--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Source/Data/Linq/Builder/DefaultIfEmptyBuilder.cs	Thu Mar 27 21:46:09 2014 +0400
@@ -0,0 +1,135 @@
+using System;
+using System.Linq;
+using System.Linq.Expressions;
+
+namespace BLToolkit.Data.Linq.Builder
+{
+	using BLToolkit.Linq;
+	using Data.Sql;
+
+	class DefaultIfEmptyBuilder : MethodCallBuilder
+	{
+		protected override bool CanBuildMethodCall(ExpressionBuilder builder, MethodCallExpression methodCall, BuildInfo buildInfo)
+		{
+			return methodCall.IsQueryable("DefaultIfEmpty");
+		}
+
+		protected override IBuildContext BuildMethodCall(ExpressionBuilder builder, MethodCallExpression methodCall, BuildInfo buildInfo)
+		{
+			var sequence     = builder.BuildSequence(new BuildInfo(buildInfo, methodCall.Arguments[0]));
+			var defaultValue = methodCall.Arguments.Count == 1 ? null : methodCall.Arguments[1].Unwrap();
+
+			if (buildInfo.Parent is SelectManyBuilder.SelectManyContext)
+			{
+				var groupJoin = ((SelectManyBuilder.SelectManyContext)buildInfo.Parent).Sequence[0] as JoinBuilder.GroupJoinContext;
+
+				if (groupJoin != null)
+				{
+					groupJoin.SqlQuery.From.Tables[0].Joins[0].JoinType = SqlQuery.JoinType.Left;
+					groupJoin.SqlQuery.From.Tables[0].Joins[0].IsWeak   = false;
+				}
+			}
+
+			return new DefaultIfEmptyContext(buildInfo.Parent, sequence, defaultValue);
+		}
+
+		protected override SequenceConvertInfo Convert(
+			ExpressionBuilder builder, MethodCallExpression methodCall, BuildInfo buildInfo, ParameterExpression param)
+		{
+			return null;
+		}
+
+		public class DefaultIfEmptyContext : SequenceContextBase
+		{
+			public DefaultIfEmptyContext(IBuildContext parent, IBuildContext sequence, Expression defaultValue) 
+				: base(parent, sequence, null)
+			{
+				_defaultValue = defaultValue;
+			}
+
+			private readonly Expression _defaultValue;
+
+			public override Expression BuildExpression(Expression expression, int level)
+			{
+				var expr = Sequence.BuildExpression(expression, level);
+
+				if (expression == null)
+				{
+					var q =
+						from col in SqlQuery.Select.Columns
+						where !col.CanBeNull()
+						select SqlQuery.Select.Columns.IndexOf(col);
+
+					var idx = q.DefaultIfEmpty(-1).First();
+
+					if (idx == -1)
+						idx = SqlQuery.Select.Add(new SqlValue((int?) 1));
+
+					var n = ConvertToParentIndex(idx, this);
+
+					var e = Expression.Call(
+						ExpressionBuilder.DataReaderParam,
+						ReflectionHelper.DataReader.IsDBNull,
+						Expression.Constant(n)) as Expression;
+
+					var defaultValue = _defaultValue ?? Expression.Constant(null, expr.Type);
+
+#if FW4 || SILVERLIGHT
+
+					if (expr.NodeType == ExpressionType.Parameter)
+					{
+						var par  = (ParameterExpression)expr;
+						var pidx = Builder.BlockVariables.IndexOf(par);
+
+						if (pidx >= 0)
+						{
+							var ex = Builder.BlockExpressions[pidx];
+
+							if (ex.NodeType == ExpressionType.Assign)
+							{
+								var bex = (BinaryExpression)ex;
+
+								if (bex.Left == expr)
+								{
+									if (bex.Right.NodeType != ExpressionType.Conditional)
+									{
+										Builder.BlockExpressions[pidx] =
+											Expression.Assign(
+											bex.Left,
+											Expression.Condition(e, defaultValue, bex.Right));
+									}
+								}
+							}
+						}
+					}
+
+#endif
+
+					expr = Expression.Condition(e, defaultValue, expr);
+				}
+
+				return expr;
+			}
+
+			public override SqlInfo[] ConvertToSql(Expression expression, int level, ConvertFlags flags)
+			{
+				return Sequence.ConvertToSql(expression, level, flags);
+			}
+
+			public override SqlInfo[] ConvertToIndex(Expression expression, int level, ConvertFlags flags)
+			{
+				return Sequence.ConvertToIndex(expression, level, flags);
+			}
+
+			public override IsExpressionResult IsExpression(Expression expression, int level, RequestFor requestFlag)
+			{
+				return Sequence.IsExpression(expression, level, requestFlag);
+			}
+
+			public override IBuildContext GetContext(Expression expression, int level, BuildInfo buildInfo)
+			{
+				return Sequence.GetContext(expression, level, buildInfo);
+			}
+		}
+	}
+}