Question

如何为聚合类构建Linq表达式？让我解释一下我想要做的事情。说，我想找到薪水不等于50000的所有员工，这里是班级结构。

    Employee e1 = new Employee { Name = "Jane", PDetail = new PayDetail { Salary = 100000 } };
    Employee e2 = new Employee { Name = "Joe", PDetail = new PayDetail { Salary = 50000 } };

    emps.Add(e1);
    emps.Add(e2);

//I started doing it this way and this code DOES NOT compile

var parameter = Expression.Parameter(typeof(PayDetail));

var where = Expression.NotEqual(Expression.Property(parameter, "Salary"), Expression.Constant(50000));

var pred = Expression.Lambda(where, parameter);

var query = Enumerable.Where(emps, (Func<PayDetail, Boolean>)pred.Compile());

编辑：有关null问题的任何帮助（如评论中所述）？感谢

Answer 1

您没有将正确的参数传递给Enumerable.Where方法。您正在传递Employee列表以及适用于PayDetail的方法。你可以用

var query = Enumerable
    .Where(emps.Select(e => e.PDetail), (Func<PayDetail, Boolean>)pred.Compile());

或更短的版本

var query = emps.Select(e => e.PDetail)
    .Where((Func<PayDetail, Boolean>)pred.Compile());

哪个会为您提供正确的PayDetails，而不是正确的Employee s'。我相信你真的在寻找：

var parameter = Expression.Parameter(typeof(Employee));

var where = Expression.NotEqual(Expression.Property(
    Expression.Property(parameter, "PDetail"), "Salary"), 
        Expression.Constant(50000));

var pred = Expression.Lambda(where, parameter);

var query = emps.Where((Func<Employee, Boolean>)pred.Compile());

Linq表达式和聚合类

1 个答案: