public override Expression Visit(DbVariableReferenceExpression expression)
        {
            string name = expression.VariableName;
            Variable context = this.currentVariables.GetVariable(name);

            return context.Expression;
        }

 internal DbGroupExpressionBinding(
     DbExpression input, DbVariableReferenceExpression inputRef, DbVariableReferenceExpression groupRef)
 {
     _expr = input;
     _varRef = inputRef;
     _groupVarRef = groupRef;
 }

        internal DbExpressionBinding(DbExpression input, DbVariableReferenceExpression varRef)
        {
            DebugCheck.NotNull(input);
            DebugCheck.NotNull(varRef);

            _expr = input;
            _varRef = varRef;
        }

        internal DbExpressionBinding(DbExpression input, DbVariableReferenceExpression varRef)
        {
            Debug.Assert(input != null, "DbExpressionBinding input cannot be null");
            Debug.Assert(varRef != null, "DbExpressionBinding variable cannot be null");

            _expr = input;
            _varRef = varRef;
        }

        /// <summary>
        ///     Replace existing var at the head of the property chain with the new <paramref name="parentVarRef" />.
        /// </summary>
        internal void ReplaceParentVar(DbVariableReferenceExpression parentVarRef)
        {
            //
            // No parent var adjustment is allowed while adjusted to group var (see AdjustToGroupVar(...) for more info).
            //
            Debug.Assert(_groupVarBasedExpression == null, "_groupVarBasedExpression == null");
            Debug.Assert(_groupAggBasedExpression == null, "_groupAggBasedExpression == null");

            if (_propRefs == null)
            {
                Debug.Assert(_varBasedExpression is DbVariableReferenceExpression, "_varBasedExpression is DbVariableReferenceExpression");
                _varBasedExpression = parentVarRef;
            }
            else
            {
                Debug.Assert(_propRefs.Count > 0, "_propRefs.Count > 0");
                _propRefs.RemoveAt(_propRefs.Count - 1);
                AddParentVar(parentVarRef);
            }
        }

        /// <summary>
        ///     Prepend <paramref name="parentVarRef" /> to the property chain.
        /// </summary>
        internal SourceScopeEntry AddParentVar(DbVariableReferenceExpression parentVarRef)
        {
            //
            // No parent var adjustment is allowed while adjusted to group var (see AdjustToGroupVar(...) for more info).
            //
            Debug.Assert(_groupVarBasedExpression == null, "_groupVarBasedExpression == null");
            Debug.Assert(_groupAggBasedExpression == null, "_groupAggBasedExpression == null");

            if (_propRefs == null)
            {
                Debug.Assert(_varBasedExpression is DbVariableReferenceExpression, "_varBasedExpression is DbVariableReferenceExpression");
                _propRefs = new List<string>(2);
                _propRefs.Add(((DbVariableReferenceExpression)_varBasedExpression).VariableName);
            }

            _varBasedExpression = parentVarRef;
            for (var i = _propRefs.Count - 1; i >= 0; --i)
            {
                _varBasedExpression = _varBasedExpression.Property(_propRefs[i]);
            }
            _propRefs.Add(parentVarRef.VariableName);

            return this;
        }

 internal SourceScopeEntry(DbVariableReferenceExpression varRef)
     : this(varRef, null)
 {
 }

        /// <summary>
        ///     Rolls back the <see cref="AdjustToGroupVar" />(...) adjustment, clears the <see
        ///      cref="IGroupExpressionExtendedInfo.GroupVarBasedExpression" />.
        /// </summary>
        internal void RollbackAdjustmentToGroupVar(DbVariableReferenceExpression pregroupParentVarRef)
        {
            Debug.Assert(_groupVarBasedExpression != null, "_groupVarBasedExpression != null");

            _groupVarBasedExpression = null;
            _groupAggBasedExpression = null;
            ReplaceParentVar(pregroupParentVarRef);
        }

        /// <summary>
        ///     Rebuild the current scope entry expression as the property chain off the <paramref name="parentVarRef" /> expression.
        ///     Also build 
        ///     - <see cref="IGroupExpressionExtendedInfo.GroupVarBasedExpression" /> off the <paramref name="parentGroupVarRef" /> expression;
        ///     - <see cref="IGroupExpressionExtendedInfo.GroupAggBasedExpression" /> off the <paramref name="groupAggRef" /> expression.
        ///     This adjustment is reversable by <see cref="RollbackAdjustmentToGroupVar" />(...).
        /// </summary>
        internal void AdjustToGroupVar(
            DbVariableReferenceExpression parentVarRef, DbVariableReferenceExpression parentGroupVarRef,
            DbVariableReferenceExpression groupAggRef)
        {
            // Adjustment is not reentrant.
            Debug.Assert(_groupVarBasedExpression == null, "_groupVarBasedExpression == null");
            Debug.Assert(_groupAggBasedExpression == null, "_groupAggBasedExpression == null");

            //
            // Let's assume this entry represents variable "x" in the following query:
            //      select x, y, z from {1, 2} as x join {2, 3} as y on x = y join {3, 4} as z on y = z
            // In this case _propRefs contains x._##join0._##join1 and the corresponding input expression looks like this:
            //     |_Input : '_##join1'
            //     | |_InnerJoin
            //     |   |_Left : '_##join0'
            //     |   | |_InnerJoin
            //     |   |   |_Left : 'x'
            //     |   |   |_Right : 'y'
            //     |   |_Right : 'z'
            // When we start processing a group by, like in this query:
            //      select k1, k2, k3 from {1, 2} as x join {2, 3} as y on x = y join {3, 4} as z on y = z group by x as k1, y as k2, z as k3
            // we are switching to the following input expression:
            //     |_Input : '_##geb2', '_##group3'
            //     | |_InnerJoin
            //     |   |_Left : '_##join0'
            //     |   | |_InnerJoin
            //     |   |   |_Left : 'x'
            //     |   |   |_Right : 'y'
            //     |   |_Right : 'z'
            // where _##join1 is replaced by _##geb2 for the regular expression and by _##group3 for the group var based expression.
            // So the switch, or the adjustment, is done by 
            //      a. replacing _##join1 with _##geb2 in _propRefs and rebuilding the regular expression accordingly to get
            //         the following property chain: _##geb2._##join1.x
            //      b. building a group var based expression using _##group3 instead of _##geb2 to get
            //         the following property chain: _##group3._##join1.x
            //

            //
            // Rebuild ScopeEntry.Expression using the new parent var.
            //
            ReplaceParentVar(parentVarRef);

            //
            // Build the GroupVarBasedExpression and GroupAggBasedExpression, 
            // take into account that parentVarRef has already been added to the _propRefs in the AdjustToParentVar(...) call, so ignore it.
            //
            _groupVarBasedExpression = parentGroupVarRef;
            _groupAggBasedExpression = groupAggRef;
            if (_propRefs != null)
            {
                for (var i = _propRefs.Count - 2 /*ignore the parentVarRef*/; i >= 0; --i)
                {
                    _groupVarBasedExpression = _groupVarBasedExpression.Property(_propRefs[i]);
                    _groupAggBasedExpression = _groupAggBasedExpression.Property(_propRefs[i]);
                }
            }
        }

 /// <summary>
 ///     Visitor pattern method for <see cref="DbVariableReferenceExpression" />.
 /// </summary>
 /// <param name="expression"> The DbVariableReferenceExpression that is being visited. </param>
 /// <exception cref="ArgumentNullException">
 ///     <paramref name="expression" />
 ///     is null
 /// </exception>
 public override void Visit(DbVariableReferenceExpression expression)
 {
     // #433613: PreSharp warning 56506: Parameter 'expression' to this public method must be validated: A null-dereference can occur here.
     Check.NotNull(expression, "expression");
 }

 public override void Visit(DbVariableReferenceExpression expression) { }

 public override void Visit(DbVariableReferenceExpression e)
 {
     _key.Append("Var(");
     VisitVariableName(e.VariableName);
     _key.Append(")");
 }

 public override void Visit(DbVariableReferenceExpression expression)
 {
     Contract.Requires(expression != null);
 }

        private static DbVariableReferenceExpression[] CreateVariables(MethodInfo lambdaMethod, params TypeUsage[] argumentTypes)
        {
            Debug.Assert(lambdaMethod != null, "Lambda function method must not be null");
            var paramNames = DbExpressionBuilder.ExtractAliases(lambdaMethod);

            Debug.Assert(paramNames.Length == argumentTypes.Length, "Lambda function method parameter count does not match argument count");
            var result = new DbVariableReferenceExpression[argumentTypes.Length];
            for (var idx = 0; idx < paramNames.Length; idx++)
            {
                Debug.Assert(argumentTypes[idx] != null, "DbLambda.Create allowed null type argument");
                result[idx] = argumentTypes[idx].Variable(paramNames[idx]);
            }
            return result;
        }

 public override void Visit(DbVariableReferenceExpression e)
 {
     Dictionary<string, object> attrs = new Dictionary<string, object>();
     attrs.Add("VariableName", e.VariableName);
     Begin(e, attrs);
     End(e);
 }

 public override DbExpression Visit(DbVariableReferenceExpression expression)
 {
     return _binding.VariableType.Variable(_binding.VariableName);
 }

 /// <summary>
 /// Heuristic check to make sure the var ref is the one we're supposed to be replacing.
 /// </summary>
 private bool IsOuterBindingVarRef(DbVariableReferenceExpression varRef)
 {
     return varRef.VariableName == m_outerBinding.VariableName;
 }

 internal FreeVariableScopeEntry(DbVariableReferenceExpression varRef)
     : base(ScopeEntryKind.FreeVar)
 {
     _varRef = varRef;
 }

		public override void Visit(DbVariableReferenceExpression expression)
		{
			throw new NotSupportedException("Visit(\"VariableReferenceExpression\") is not supported.");
		}

        // <summary>
        // Rolls back the <see cref="AdjustToGroupVar" />(...) adjustment, clears the
        // <see
        //     cref="IGroupExpressionExtendedInfo.GroupVarBasedExpression" />
        // .
        // </summary>
        internal void RollbackAdjustmentToGroupVar(DbVariableReferenceExpression pregroupParentVarRef)
        {
            DebugCheck.NotNull(_groupVarBasedExpression);

            _groupVarBasedExpression = null;
            _groupAggBasedExpression = null;
            ReplaceParentVar(pregroupParentVarRef);
        }