Imported Upstream version 4.0.0~alpha1

Former-commit-id: 806294f5ded97629b74c85c09952f2a74fe182d9

external/referencesource/System.Web/Util/Utf16StringValidator.cs

@@ -0,0 +1,76 @@
+//------------------------------------------------------------------------------
+// <copyright file="Utf16StringValidator.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>                                                                
+//------------------------------------------------------------------------------
+
+namespace System.Web.Util {
+    using System;
+
+    // This class contains utility methods for dealing with security contexts when crossing AppDomain boundaries.
+
+    internal static class Utf16StringValidator {
+
+        private const char UNICODE_NULL_CHAR = '\0';
+        private const char UNICODE_REPLACEMENT_CHAR = '\uFFFD';
+
+        private static readonly bool _skipUtf16Validation = AppSettings.AllowRelaxedUnicodeDecoding;
+
+        public static string ValidateString(string input) {
+            return ValidateString(input, _skipUtf16Validation);
+        }
+
+        // only internal for unit testing
+        internal static string ValidateString(string input, bool skipUtf16Validation) {
+            if (skipUtf16Validation || String.IsNullOrEmpty(input)) {
+                return input;
+            }
+
+            // locate the first surrogate character
+            int idxOfFirstSurrogate = -1;
+            for (int i = 0; i < input.Length; i++) {
+                if (Char.IsSurrogate(input[i])) {
+                    idxOfFirstSurrogate = i;
+                    break;
+                }
+            }
+
+            // fast case: no surrogates = return input string
+            if (idxOfFirstSurrogate < 0) {
+                return input;
+            }
+
+            // slow case: surrogates exist, so we need to validate them
+            char[] chars = input.ToCharArray();
+            for (int i = idxOfFirstSurrogate; i < chars.Length; i++) {
+                char thisChar = chars[i];
+
+                // If this character is a low surrogate, then it was not preceded by
+                // a high surrogate, so we'll replace it.
+                if (Char.IsLowSurrogate(thisChar)) {
+                    chars[i] = UNICODE_REPLACEMENT_CHAR;
+                    continue;
+                }
+
+                if (Char.IsHighSurrogate(thisChar)) {
+                    // If this character is a high surrogate and it is followed by a
+                    // low surrogate, allow both to remain.
+                    if (i + 1 < chars.Length && Char.IsLowSurrogate(chars[i + 1])) {
+                        i++; // skip the low surrogate also
+                        continue;
+                    }
+
+                    // If this character is a high surrogate and it is not followed
+                    // by a low surrogate, replace it.
+                    chars[i] = UNICODE_REPLACEMENT_CHAR;
+                    continue;
+                }
+
+                // Otherwise, this is a non-surrogate character and just move to the
+                // next character.
+            }
+            return new String(chars);
+        }
+
+    }
+}