Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System/misc/CompatibleIComparer.cs

@@ -0,0 +1,57 @@
+//------------------------------------------------------------------------------
+// <copyright file="CompatibleComparer.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>                                                                
+//------------------------------------------------------------------------------
+
+/*
+ * This class is used to create hashcodes that are Everett Compatible.
+ * 
+ * Copyright (c) 2004 Microsoft Corporation
+ */
+
+namespace System.Collections.Specialized {
+
+    using Microsoft.Win32;
+    using System.Collections;
+    using System.Runtime.Serialization;
+    using System.Globalization;
+
+    internal class BackCompatibleStringComparer : IEqualityComparer {
+
+        static internal IEqualityComparer Default = new BackCompatibleStringComparer();
+
+        internal BackCompatibleStringComparer() {
+        }
+
+        //This comes from VS# 434837 and is specifically written to get backcompat
+        public static int GetHashCode(string obj) {
+            unsafe {
+                fixed (char* src = obj) {
+                    int hash = 5381;
+                    int c;
+                    char* szStr = src;
+
+                    while ((c = *szStr) != 0) {
+                        hash = ((hash << 5) + hash) ^ c;
+                        ++szStr;
+                    }
+                    return hash;
+                }
+            }
+        }
+
+        bool IEqualityComparer.Equals(Object a, Object b) {
+            return Object.Equals(a, b);
+        }
+
+        public virtual int GetHashCode(Object o) {
+            String obj = o as string;
+            if (obj == null) {
+                return o.GetHashCode();
+            }
+
+            return BackCompatibleStringComparer.GetHashCode(obj);
+        }
+    }
+}