Each command can format the mpd response. User response is a MpdMessage which conatins the requested command and the formatted response.

LibMpc/Message/MpdMessage.cs

@@ -1,24 +1,69 @@
+using System.Collections.Generic;
+using System.Linq;
+using System.Text.RegularExpressions;
+using Newtonsoft.Json;
+
 namespace LibMpc
 {
-    public interface IMpdMessage
+    public interface IMpdMessage<T>
     {
-        IMpdRequest Request { get; }
-        IMpdResponse Response { get; }
+        IMpdRequest<T> Request { get; }
+        IMpdResponse<T> Response { get; }
     }
 
-    public class MpdMessage : IMpdMessage
+    public class MpdMessage<T> : IMpdMessage<T>
     {
-        public MpdMessage(IMpcCommand command)
+        private readonly Regex _linePattern = new Regex("^(?<key>[A-Za-z]*):[ ]{0,1}(?<value>.*)$");
+        private readonly IList<string> _rawResponse;
+
+        public MpdMessage(IMpcCommand<T> command, bool connected, IReadOnlyCollection<string> response)
         {
-            Request = new MpdRequest(command);
+            Request = new MpdRequest<T>(command);
+
+            var endLine = response.Skip(response.Count - 1).Single();
+            _rawResponse = response.Take(response.Count - 1).ToList();
+
+            var values = Request.Command.FormatResponse(GetValuesFromResponse());
+            Response = new MpdResponse<T>(endLine, values, connected);
         }
 
-        public void AddResponse(IMpdResponse response)
-        {
-            Response = response;
+        public IMpdRequest<T> Request { get; }
+        public IMpdResponse<T> Response { get; }
+
+        private IReadOnlyDictionary<string, IList<string>> GetValuesFromResponse()
+        {
+            var result = new Dictionary<string, IList<string>>();
+
+            foreach (var line in _rawResponse)
+            {
+                var match = _linePattern.Match(line);
+                if (match.Success)
+                {
+                    var mpdKey = match.Result("${key}");
+                    if (!string.IsNullOrEmpty(mpdKey))
+                    {
+                        var mpdValue = match.Result("${value}");
+                        if (!string.IsNullOrEmpty(mpdValue))
+                        {
+                            if (!result.ContainsKey(mpdKey))
+                            {
+                                result.Add(mpdKey, new List<string>() { mpdValue });
+                            }
+                            else
+                            {
+                                result[mpdKey].Add(mpdValue);
+                            }
+                        }
+                    }
+                }
+            }
+
+            return result;
         }
 
-        public IMpdRequest Request { get; }
-        public IMpdResponse Response { get; private set; }
+        public override string ToString()
+        {
+            return JsonConvert.SerializeObject(this, Formatting.Indented);
+        }
     }
 }