如何將謂詞傳遞給linq表達式

Question

給定這樣的方法簽名...

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

如何將謂詞參數傳遞給linq表達式？

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

由於predicate是一個將DataRow作為輸入的函數-像where predicate(student)一樣使用它。

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

Answer 2

我想你要：

where predicate == null || predicate(student)