Linq语句 动态组建

       public override System.Linq.Expressions.Expression<Func<I_Position_Access, bool>> GetWhereLambda(I_Position_Access t)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(I_Position_Access), "b");//创建参数b
            System.Linq.Expressions.ConstantExpression constant = Expression.Constant(t.I_Position_AccessID);//创建常数 
            MemberExpression member = Expression.PropertyOrField(parameter, "I_Position_AccessID");

            System.Linq.Expressions.BinaryExpression bin =
                Expression.GreaterThan(member, constant);//创建 i > 常数 

            if (t.I_Position_AccessID > 0)
            {
                constant = Expression.Constant(t.I_Position_AccessID);
                member = Expression.PropertyOrField(parameter, "I_Position_AccessID");
                bin = Expression.Equal(member, constant);
            }
            if (t.I_PositionID > 0)
            {
                constant = Expression.Constant(Convert.ToInt32(t.I_PositionID), typeof(int?));
                member = Expression.PropertyOrField(parameter, "I_PositionID");
                System.Linq.Expressions.BinaryExpression where2 = Expression.Equal(member, constant);

                bin = Expression.And(bin, where2);
            }
            if (t.I_AccessID > 0)
            {
                constant = Expression.Constant(Convert.ToInt32(t.I_AccessID), typeof(int?));
                member = Expression.PropertyOrField(parameter, "I_AccessID");
                System.Linq.Expressions.BinaryExpression whereI_AccessID = Expression.Equal(member, constant);
                bin = Expression.And(bin, whereI_AccessID);
            }
            if (t.OpenState > 0)
            {
                constant = Expression.Constant(Convert.ToInt32(t.OpenState), typeof(int?));
                member = Expression.PropertyOrField(parameter, "OpenState");
                System.Linq.Expressions.BinaryExpression whereOpenState = Expression.Equal(member, constant);
                bin = Expression.And(bin, whereOpenState);
            }
            return Expression.Lambda<Func<I_Position_Access, bool>>(bin, parameter);
            //获取Lambda表达式
        }