Fix form key accumulator

src/Http/Headers/src/HeaderUtilities.cs

@@ -241,7 +241,7 @@ public static bool TryParseSeconds(StringValues headerValues, string targetValue
 
             for (var i = 0; i < headerValues.Count; i++)
             {
-                var segment = headerValues[i]!;
+                var segment = headerValues[i] ?? string.Empty;
 
                 // Trim leading white space
                 var current = HttpRuleParser.GetWhitespaceLength(segment, 0);
@@ -297,7 +297,7 @@ public static bool ContainsCacheDirective(StringValues cacheControlDirectives, s
 
             for (var i = 0; i < cacheControlDirectives.Count; i++)
             {
-                var segment = cacheControlDirectives[i]!;
+                var segment = cacheControlDirectives[i] ?? string.Empty;
 
                 // Trim leading white space
                 var current = HttpRuleParser.GetWhitespaceLength(segment, 0);

src/Http/Http/src/Internal/ResponseCookies.cs

@@ -192,7 +192,7 @@ public void Delete(string key, CookieOptions options)
 
                 for (var i = 0; i < values.Length; i++)
                 {
-                    var value = values[i]!;
+                    var value = values[i] ?? string.Empty;
                     if (!rejectPredicate(value, encodedKeyPlusEquals, options))
                     {
                         newValues.Add(value);

src/Http/WebUtilities/src/KeyValueAccumulator.cs

@@ -30,7 +30,7 @@ public void Append(string key, string value)
             StringValues values;
             if (_accumulator.TryGetValue(key, out values))
             {
-                if (StringValues.IsNullOrEmpty(values))
+                if (values.Count == 0)
                 {
                     // Marker entry for this key to indicate entry already in expanding list dictionary
                     _expandingAccumulator[key].Add(value);

src/Http/WebUtilities/test/FormPipeReaderTests.cs

@@ -1,15 +1,10 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System;
 using System.Buffers;
-using System.Collections.Generic;
-using System.IO;
 using System.IO.Pipelines;
 using System.Text;
-using System.Threading.Tasks;
 using Microsoft.Extensions.Primitives;
-using Xunit;
 
 namespace Microsoft.AspNetCore.WebUtilities
 {
@@ -567,6 +562,16 @@ public void ParseFormWithIncompleteValueWhenIsFinalBlockSucceeds(ReadOnlySequenc
             Assert.Equal("", values["b"]);
         }
 
+        [Fact]
+        public async Task ReadFormAsync_AccumulatesEmptyKeys()
+        {
+            var bodyPipe = await MakePipeReader("&&&");
+
+            var formCollection = await ReadFormAsync(new FormPipeReader(bodyPipe));
+
+            Assert.Single(formCollection);
+        }
+
         public static TheoryData<ReadOnlySequence<byte>> IncompleteFormKeys =>
             new TheoryData<ReadOnlySequence<byte>>
             {

src/Middleware/ResponseCaching/src/ResponseCachingKeyProvider.cs

@@ -120,7 +120,7 @@ public string CreateStorageVaryByKey(ResponseCachingContext context)
                     var requestHeaders = context.HttpContext.Request.Headers;
                     for (var i = 0; i < headersCount; i++)
                     {
-                        var header = varyByRules!.Headers[i]!;
+                        var header = varyByRules!.Headers[i] ?? string.Empty;
                         var headerValues = requestHeaders[header];
                         builder.Append(KeyDelimiter)
                             .Append(header)
@@ -174,7 +174,7 @@ public string CreateStorageVaryByKey(ResponseCachingContext context)
                     {
                         for (var i = 0; i < varyByRules.QueryKeys.Count; i++)
                         {
-                            var queryKey = varyByRules.QueryKeys[i]!;
+                            var queryKey = varyByRules.QueryKeys[i] ?? string.Empty;
                             var queryKeyValues = context.HttpContext.Request.Query[queryKey];
                             builder.Append(KeyDelimiter)
                                 .Append(queryKey)

src/Servers/IIS/IIS/src/Core/IISHttpContext.cs

@@ -431,13 +431,16 @@ public unsafe void SetResponseHeaders()
                 var knownHeaderIndex = HttpApiTypes.HTTP_RESPONSE_HEADER_ID.IndexOfKnownHeader(headerPair.Key);
                 for (var i = 0; i < headerValues.Count; i++)
                 {
-                    if (string.IsNullOrEmpty(headerValues[i]))
+                    var headerValue = headerValues[i];
+
+                    if (string.IsNullOrEmpty(headerValue))
                     {
                         continue;
                     }
 
                     var isFirst = i == 0;
-                    var headerValueBytes = Encoding.UTF8.GetBytes(headerValues[i]!);
+                    var headerValueBytes = Encoding.UTF8.GetBytes(headerValue);
+
                     fixed (byte* pHeaderValue = headerValueBytes)
                     {
                         if (knownHeaderIndex == -1)

src/Servers/Kestrel/Core/src/Internal/Http/HttpHeaders.cs

@@ -268,7 +268,7 @@ public static void ValidateHeaderValueCharacters(string headerName, StringValues
             var count = headerValues.Count;
             for (var i = 0; i < count; i++)
             {
-                ValidateHeaderValueCharacters(headerValues[i]!, requireAscii);
+                ValidateHeaderValueCharacters(headerValues[i] ?? string.Empty, requireAscii);
             }
         }
 

src/Shared/RangeHelper/RangeHelper.cs

@@ -45,7 +45,7 @@ public static (bool isRangeRequest, RangeItemHeaderValue? range) ParseRange(
             }
 
             // Perf: Check for a single entry before parsing it
-            if (rawRangeHeader.Count > 1 || rawRangeHeader[0]!.IndexOf(',') >= 0)
+            if (rawRangeHeader.Count > 1 || (rawRangeHeader[0] ?? string.Empty).IndexOf(',') >= 0)
             {
                 logger.LogDebug("Multiple ranges are not supported.");