asp.net core app.UseExceptionHandler() to handle exceptions for certain endpoints?

user_178757 0
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I'm adding some web API methods to an existing ASP.NET Core web app. This means my new web API controller(s) will have the attribute [ApiController] and calling clients will expect the return output to be JSON instead of regular HTML.

In another project, I implement a custom exception handler using app.UseExceptionHandler(...) which encodes the response in JSON, giving information about the exception. In this one, I want to do that, but only when the exception came from an action method inside an [ApiController] controller; otherwise I still want to use app.UseExceptionHandler("/Home/Error");, as the exception came from a regular page and the response should be HTML.

Is there a way I can set up an exception handler so that it knows where the exception came from, and render the response accordingly?

asp.net-core asp.net-core-webapi exception json c#
3 Answers

In this one, I want to do that, but only when the exception came from an action method inside an [ApiController] controller; otherwise I still want to use app.UseExceptionHandler("/Home/Error");, as the exception came from a regular page and the response should be HTML.

From your description, it seems that you want to detect where the exception come from, and if the exception come from the API method, you will use the custom exception handler with custom response, otherwise it will show the error message via the Home/Error page.

In this scenario, an alternative to a custom exception handler page is to provide a lambda to UseExceptionHandler. Using a lambda allows access to the path of the request that made the error before returning the response.

For example:

//app.UseExceptionHandler("/Home/Error");
app.UseExceptionHandler(errorApp =>
{
    errorApp.Run(async context =>
    { 
        var exceptionHandlerPathFeature =
            context.Features.Get<IExceptionHandlerPathFeature>();
        //check if the handler path contains api or not.
        if (exceptionHandlerPathFeature.Path.Contains("api"))
        { 
            context.Response.StatusCode = (int)HttpStatusCode.InternalServerError; ;
            context.Response.ContentType = "text/html";

            await context.Response.WriteAsync("<html lang=\"en\"><body>\r\n");
            await context.Response.WriteAsync("ERROR From API!<br><br>\r\n");

            await context.Response.WriteAsync(
                                            "<a href=\"/\">Home</a><br>\r\n");
            await context.Response.WriteAsync("</body></html>\r\n"); 
        }
        else
        {
            context.Response.Redirect("/Home/Error");
        }
    });
});

The result as below:

enter image description here

More detail information, see Exception handler lambda and Asp .Net Core How to handle error pages in area.

user_70622910

In ASP.Net Core 6 you could get the Endpoint from the IExceptionHandlerFeature, until then you can capture the Endpoint yourself after Routing middleware and store it in a feature and use that in exception handler code.

app.UseExceptionHandler("/Error");
app.UseRouting();
app.Use(async (context, next) =>
{
    var feature = new EndpointFeature
    {
        Endpoint = context.GetEndpoint()
    };
    context.Features.Set(feature);
    await next();
});
app.UseEndpoints(endpoints =>
{
    endpoints.Map("/Error", context =>
    {
        var feature = context.Features.Get<EndpointFeature>();
        var endpointMetadata = feature.Endpoint.Metadata;
    });
});
user_36679380

May I know which version are you using ASP.Net Core? link might be helpful for you. Exception Handler Middleware for ASP.Net Core Web Api

var exceptionHandlerPathFeature =
        HttpContext.Features.Get<IExceptionHandlerPathFeature>();
     var context = HttpContext.Features.Get<IExceptionHandlerFeature>();
     var detail = context.Error.StackTrace;
     var message = context.Error.Message;
        if (exceptionHandlerPathFeature?.Error is FileNotFoundException)
        {
            ExceptionMessage = "File error thrown";
            _logger.LogError(ExceptionMessage);
        }
        if (exceptionHandlerPathFeature?.Path == "/index")
        {
            ExceptionMessage += " from home page";
        }
user_61002250
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