How Should ASP.NET Web APIs Return Errors: Immediately or Collectively?

Best Practices for Returning Errors in ASP.NET Web API

When dealing with errors in ASP.NET Web APIs, there are two primary approaches: returning errors immediately or accumulating errors and sending them back collectively. This article examines the pros and cons of each approach and provides the recommended best practice.

1. Returning Errors Immediately

In the first approach, errors are returned immediately using HttpResponseExceptions. This is suitable when:

• The error is fatal: This prevents the API from continuing processing, as further execution is impossible.
• The error is specific and actionable: It provides a clear indication of the problem and actionable steps for the client.

Example:

public void Post(Customer customer)
{
    if (string.IsNullOrEmpty(customer.Name))
    {
        throw new HttpResponseException("Customer Name cannot be empty", HttpStatusCode.BadRequest);
    }
    if (customer.Accounts.Count == 0)
    {
        throw new HttpResponseException("Customer does not have any account", HttpStatusCode.BadRequest);
    }
}

2. Accumulating and Sending Back Errors

In the second approach, errors are accumulated and returned collectively at the end of the action. This is recommended when:

• The errors are non-fatal: They do not prevent the action from being completed, but still need to be communicated to the client.
• The errors may be numerous and cover different parts of the input: Consolidating them allows for a comprehensive error message.

Example:

public void Post(Customer customer)
{
    List<string> errors = new List<string>();
    if (string.IsNullOrEmpty(customer.Name))
    {
        errors.Add("Customer Name cannot be empty");
    }
    if (customer.Accounts.Count == 0)
    {
        errors.Add("Customer does not have any account");
    }
    var responseMessage = new HttpResponseMessage<List<string>>(errors, HttpStatusCode.BadRequest);
    throw new HttpResponseException(responseMessage);
}

Best Practice

While both approaches have their merits, the recommended best practice is to return errors immediately. This:

• Provides instant feedback: The client is notified of the error as soon as it occurs, enabling them to address it promptly.
• Simplifies error handling: Catching and handling exceptions at the appropriate location avoids the need for complex error propagation mechanisms.
• Promotes maintainability: Clearly demarcated error handling logic makes code easier to understand and maintain.

However, for non-fatal errors that are part of a larger validation or processing stage, accumulating errors and returning them collectively may be more appropriate.

Updates

This article has been updated over time with insights from blog posts and changes in best practices:

• Utilize HttpResponseExceptions for immediate error returns and model state errors.
• Handle server-level errors in a global exception filter, providing appropriate HTTP status codes and friendly error messages.
• Leverage IHttpActionResult interface and built-in classes for general error returns, such as NotFound and BadRequest.

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