Logging to elmah.io from HttpClient

When using the HttpClient class to make HTTP requests, log messages will be automatically published to any ILogger registered in your application. All requests will cause 4 log messages to be produced in your elmah.io log if you have Information logging enabled. While some prefer to have an extended level of log messages produced, others might want just a single log message per request or only log failing requests. This can be done by implementing a custom logger for HttpClient:

public class HttpClientLogger : IHttpClientLogger
{
    private readonly ILogger<HttpClientLogger> _logger;

    public HttpClientLogger(ILogger<HttpClientLogger> logger)
    {
        _logger = logger;
    }

    public object? LogRequestStart(HttpRequestMessage request)
    {
        // We only log when there's a response or an error in this example.
        return null;
    }

    public void LogRequestStop(
        object? context,
        HttpRequestMessage request,
        HttpResponseMessage response,
        TimeSpan elapsed)
    {
        _logger.LogInformation(
            "HttpClient made a {method} request to {url} and received {statusCode}",
            request.Method,
            request.RequestUri,
            (int?)response.StatusCode);
    }

    public void LogRequestFailed(
        object? context,
        HttpRequestMessage request,
        HttpResponseMessage? response,
        Exception exception,
        TimeSpan elapsed)
    {
        _logger.LogError(
            exception,
            "HttpClient made a failing {method} request to {url} and received {statusCode}",
            request.Method,
            request.RequestUri,
            (int?)response?.StatusCode);
    }
}

The HttpClientLogger class implements the IHttpClientLogger interface. Three hooks are provided as part of this interface: request starting, request stopping, and request failing. In the code above, we create a single Information message when a response is received and a single Error message in case of an error. By using structured properties like {method} and {statusCode} we make sure that elmah.io shows these pieces of data in the right fields. The format and wording of each log message can be tailored to your preference.

Registering the custom logger can be done like this:

builder
    .Services
    .AddHttpClient("myclient")
    .RemoveAllLoggers()
    .AddLogger<HttpClientLogger>();

This example registers a new HttpClient named myclient. By calling the RemoveAllLoggers method we make sure that the four default log messages are not logged. In the last line, the HttpClientLogger class that we implemented is set as the logger for this HttpClient.

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