Preserve parsed & raw header value ordering (#67227)

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeaders.cs

@@ -1162,8 +1162,8 @@ internal static void GetStoreValuesAsStringOrStringArray(HeaderDescriptor descri
             }
 
             int currentIndex = 0;
-            ReadStoreValues<string?>(values, info.RawValue, null, ref currentIndex);
             ReadStoreValues<object?>(values, info.ParsedAndInvalidValues, descriptor.Parser, ref currentIndex);
+            ReadStoreValues<string?>(values, info.RawValue, null, ref currentIndex);
 
             Debug.Assert(currentIndex == length);
         }
@@ -1196,8 +1196,8 @@ internal static int GetStoreValuesIntoStringArray(HeaderDescriptor descriptor, o
                 }
 
                 int currentIndex = 0;
-                ReadStoreValues<string?>(values, info.RawValue, null, ref currentIndex);
                 ReadStoreValues<object?>(values, info.ParsedAndInvalidValues, descriptor.Parser, ref currentIndex);
+                ReadStoreValues<string?>(values, info.RawValue, null, ref currentIndex);
                 Debug.Assert(currentIndex == length);
             }
 
@@ -1208,9 +1208,7 @@ private static int GetValueCount(HeaderStoreItemInfo info)
         {
             Debug.Assert(info != null);
 
-            int valueCount = Count<string>(info.RawValue);
-            valueCount += Count<object>(info.ParsedAndInvalidValues);
-            return valueCount;
+            return Count<object>(info.ParsedAndInvalidValues) + Count<string>(info.RawValue);
 
             static int Count<T>(object? valueStore) =>
                 valueStore is null ? 0 :

src/libraries/System.Net.Http/tests/UnitTests/Headers/HttpHeadersTest.cs

@@ -51,6 +51,25 @@ public void TryAddWithoutValidation_ValidAndInvalidValues_InsertionOrderIsPreser
             Assert.Equal(new[] { "text/bar", "invalid", "text/baz" }, accept);
         }
 
+        [Fact]
+        public void MixedAddAndTryAddWithoutValidation_ParsedAndRawValues_InsertionOrderIsPreserved()
+        {
+            HttpHeaders headers = new HttpRequestMessage().Headers;
+
+            headers.Add("Accept", "text/foo");
+            headers.TryAddWithoutValidation("Accept", "text/bar");
+            headers.TryAddWithoutValidation("Accept", "invalid");
+
+            var expectedValues = new string[] { "text/foo", "text/bar", "invalid" };
+
+            Assert.Equal(expectedValues, headers.NonValidated["Accept"]);
+
+            Assert.True(headers.TryGetValues("Accept", out IEnumerable<string>? values));
+            Assert.Equal(expectedValues, values);
+
+            Assert.Equal(expectedValues, headers.NonValidated["Accept"]);
+        }
+
         [Theory]
         [InlineData(null)]
         [InlineData("")]
@@ -1431,7 +1450,7 @@ public void NonValidated_SetValidAndInvalidHeaderValues_AllHeaderValuesReturned(
             headers.TryAddWithoutValidation(headers.Descriptor, "value2,value3");
             headers.TryAddWithoutValidation(headers.Descriptor, invalidHeaderValue);
 
-            string expectedValue = "value2,value3---" + invalidHeaderValue + "---" + parsedPrefix + "1";
+            string expectedValue = $"{parsedPrefix}1---value2,value3---{invalidHeaderValue}";
 
             Assert.Equal(1, headers.NonValidated.Count);