Question

给出类似的方法签名......

IEnumerable<Student> get(Func<DataRow, bool> predicate);

如何将谓词参数传递给linq表达式？

public override IEnumerable<Student> get(Func<System.Data.DataRow, bool> predicate = null) {
    var students = from student in _dataSet.Tables[0].AsEnumerable()
              join attendance in _dataSet.Tables[1].AsEnumerable() 
              on student.Field<int>("id") equals attendance.Field<int>("idStudent")
              join grade in _dataSet.Tables[2].AsEnumerable()
              on student.Field<int>("id") equals grade.Field<int>("idStudent")
              join assignment in _dataSet.Tables[3].AsEnumerable()
              on student.Field<int>("id") equals assignment.Field<int>("idStudent")
              // where predicate??
              // select blahblahblah
    // return statement

}

Answer 1

由于predicate是一个以DataRow为输入的函数，因此请使用where predicate(student)。

var students = from student in _dataSet.Tables[0].AsEnumerable()
    ...
    where predicate == null || predicate(student)
    select student

Answer 2

我想你想要：

where predicate == null || predicate(student)

如何将谓词传递给linq表达式

2 个答案: