Fix issue with not resetting valueLength (#78890)

* Fix issue with not resetting valueLength

#78881 was just merged with a small bug, in that valueLength wasn't reset before it was used a second time. If the typeName is > 64 chars, then this would have thrown a different exception than it should have down a couple lines.

Author: ToddGrun

src/LanguageServer/Protocol/Protocol/Internal/Converters/ImageElementConverter.cs

@@ -33,10 +33,7 @@ public override ImageElement Read(ref Utf8JsonReader reader, Type typeToConvert,
 
                 if (reader.TokenType == JsonTokenType.PropertyName)
                 {
-                    var valueLength = reader.HasValueSequence ? reader.ValueSequence.Length : reader.ValueSpan.Length;
-
-                    var propertyNameLength = valueLength <= scratchChars.Length ? reader.CopyString(scratchChars) : -1;
-                    var propertyName = propertyNameLength >= 0 ? scratchChars[..propertyNameLength] : reader.GetString().AsSpan();
+                    var propertyName = reader.GetStringSpan(scratchChars);
 
                     reader.Read();
                     switch (propertyName)
@@ -48,10 +45,9 @@ public override ImageElement Read(ref Utf8JsonReader reader, Type typeToConvert,
                             automationName = reader.GetString();
                             break;
                         case ObjectContentConverter.TypeProperty:
-                            var typePropertyLength = valueLength <= scratchChars.Length ? reader.CopyString(scratchChars) : -1;
-                            var typeProperty = typePropertyLength >= 0 ? scratchChars[..typePropertyLength] : reader.GetString().AsSpan();
+                            var typePropertyValue = reader.GetStringSpan(scratchChars);
 
-                            if (!typeProperty.SequenceEqual(nameof(ImageElement).AsSpan()))
+                            if (!typePropertyValue.SequenceEqual(nameof(ImageElement).AsSpan()))
                                 throw new JsonException($"Expected {ObjectContentConverter.TypeProperty} property value {nameof(ImageElement)}");
                             break;
                         default:

src/LanguageServer/Protocol/Protocol/Internal/Converters/ImageIdConverter.cs

@@ -34,10 +34,7 @@ public override ImageId Read(ref Utf8JsonReader reader, Type objectType, JsonSer
 
                 if (reader.TokenType == JsonTokenType.PropertyName)
                 {
-                    var valueLength = reader.HasValueSequence ? reader.ValueSequence.Length : reader.ValueSpan.Length;
-
-                    var propertyNameLength = valueLength <= scratchChars.Length ? reader.CopyString(scratchChars) : -1;
-                    var propertyName = propertyNameLength >= 0 ? scratchChars[..propertyNameLength] : reader.GetString().AsSpan();
+                    var propertyName = reader.GetStringSpan(scratchChars);
 
                     reader.Read();
                     switch (propertyName)
@@ -49,10 +46,9 @@ public override ImageId Read(ref Utf8JsonReader reader, Type objectType, JsonSer
                             id = reader.GetInt32();
                             break;
                         case ObjectContentConverter.TypeProperty:
-                            var typePropertyLength = valueLength <= scratchChars.Length ? reader.CopyString(scratchChars) : -1;
-                            var typeProperty = typePropertyLength >= 0 ? scratchChars[..typePropertyLength] : reader.GetString().AsSpan();
+                            var typePropertyValue = reader.GetStringSpan(scratchChars);
 
-                            if (!typeProperty.SequenceEqual(nameof(ImageId).AsSpan()))
+                            if (!typePropertyValue.SequenceEqual(nameof(ImageId).AsSpan()))
                                 throw new JsonException($"Expected {ObjectContentConverter.TypeProperty} property value {nameof(ImageId)}");
                             break;
                         default:

src/LanguageServer/Protocol/Protocol/Internal/Converters/VSInternalExtensionUtilities.cs

@@ -2,6 +2,7 @@
 // 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;
 using System.Collections.Generic;
 using System.Text.Json;
 using System.Text.Json.Serialization;
@@ -71,4 +72,26 @@ void AddOrReplaceConverter<TBase, TExtension>()
             converters.Add(new VSExtensionConverter<TBase, TExtension>());
         }
     }
+
+    /// <summary>
+    /// Returns a string read from the <see cref="Utf8JsonReader"/>. If the string is small enough
+    /// to fit into the provided <paramref name="scratchChars"/>, no allocations will be needed.
+    /// </summary>
+    internal static ReadOnlySpan<char> GetStringSpan(this ref readonly Utf8JsonReader reader, Span<char> scratchChars)
+    {
+        var valueLength = reader.HasValueSequence ? reader.ValueSequence.Length : reader.ValueSpan.Length;
+
+        if (valueLength <= scratchChars.Length)
+        {
+            // If the value fits into the scratch buffer, copy it there, and return a span from that.
+            var actualLength = reader.CopyString(scratchChars);
+
+            return scratchChars.Slice(0, actualLength);
+        }
+        else
+        {
+            // Otherwise, ask the reader to allocate a string and return a span from that.
+            return reader.GetString().AsSpan();
+        }
+    }
 }