Fix potential multiple enumerations (#742)

There are more of these in unit tests and samples, where I think it's
not a big deal. The PR fixes code in the SDK (one false positive simply
resolved using ICollection) and code that could be copied into other
projects.

Co-authored-by: Adrian Bonar <[email protected]>

dotnet/src/SemanticKernel/Memory/VolatileMemoryStore.cs

@@ -147,7 +147,7 @@ public class VolatileMemoryStore : IMemoryStore
             return AsyncEnumerable.Empty<(MemoryRecord, double)>();
         }
 
-        IEnumerable<MemoryRecord>? embeddingCollection = null;
+        ICollection<MemoryRecord>? embeddingCollection = null;
         if (this.TryGetCollection(collectionName, out var collectionDict))
         {
             embeddingCollection = collectionDict.Values;

samples/apps/copilot-chat-app/importdocument/Program.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System;
+using System.Collections.Generic;
 using System.CommandLine;
 using System.IO;
 using System.Linq;
@@ -67,14 +68,16 @@ public static class Program
                 .WithRedirectUri(config.RedirectUri)
                 .Build();
             var result = await app.AcquireTokenInteractive(scopes).ExecuteAsync();
-            var accounts = await app.GetAccountsAsync();
-            if (!accounts.Any())
+            IEnumerable<IAccount>? accounts = await app.GetAccountsAsync();
+            IAccount? first = accounts.FirstOrDefault();
+
+            if (first is null)
             {
                 Console.WriteLine("Error: No accounts found");
                 return null;
             }
 
-            return accounts.First().HomeAccountId.Identifier;
+            return first.HomeAccountId.Identifier;
         }
         catch (Exception ex) when (ex is MsalServiceException || ex is MsalClientException)
         {

samples/apps/copilot-chat-app/webapi/Controllers/ChatHistoryController.cs

@@ -130,22 +130,16 @@ public class ChatHistoryController : ControllerBase
         [FromQuery] int startIdx = 0,
         [FromQuery] int count = -1)
     {
+        // TODO: the code mixes strings and Guid without being explicit about the serialization format
         var chatMessages = await this._chatMessageRepository.FindByChatIdAsync(chatId.ToString());
         if (chatMessages == null)
         {
-            return this.NotFound($"No messages found for chat of id {chatId}.");
+            return this.NotFound($"No messages found for chat id '{chatId}'.");
         }
 
-        if (startIdx >= chatMessages.Count())
-        {
-            return this.BadRequest($"Start index {startIdx} is out of range.");
-        }
-        else if (startIdx + count > chatMessages.Count() || count == -1)
-        {
-            count = chatMessages.Count() - startIdx;
-        }
+        chatMessages = chatMessages.OrderByDescending(m => m.Timestamp).Skip(startIdx);
+        if (count >= 0) { chatMessages = chatMessages.Take(count); }
 
-        chatMessages = chatMessages.OrderByDescending(m => m.Timestamp).Skip(startIdx).Take(count);
         return this.Ok(chatMessages);
     }
 

samples/apps/copilot-chat-app/webapi/Storage/ChatMessageRepository.cs

@@ -28,12 +28,13 @@ public class ChatMessageRepository : Repository<ChatMessage>
 
     public async Task<ChatMessage> FindLastByChatIdAsync(string chatId)
     {
-        var messages = await this.FindByChatIdAsync(chatId);
-        if (!messages.Any())
+        var chatMessages = await this.FindByChatIdAsync(chatId);
+        var first = chatMessages.MaxBy(e => e.Timestamp);
+        if (first is null)
         {
-            throw new KeyNotFoundException($"No messages found for chat {chatId}.");
+            throw new KeyNotFoundException($"No messages found for chat '{chatId}'.");
         }
 
-        return messages.OrderByDescending(e => e.Timestamp).First();
+        return first;
     }
 }