Handle 32bits Unicode escaped sequences

Author: mlaily

YamlDotNet/Core/Emitter.cs

@@ -1160,12 +1160,25 @@ private void WriteDoubleQuotedScalar(string value, bool allowBreaks)
                             break;
 
                         default:
-                            var code = (short)character;
+                            var code = (ushort)character;
                             if (code <= 0xFF)
                             {
                                 Write('x');
                                 Write(code.ToString("X02", CultureInfo.InvariantCulture));
                             }
+                            else if (IsHighSurrogate(character))
+                            {
+                                if (index + 1 < value.Length && IsLowSurrogate(value[index + 1]))
+                                {
+                                    Write('U');
+                                    Write(char.ConvertToUtf32(character, value[index + 1]).ToString("X08", CultureInfo.InvariantCulture));
+                                    index++;
+                                }
+                                else
+                                {
+                                    throw new SyntaxErrorException("While writing a quoted scalar, found an orphaned high surrogate.");
+                                }
+                            }
                             else
                             {
                                 Write('u');
@@ -1342,6 +1355,10 @@ private static bool IsPrintable(char character)
                     (character >= '\xE000' && character <= '\xFFFD');
         }
 
+        private static bool IsHighSurrogate(char c) => 0xD800 <= c && c <= 0xDBFF;
+
+        private static bool IsLowSurrogate(char c) => 0xDC00 <= c && c <= 0xDFFF;
+
         #endregion
 
         /// <summary>