evaluation

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

The ProblemDetailsService implementation uses the last registered IProblemDetailsWriter where CanWrite returns true as seen at:

https://github.com/dotnet/aspnetcore/blob/5918bc9db2099f90cc6f5b5db43a8d16f5dc89df/src/Http/Http.Extensions/src/ProblemDetailsService.cs#L42

Although a micro-optimization, the loop should be processed in reverse. As soon as a writer on the tail end can write the ProblemDetails, then the loop can break. In fact, the entire implementation can be simplified to:

    public ValueTask WriteAsync(ProblemDetailsContext context)
    {
        ArgumentNullException.ThrowIfNull(context);
        ArgumentNullException.ThrowIfNull(context.ProblemDetails);
        ArgumentNullException.ThrowIfNull(context.HttpContext);

        if (context.HttpContext.Response.HasStarted ||
            context.HttpContext.Response.StatusCode < 400 ||
            _writers.Length == 0)
        {
            return ValueTask.CompletedTask;
        }

-        IProblemDetailsWriter? selectedWriter = null;

-        if (_writers.Length == 1)
-        {
-            selectedWriter = _writers[0];

-            return selectedWriter.CanWrite(context) ?
-                selectedWriter.WriteAsync(context) :
-                ValueTask.CompletedTask;
-        }

-       for (var i = 0; i < _writers.Length; i++)
+       for (var i = _writers.Length - 1; i >= 0; i--)
        {
+            var selectedWriter = _writers[i];
-            if (_writers[i].CanWrite(context))
+            if (selectedWriter.CanWrite(context))
-            {
-                selectedWriter = _writers[i];
-                break;
+                return selectedWriter.WriteAsync(context);
-            }
        }

-        if (selectedWriter != null)
-        {
-            return selectedWriter.WriteAsync(context);
-        }

        return ValueTask.CompletedTask;
    }

Expected Behavior

IProblemDetailsService.WriteAsync should execute with the fewest number of operations necessary.

Steps To Reproduce

var builder = WebApplication.CreateBuilder( args );

builder.Services.AddProblemDetails();

Exceptions (if any)

No response

.NET Version

7.0.100

Anything else?

Happy to submit a PR if the proposed changes are accepted.

[brunolins16]

@commonsensesoftware that is right. I ended up implementing everything as part of another work I did. Sorry if you spent time in this PR.

[brunolins16]

Sure. I can spin up a PR.

What is gained (or retained) by keeping the branch for if (_writers.Length == 1)? Since `writers supports indexing, I don’t see any additional allocations or other benefits. I’m just seeing another branch that does the same thing as the loop. The net effect will be the same and is seeming unnecessary.

Just curious. If there is some performance benefit, I would like to learn about it for future reference.

Actually, you are right. I just noticed that writers is an array already, so, I think no extra allocations.