Append directly to the request headers StringBuilder

Avoid intermediate allocations associated with string.Format and
string.Join when building up the request headers buffer by appending
directly to the StringBuilder in WinHttpWebSocket.


Commit migrated from https://github.com/dotnet/corefx/commit/c1ee50f7f03833e782e0d7f30f8d89ffe4e709fa

src/libraries/System.Net.WebSockets.Client/src/System/Net/WebSockets/WinHttpWebSocket.cs

@@ -2,14 +2,13 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Collections.Generic;
+using System.Diagnostics;
 using System.Net.Http;
+using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
-using System.ComponentModel;
-using System.Runtime.InteropServices;
-using System.Diagnostics;
-using System.Globalization;
 
 namespace System.Net.WebSockets
 {
@@ -590,23 +589,29 @@ private void AddRequestHeaders(Uri uri, ClientWebSocketOptions options)
             // Manually add cookies.
             if (options.Cookies != null)
             {
-                string cookieHeader = GetCookieHeader(uri, options.Cookies);
-                if (!string.IsNullOrEmpty(cookieHeader))
-                {
-                    requestHeadersBuffer.AppendLine(cookieHeader);
-                }
+                AppendCookieHeaderLine(uri, options.Cookies, requestHeadersBuffer);
             }
 
             // Serialize general request headers.
             requestHeadersBuffer.AppendLine(options.RequestHeaders.ToString());
 
-            var subProtocols = options.RequestedSubProtocols;
-            if (subProtocols.Count > 0)
+            using (List<string>.Enumerator e = options.RequestedSubProtocols.GetEnumerator())
             {
-                requestHeadersBuffer.AppendLine(string.Format("{0}: {1}", HeaderNameWebSocketProtocol,
-                    string.Join(", ", subProtocols)));
+                if (e.MoveNext())
+                {
+                    requestHeadersBuffer.Append(HeaderNameWebSocketProtocol + ": ");
+                    requestHeadersBuffer.Append(e.Current);
+
+                    while (e.MoveNext())
+                    {
+                        requestHeadersBuffer.Append(", ");
+                        requestHeadersBuffer.Append(e.Current);
+                    }
+
+                    requestHeadersBuffer.AppendLine();
+                }
             }
-            
+
             // Add request headers to WinHTTP request handle.
             if (!Interop.WinHttp.WinHttpAddRequestHeaders(
                 _operation.RequestHandle,
@@ -618,19 +623,17 @@ private void AddRequestHeaders(Uri uri, ClientWebSocketOptions options)
             }
         }
 
-        private static string GetCookieHeader(Uri uri, CookieContainer cookies)
+        private static void AppendCookieHeaderLine(Uri uri, CookieContainer cookies, StringBuilder requestHeadersBuffer)
         {
-            string cookieHeader = null;
-
             Debug.Assert(cookies != null);
+            Debug.Assert(requestHeadersBuffer != null);
 
             string cookieValues = cookies.GetCookieHeader(uri);
             if (!string.IsNullOrEmpty(cookieValues))
             {
-                cookieHeader = string.Format(CultureInfo.InvariantCulture, "{0}: {1}", HeaderNameCookie, cookieValues);
+                requestHeadersBuffer.Append(HeaderNameCookie + ": ");
+                requestHeadersBuffer.AppendLine(cookieValues);
             }
-
-            return cookieHeader;
         }
 
         private HttpStatusCode GetHttpStatusCode()