Intercept bad requests before reaching controller in ASP.NET Core

An example of intersection for logging is describe in Log automatic 400 responses Add configuration in Startup.ConfigureServices.

services.AddControllers()
    .ConfigureApiBehaviorOptions(options =>
    {
        // To preserve the default behavior, capture the original delegate to call later.
        var builtInFactory = options.InvalidModelStateResponseFactory;

        options.InvalidModelStateResponseFactory = context =>
        {
            var logger = context.HttpContext.RequestServices.GetRequiredService<ILogger<Startup>>();

            // Perform logging here.
//E.g. logger.LogError($”{context.ModelState}”);
    logger.LogWarning(context.ModelState.ModelStateErrorsToString());

            // Invoke the default behavior, which produces a ValidationProblemDetails response.
            // To produce a custom response, return a different implementation of IActionResult instead.
            return builtInFactory(context);
        };
    });

public static String ModelStateErrorsToString(this ModelStateDictionary modelState)
        {
            IEnumerable<ModelError> allErrors = modelState.Values.SelectMany(v => v.Errors);
            StringBuilder sb = new StringBuilder();
            foreach (ModelError error in allErrors)
            {
                sb.AppendLine($"error {error.ErrorMessage} {error.Exception}");
            }
            return sb.ToString();
        }

The [ApiController] attribute that you've applied to your controller adds Automatic HTTP 400 Responses to the MVC pipeline, which means that your custom filter and action aren't executed if ModelState is invalid.

I see a few options for affecting how this works:

1. Remove the [ApiController] attribute

   Although you can just remove the [ApiController] attribute, this would also cause the loss of some of the other features it provides, such as Binding source parameter inference.

2. Disable only the Automatic HTTP 400 Responses

   Here's an example from the docs that shows how to disable just this feature:

   services.AddControllers()
       .ConfigureApiBehaviorOptions(options =>
       {
           // ...
           options.SuppressModelStateInvalidFilter = true;
           // ...
       }
   

   This code goes inside of your Startup's ConfigureServices method.

3. Customise the automatic response that gets generated

   If you just want to provide a custom response to the caller, you can customise what gets returned. I've already described how this works in another answer, here.