[.Net][AutoGen.OpenAI] Allow nullable system message, temperature and…

… max token to support o1-preview model (#3524)

* add connect to o1 example

* Update Connect_To_OpenAI_o1_preview.cs

dotnet/sample/AutoGen.OpenAI.Sample/Connect_To_OpenAI_o1_preview.cs

@@ -0,0 +1,37 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Connect_To_OpenAI_o1_preview.cs
+
+using AutoGen.Core;
+using OpenAI;
+
+namespace AutoGen.OpenAI.Sample;
+
+public class Connect_To_OpenAI_o1_preview
+{
+    public static async Task RunAsync()
+    {
+        #region create_agent
+        var apiKey = Environment.GetEnvironmentVariable("OPENAI_API_KEY") ?? throw new InvalidOperationException("Please set environment variable OPENAI_API_KEY");
+        var openAIClient = new OpenAIClient(apiKey);
+
+        // until 2024/09/12
+        // openai o1-preview doesn't support systemMessage, temperature, maxTokens, streaming output
+        // so in order to use OpenAIChatAgent with o1-preview, you need to set those parameters to null
+        var agent = new OpenAIChatAgent(
+            chatClient: openAIClient.GetChatClient("o1-preview"),
+            name: "assistant",
+            systemMessage: null,
+            temperature: null,
+            maxTokens: null,
+            seed: 0)
+            // by using RegisterMiddleware instead of RegisterStreamingMiddleware
+            // it turns an IStreamingAgent into an IAgent and disables streaming
+            .RegisterMiddleware(new OpenAIChatRequestMessageConnector())
+            .RegisterPrintMessage();
+        #endregion create_agent
+
+        #region send_message
+        await agent.SendAsync("Can you write a piece of C# code to calculate 100th of fibonacci?");
+        #endregion send_message
+    }
+}

dotnet/src/AutoGen.OpenAI/Agent/OpenAIChatAgent.cs

@@ -34,7 +34,7 @@ public class OpenAIChatAgent : IStreamingAgent
 {
     private readonly ChatClient chatClient;
     private readonly ChatCompletionOptions options;
-    private readonly string systemMessage;
+    private readonly string? systemMessage;
 
     /// <summary>
     /// Create a new instance of <see cref="OpenAIChatAgent"/>.
@@ -50,9 +50,9 @@ public class OpenAIChatAgent : IStreamingAgent
     public OpenAIChatAgent(
         ChatClient chatClient,
         string name,
-        string systemMessage = "You are a helpful AI assistant",
-        float temperature = 0.7f,
-        int maxTokens = 1024,
+        string? systemMessage = "You are a helpful AI assistant",
+        float? temperature = null,
+        int? maxTokens = null,
         int? seed = null,
         ChatResponseFormat? responseFormat = null,
         IEnumerable<ChatTool>? functions = null)
@@ -75,7 +75,7 @@ public OpenAIChatAgent(
         ChatClient chatClient,
         string name,
         ChatCompletionOptions options,
-        string systemMessage = "You are a helpful AI assistant")
+        string? systemMessage = "You are a helpful AI assistant")
     {
         this.chatClient = chatClient;
         this.Name = name;
@@ -124,7 +124,7 @@ private IEnumerable<ChatMessage> CreateChatMessages(IEnumerable<IMessage> messag
         });
 
         // add system message if there's no system message in messages
-        if (!oaiMessages.Any(m => m is SystemChatMessage))
+        if (!oaiMessages.Any(m => m is SystemChatMessage) && systemMessage is not null)
         {
             oaiMessages = new[] { new SystemChatMessage(systemMessage) }.Concat(oaiMessages);
         }
@@ -192,8 +192,8 @@ private ChatCompletionOptions CreateChatCompletionsOptions(GenerateReplyOptions?
     }
 
     private static ChatCompletionOptions CreateChatCompletionOptions(
-        float temperature = 0.7f,
-        int maxTokens = 1024,
+        float? temperature = 0.7f,
+        int? maxTokens = 1024,
         int? seed = null,
         ChatResponseFormat? responseFormat = null,
         IEnumerable<ChatTool>? functions = null)