Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Web/Util/DoubleLink.cs

@@ -0,0 +1,65 @@
+//------------------------------------------------------------------------------
+// <copyright file="DoubleLink.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>                                                                
+//------------------------------------------------------------------------------
+
+/*
+ * DoubleLink
+ * 
+ * Copyright (c) 1998-1999, Microsoft Corporation
+ * 
+ */
+
+namespace System.Web.Util {
+    using System.Runtime.Serialization.Formatters;
+
+    internal class DoubleLink {
+        internal DoubleLink    _next, _prev;
+        internal Object           Item;
+
+        internal DoubleLink() {
+            _next = _prev = this;
+        }
+        internal DoubleLink(Object item)  : this() {
+            this.Item = item;
+        }
+        internal DoubleLink Next {get {return _next;}}
+
+        internal void InsertAfter(DoubleLink after) {
+            this._prev = after;
+            this._next = after._next;
+            after._next = this;
+            this._next._prev = this;
+        }
+
+        internal void InsertBefore(DoubleLink before) {
+            this._prev = before._prev;
+            this._next = before;
+            before._prev = this;
+            this._prev._next = this;
+        }
+
+        internal void Remove() {
+            this._prev._next = this._next;
+            this._next._prev = this._prev;
+            _next = _prev = this;
+        }
+
+#if DBG
+        internal virtual void DebugValidate() {
+            Debug.CheckValid(this._next != this || this._prev == this, "Invalid link");
+        }
+
+        internal virtual string DebugDescription(string indent) {
+            string desc;
+
+            desc = indent + "_next=" + _next + ", _prev=" + _prev + "\nItem=";
+            desc += Debug.GetDescription(Item, indent + "    ");
+
+            return desc;
+        }
+#endif
+    }
+
+}