Add custom client interceptions for any client WebRequest

AddClientHeader.cs

// <configuration>  
//  <system.net>  
//    <webRequestModules>  
//      <add prefix="http"  
//           type="UserAgentRequestCreator, namespace, Version=1.0.0.0,  
//           Culture=neutral, PublicKeyToken=9999999999999999"  
//      />  
//    </webRequestModules>  
//  </system.net>  
// </configuration>    
    
        public class UserAgentRequestCreator : IWebRequestCreate
    {
        public const string UserAgentKey = "HTTP_WEB_REQUEST_USER_AGENT";
        private static string _defaultUserAgentName = new Lazy<string>(() =>
        {
            var agentValue = Environment.GetEnvironmentVariable(UserAgentKey);
            return !string.IsNullOrWhiteSpace(agentValue) ? agentValue : ConfigurationManager.AppSettings[UserAgentKey];
        }).Value;

        private readonly string _userAgentName;

        public UserAgentRequestCreator() : this(_defaultUserAgentName) { }

        public UserAgentRequestCreator(string userAgentName)
        {
            _userAgentName = userAgentName;
        }

        public static void SetDefaultUserAgent(string defaultValue)
        {
            _defaultUserAgentName = defaultValue;
        }

        public static void RegisterUserAgent(string url, string agentName)
        {
            WebRequest.RegisterPrefix(url, new UserAgentRequestCreator(agentName));
        }

        #region Implementation of IWebRequestCreate

        public WebRequest Create(Uri uri)
        {
            if (!(WebRequest.CreateDefault(uri) is HttpWebRequest httpWebRequest))
                throw new InvalidOperationException("The URI prefix is not recognized.");

            if (string.IsNullOrWhiteSpace(_userAgentName)) return httpWebRequest;

            var methodInfo = typeof(WebHeaderCollection).GetMethod("AddWithoutValidate", BindingFlags.Instance | BindingFlags.NonPublic);
            methodInfo.Invoke(httpWebRequest.Headers, new object[] { "User-Agent", _userAgentName });
            return httpWebRequest;
        }

        #endregion
    }