Adds DnsNotImplementedClient as a "no op" responder.

src/Ae.Dns.Protocol/DnsNotImplementedClient.cs

@@ -0,0 +1,31 @@
+using Ae.Dns.Protocol.Enums;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Ae.Dns.Protocol
+{
+    /// <summary>
+    /// A client which only returns a <see cref="DnsResponseCode.NotImp"/> code.
+    /// </summary>
+    public sealed class DnsNotImplementedClient : IDnsClient
+    {
+        /// <summary>
+        /// A static instance of this client which can be used to avoid allocating a new one.
+        /// </summary>
+        public static readonly IDnsClient Instance = new DnsNotImplementedClient();
+
+        /// <inheritdoc/>
+        public void Dispose()
+        {
+        }
+
+        /// <inheritdoc/>
+        public Task<DnsMessage> Query(DnsMessage query, CancellationToken token = default)
+        {
+            return Task.FromResult(query.CreateErrorMessage(DnsResponseCode.NotImp, ToString()));
+        }
+
+        /// <inheritdoc/>
+        public override string ToString() => nameof(DnsNotImplementedClient);
+    }
+}

src/Ae.Dns.Protocol/DnsOperationRouter.cs

@@ -25,14 +25,12 @@ public DnsOperationRouter(IReadOnlyDictionary<DnsOperationCode, IDnsClient> rout
         /// <inheritdoc/>
         public async Task<DnsMessage> Query(DnsMessage query, CancellationToken token = default)
         {
-            _routes.TryGetValue(query.Header.OperationCode, out IDnsClient? client);
-
-            if (client == null)
+            if (!_routes.TryGetValue(query.Header.OperationCode, out IDnsClient? operationClient))
             {
-                return query.CreateErrorMessage(DnsResponseCode.NotImp, ToString());
+                operationClient = DnsNotImplementedClient.Instance;
             }
 
-            return await client.Query(query, token);
+            return await operationClient.Query(query, token);
         }
 
         /// <inheritdoc/>