LINQ To DataSet:联合查询

Student[] students = {  
  new Student { Id = 1, Name = "Joe Rattz" }, 
  new Student { Id = 7, Name = "Anthony Adams" }, 
  new Student { Id = 13, Name = "Stacy Sinclair" }, 
  new Student { Id = 72, Name = "Dignan Stephens" } 
}; 
 
StudentClass[] classDesignations = {  
  new StudentClass { Id = 1, Class = "Sophmore" }, 
  new StudentClass { Id = 7, Class = "Freshman" }, 
  new StudentClass { Id = 13, Class = "Graduate" }, 
  new StudentClass { Id = 72, Class = "Senior" } 
}; 
 
DataTable dt1 = GetDataTable(students); 
IEnumerable<DataRow> seq1 = dt1.AsEnumerable(); 
DataTable dt2 = GetDataTable2(classDesignations); 
IEnumerable<DataRow> seq2 = dt2.AsEnumerable(); 
 
string anthonysClass = (from s in seq1 
                        where s.Field<string>("Name") == "
                        from c in seq2 
                        where c["Id"] == s["Id"] 
                        select (string)c["Class"]). 
                       SingleOrDefault<string>(); 
 
Console.WriteLine("Anthony's Class is: {0}", 
  anthonysClass != null ? anthonysClass : "null"); 
 

输出

Anthony's Class is: null

这是为什么呢?因为这里发生的了装箱操作,

This is because of the boxing of the Id field when it is retrieved using the

DataRow indexer

在用DataRow索引器对Id字段进行检索的时候，该字段发生了装箱操作

导致join没有成功 "where c["Id"] == s["Id"]"

我们要做的就是给它拆箱，改为

where (int)c["Id"] == (int)s["Id"]

输出

Anthony's Class is: Freshman

上面人为的拆箱并不是很好的做法，所幸的是LiNQ为我们提供了泛型支持，看下面

Student[] students = {  
  new Student { Id = 1, Name = "Joe Rattz" }, 
  new Student { Id = 7, Name = "Anthony Adams" }, 
  new Student { Id = 13, Name = "Stacy Sinclair" }, 
  new Student { Id = 72, Name = "Dignan Stephens" } 
}; 
 
StudentClass[] classDesignations = {  
  new StudentClass { Id = 1, Class = "Sophmore" }, 
  new StudentClass { Id = 7, Class = "Freshman" }, 
  new StudentClass { Id = 13, Class = "Graduate" }, 
  new StudentClass { Id = 72, Class = "Senior" } 
}; 
 
DataTable dt1 = GetDataTable(students); 
IEnumerable<DataRow> seq1 = dt1.AsEnumerable(); 
DataTable dt2 = GetDataTable2(classDesignations); 
IEnumerable<DataRow> seq2 = dt2.AsEnumerable(); 
 
string anthonysClass = (from s in seq1 
                        where s.Field<string>("Name") == "Anthony Adams" 
                        from c in seq2 
                        where c.Field<int>("Id") == s.Field<int>("Id") 
                        select (string)c["Class"]). 
                       SingleOrDefault<string>(); 
 
Console.WriteLine("Anthony's Class is: {0}", 
  anthonysClass != null ? anthonysClass : "null"); 

注意，已经改成使用泛型的形式

where c.Field<int>("Id") == s.Field<int>("Id")