Performance optimizations

- Calculate age using operations on long
- Compute content length of resposne on store
- Format age using the new HeaderUtility
- Lazily create HeaderDictionary
- Use for instead of foreach to reduce allocations from enumerators

src/Microsoft.AspNetCore.ResponseCaching/Internal/CacheEntry/CachedResponse.cs

@@ -13,7 +13,7 @@ namespace Microsoft.AspNetCore.ResponseCaching.Internal
 
         public int StatusCode { get; set; }
 
-        public IHeaderDictionary Headers { get; set; } = new HeaderDictionary();
+        public IHeaderDictionary Headers { get; set; }
 
         public Stream Body { get; set; }
     }

src/Microsoft.AspNetCore.ResponseCaching/Internal/MemoryResponseCache.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Threading.Tasks;
 using Microsoft.Extensions.Caching.Memory;
+using Microsoft.Net.Http.Headers;
 
 namespace Microsoft.AspNetCore.ResponseCaching.Internal
 {

src/Microsoft.AspNetCore.ResponseCaching/Internal/ResponseCachingKeyProvider.cs

@@ -107,13 +107,18 @@ namespace Microsoft.AspNetCore.ResponseCaching.Internal
                     builder.Append(KeyDelimiter)
                         .Append('H');
 
-                    foreach (var header in varyByRules.Headers)
+                    for (var i = 0; i < varyByRules.Headers.Count; i++)
                     {
+                        var header = varyByRules.Headers[i];
+                        var headerValues = context.HttpContext.Request.Headers[header];
                         builder.Append(KeyDelimiter)
                             .Append(header)
-                            .Append("=")
-                            // TODO: Perf - iterate the string values instead?
-                            .Append(context.HttpContext.Request.Headers[header]);
+                            .Append("=");
+
+                        for (var j = 0; j < headerValues.Count; j++)
+                        {
+                            builder.Append(headerValues[j]);
+                        }
                     }
                 }
 
@@ -131,19 +136,28 @@ namespace Microsoft.AspNetCore.ResponseCaching.Internal
                         {
                             builder.Append(KeyDelimiter)
                                 .AppendUpperInvariant(query.Key)
-                                .Append("=")
-                                .Append(query.Value);
+                                .Append("=");
+
+                            for (var i = 0; i < query.Value.Count; i++)
+                            {
+                                builder.Append(query.Value[i]);
+                            }
                         }
                     }
                     else
                     {
-                        foreach (var queryKey in varyByRules.QueryKeys)
+                        for (var i = 0; i < varyByRules.QueryKeys.Count; i++)
                         {
+                            var queryKey = varyByRules.QueryKeys[i];
+                            var queryKeyValues = context.HttpContext.Request.Query[queryKey];
                             builder.Append(KeyDelimiter)
                                 .Append(queryKey)
-                                .Append("=")
-                                // TODO: Perf - iterate the string values instead?
-                                .Append(context.HttpContext.Request.Query[queryKey]);
+                                .Append("=");
+
+                            for (var j = 0; j < queryKeyValues.Count; j++)
+                            {
+                                builder.Append(queryKeyValues[j]);
+                            }
                         }
                     }
                 }

src/Microsoft.AspNetCore.ResponseCaching/ResponseCachingMiddleware.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Globalization;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
@@ -142,18 +141,15 @@ namespace Microsoft.AspNetCore.ResponseCaching
                         response.Headers.Add(header);
                     }
 
-                    response.Headers[HeaderNames.Age] = context.CachedEntryAge.Value.TotalSeconds.ToString("F0", CultureInfo.InvariantCulture);
+                    // Note: int64 division truncates result and errors may be up to 1 second. This reduction in
+                    // accuracy of age calculation is considered appropriate since it is small compared to clock
+                    // skews and the "Age" header is an estimate of the real age of cached content.
+                    response.Headers[HeaderNames.Age] = HeaderUtilities.FormatInt64(context.CachedEntryAge.Value.Ticks / TimeSpan.TicksPerSecond);
 
                     // Copy the cached response body
                     var body = context.CachedResponse.Body;
                     if (body.Length > 0)
                     {
-                        // Add a content-length if required
-                        if (!response.ContentLength.HasValue && StringValues.IsNullOrEmpty(response.Headers[HeaderNames.TransferEncoding]))
-                        {
-                            response.ContentLength = body.Length;
-                        }
-
                         try
                         {
                             await body.CopyToAsync(response.Body, StreamUtilities.BodySegmentSize, context.HttpContext.RequestAborted);
@@ -263,7 +259,8 @@ namespace Microsoft.AspNetCore.ResponseCaching
                 context.CachedResponse = new CachedResponse
                 {
                     Created = context.ResponseDate.Value,
-                    StatusCode = context.HttpContext.Response.StatusCode
+                    StatusCode = context.HttpContext.Response.StatusCode,
+                    Headers = new HeaderDictionary()
                 };
 
                 foreach (var header in context.HttpContext.Response.Headers)
@@ -288,6 +285,13 @@ namespace Microsoft.AspNetCore.ResponseCaching
                 var bufferStream = context.ResponseCachingStream.GetBufferStream();
                 if (!contentLength.HasValue || contentLength == bufferStream.Length)
                 {
+                    var response = context.HttpContext.Response;
+                    // Add a content-length if required
+                    if (!response.ContentLength.HasValue && StringValues.IsNullOrEmpty(response.Headers[HeaderNames.TransferEncoding]))
+                    {
+                        context.CachedResponse.Headers[HeaderNames.ContentLength] = HeaderUtilities.FormatInt64(bufferStream.Length);
+                    }
+
                     context.CachedResponse.Body = bufferStream;
                     _logger.LogResponseCached();
                     await _cache.SetAsync(context.StorageVaryKey ?? context.BaseKey, context.CachedResponse, context.CachedResponseValidFor);
@@ -371,8 +375,9 @@ namespace Microsoft.AspNetCore.ResponseCaching
                     && EntityTagHeaderValue.TryParse(cachedResponseHeaders[HeaderNames.ETag], out eTag)
                     && EntityTagHeaderValue.TryParseList(ifNoneMatchHeader, out ifNoneMatchEtags))
                 {
-                    foreach (var requestETag in ifNoneMatchEtags)
+                    for (var i = 0; i < ifNoneMatchEtags.Count; i++)
                     {
+                        var requestETag = ifNoneMatchEtags[i];
                         if (eTag.Compare(requestETag, useStrongComparison: false))
                         {
                             context.Logger.LogNotModifiedIfNoneMatchMatched(requestETag);

test/Microsoft.AspNetCore.ResponseCaching.Tests/ResponseCachingMiddlewareTests.cs

@@ -61,6 +61,7 @@ namespace Microsoft.AspNetCore.ResponseCaching.Tests
                 "BaseKey",
                 new CachedResponse()
                 {
+                    Headers = new HeaderDictionary(),
                     Body = new SegmentReadStream(new List<byte[]>(0), 0)
                 },
                 TimeSpan.Zero);
@@ -108,6 +109,7 @@ namespace Microsoft.AspNetCore.ResponseCaching.Tests
                 "BaseKeyVaryKey2",
                 new CachedResponse()
                 {
+                    Headers = new HeaderDictionary(),
                     Body = new SegmentReadStream(new List<byte[]>(0), 0)
                 },
                 TimeSpan.Zero);
@@ -666,7 +668,10 @@ namespace Microsoft.AspNetCore.ResponseCaching.Tests
             await context.HttpContext.Response.WriteAsync(new string('0', 10));
 
             context.ShouldCacheResponse = true;
-            context.CachedResponse = new CachedResponse();
+            context.CachedResponse = new CachedResponse()
+            {
+                Headers = new HeaderDictionary()
+            };
             context.BaseKey = "BaseKey";
             context.CachedResponseValidFor = TimeSpan.FromSeconds(10);