Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Data.Entity/System/Data/SqlClient/SqlGen/SymbolPair.cs

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+//---------------------------------------------------------------------
+// <copyright file="SymbolPair.cs" company="Microsoft">
+//      Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>
+//
+// @owner  [....]
+// @backupOwner [....]
+//---------------------------------------------------------------------
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Text;
+using System.Data.SqlClient;
+using System.Data.Metadata.Edm;
+using System.Data.Common.CommandTrees;
+
+namespace System.Data.SqlClient.SqlGen
+{
+    /// <summary>
+    /// The SymbolPair exists to solve the record flattening problem.
+    /// <see cref="SqlGenerator.Visit(DbPropertyExpression)"/>
+    /// Consider a property expression D(v, "j3.j2.j1.a.x")
+    /// where v is a VarRef, j1, j2, j3 are joins, a is an extent and x is a columns.
+    /// This has to be translated eventually into {j'}.{x'}
+    /// 
+    /// The source field represents the outermost SqlStatement representing a join
+    /// expression (say j2) - this is always a Join symbol.
+    /// 
+    /// The column field keeps moving from one join symbol to the next, until it
+    /// stops at a non-join symbol.
+    /// 
+    /// This is returned by <see cref="SqlGenerator.Visit(DbPropertyExpression)"/>,
+    /// but never makes it into a SqlBuilder.
+    /// </summary>
+    class SymbolPair : ISqlFragment
+    {
+        public Symbol Source;
+        public Symbol Column;
+
+        public SymbolPair(Symbol source, Symbol column)
+        {
+            this.Source = source;
+            this.Column = column;
+        }
+
+        #region ISqlFragment Members
+
+        public void WriteSql(SqlWriter writer, SqlGenerator sqlGenerator)
+        {
+            // Symbol pair should never be part of a SqlBuilder.
+            Debug.Assert(false);
+        }
+
+        #endregion
+    }
+}