Changes based on review feedback:

 - Returning template for chaining method calls
 - Returning a `TextMessage` object instead of a tuple

LLama.Unittest/TemplateTests.cs

@@ -199,15 +199,21 @@ public void RemoveMid()
         templater.Add("user", "4b");
         templater.Add("assistant", "5");
 
-        Assert.Equal(("user", "4a"), templater[3]);
-        Assert.Equal(("assistant", "5"), templater[5]);
+        Assert.Equal("user", templater[3].Role);
+        Assert.Equal("4a", templater[3].Content);
+
+        Assert.Equal("assistant", templater[5].Role);
+        Assert.Equal("5", templater[5].Content);
 
         Assert.Equal(6, templater.Count);
         templater.RemoveAt(3);
         Assert.Equal(5, templater.Count);
 
-        Assert.Equal(("user", "4b"), templater[3]);
-        Assert.Equal(("assistant", "5"), templater[4]);
+        Assert.Equal("user", templater[3].Role);
+        Assert.Equal("4b", templater[3].Content);
+
+        Assert.Equal("assistant", templater[4].Role);
+        Assert.Equal("5", templater[4].Content);
     }
 
     [Fact]
@@ -226,7 +232,8 @@ public void RemoveLast()
         templater.RemoveAt(5);
         Assert.Equal(5, templater.Count);
 
-        Assert.Equal(("user", "4b"), templater[4]);
+        Assert.Equal("user", templater[4].Role);
+        Assert.Equal("4b", templater[4].Content);
     }
 
     [Fact]

LLama/LLamaTemplate.cs

@@ -31,7 +31,7 @@ public sealed class LLamaTemplate
     /// <summary>
     /// Array of messages. The <see cref="Count"/> property indicates how many messages there are
     /// </summary>
-    private Message[] _messages = new Message[4];
+    private TextMessage?[] _messages = new TextMessage[4];
 
     /// <summary>
     /// Backing field for <see cref="AddAssistant"/>
@@ -71,7 +71,7 @@ public sealed class LLamaTemplate
     /// <param name="index"></param>
     /// <returns></returns>
     /// <exception cref="ArgumentOutOfRangeException">Thrown if index is less than zero or greater than or equal to <see cref="Count"/></exception>
-    public (string role, string content) this[int index]
+    public TextMessage this[int index]
     {
         get
         {
@@ -80,7 +80,7 @@ public sealed class LLamaTemplate
             if (index >= Count)
                 throw new ArgumentOutOfRangeException(nameof(index), "Index must be < Count");
 
-            return (_messages[index].Role, _messages[index].Content);
+            return _messages[index]!;
         }
     }
 
@@ -131,30 +131,45 @@ public LLamaTemplate(string customTemplate)
     }
     #endregion
 
+    #region modify
     /// <summary>
     /// Add a new message to the end of this template
     /// </summary>
     /// <param name="role"></param>
     /// <param name="content"></param>
-    public void Add(string role, string content)
+    /// <returns>This template, for chaining calls.</returns>
+    public LLamaTemplate Add(string role, string content)
+    {
+        return Add(new TextMessage(role, content, _roleCache));
+    }
+
+    /// <summary>
+    /// Add a new message to the end of this template
+    /// </summary>
+    /// <param name="message"></param>
+    /// <returns>This template, for chaining calls.</returns>
+    public LLamaTemplate Add(TextMessage message)
     {
         // Expand messages array if necessary
         if (Count == _messages.Length)
             Array.Resize(ref _messages, _messages.Length * 2);
 
         // Add message
-        _messages[Count] = new Message(role, content, _roleCache);
+        _messages[Count] = message;
         Count++;
 
         // Mark as dirty to ensure template is recalculated
         _dirty = true;
+
+        return this;
     }
 
     /// <summary>
     /// Remove a message at the given index
     /// </summary>
     /// <param name="index"></param>
-    public void RemoveAt(int index)
+    /// <returns>This template, for chaining calls.</returns>
+    public LLamaTemplate RemoveAt(int index)
     {
         if (index < 0)
             throw new ArgumentOutOfRangeException(nameof(index), "Index must be greater than or equal to zero");
@@ -169,7 +184,10 @@ public void RemoveAt(int index)
             Array.Copy(_messages, index + 1, _messages, index, Count - index);
 
         _messages[Count] = default;
+
+        return this;
     }
+    #endregion
 
     /// <summary>
     /// Apply the template to the messages and write it into the output buffer
@@ -192,7 +210,8 @@ public int Apply(Memory<byte> dest)
                     Array.Resize(ref _nativeChatMessages, _messages.Length);
                 for (var i = 0; i < Count; i++)
                 {
-                    ref var m = ref _messages[i];
+                    ref var m = ref _messages[i]!;
+                    Debug.Assert(m != null);
                     totalInputBytes += m.RoleBytes.Length + m.ContentBytes.Length;
 
                     // Pin byte arrays in place
@@ -258,17 +277,24 @@ unsafe int ApplyInternal(Span<LLamaChatMessage> messages, byte[] output)
     }
 
     /// <summary>
-    /// A message that has been added to the template, contains role and content converted into UTF8 bytes.
+    /// A message that has been added to a template
     /// </summary>
-    private readonly record struct Message
+    public sealed class TextMessage
     {
+        /// <summary>
+        /// The "role" string for this message
+        /// </summary>
         public string Role { get; }
+
+        /// <summary>
+        /// The text content of this message
+        /// </summary>
         public string Content { get; }
 
-        public ReadOnlyMemory<byte> RoleBytes { get; }
-        public ReadOnlyMemory<byte> ContentBytes { get; }
+        internal ReadOnlyMemory<byte> RoleBytes { get; }
+        internal ReadOnlyMemory<byte> ContentBytes { get; }
 
-        public Message(string role, string content, Dictionary<string, ReadOnlyMemory<byte>> roleCache)
+        internal TextMessage(string role, string content, IDictionary<string, ReadOnlyMemory<byte>> roleCache)
         {
             Role = role;
             Content = content;
@@ -297,5 +323,16 @@ public Message(string role, string content, Dictionary<string, ReadOnlyMemory<by
             Debug.Assert(contentArray.Length == encodedContentLength + 1);
             ContentBytes = contentArray;
         }
+
+        /// <summary>
+        /// Deconstruct this message into role and content
+        /// </summary>
+        /// <param name="role"></param>
+        /// <param name="content"></param>
+        public void Deconstruct(out string role, out string content)
+        {
+            role = Role;
+            content = Content;
+        }
     }
 }