LINQ提供的一些函数操作

class Doctor
...{
public string Name;
public int Initials;
}
public static bool dump(Doctor d)
...{
System.Console.WriteLine(d.Initials);
return true;
}
Doctor[] doctors=...{new Doctor...{Name="Mary",Initials=123},
new Doctor...{Name="Jack",Initials=456},
new Doctor...{Name="Maggie",Initials=789}
};
var query = from d in doctors
where dump(d)
select d;

foreach(var q in query);

//查询是否有一个名字叫Lary的医生，有返回true，没有返回false

bool inLakeForest = doctors.Any(doc => doc.Name == "Lary");
//等价于
var query = from doc in doctors
where doc.Name == "Lary"
select doc;
bool inLakeForest = query.Any();

System.Collections.ArrayList al = new System.Collections.ArrayList();
al.Add("abc");
al.Add("def");
al.Add("ghi");
var strings = al.Cast<string>();
foreach(string s in strings)   // "abc", "def", "ghi"
  Console.WriteLine(s);
//Cast主要作用是让LINQ可以应用在非泛型集合上

int[] ints  = ...{ 1, 2, 2, 3, 2, 3, 4 };
var distinctInts    = ints.Distinct();
foreach(var x in distinctInts)  // 1, 2, 3, 4
  Console.WriteLine(x);
//主要是返回一个集合中不同的元素

int[] intsS1 = ...{ 1, 2, 2, 3, 2, 3, 4, 5, 6 };
int[] intsS2 = ...{ 1, 3, 6, 7 };
var diffInts    = intsS1.Except(intsS2);
foreach(var x in diffInts)  // 2, 4, 5
  Console.WriteLine(x);
//可以查询在一个集合中出现却不能在另一个集合出现的元素

int[]   ints    = ...{ 1, 2, 3, 4, 5, 6 };
int    first  = ints.First();
Doctor doctor = doctors.First(doc => doc.Name == "Mary");
Console.WriteLine(first);             // 1
Console.WriteLine(doctor.Initials);   // 123
//返回集合中第一个满足条件的元素

int[] ints      = ...{3, 1, 6, 4, 2, 5};
var sorted = ints.OrderBy(x => x);
foreach(var x in sorted)   // 1, 2, 3, 4, 5, 6
  Console.WriteLine(x);
//排序，还可以用OrderByDescending，逆序

int[] ints      = ...{1, 2, 3, 4, 5, 6};
var sameInts = ints.Select(x => x>=3);
foreach(var x in sameInts)   // 3, 4, 5, 6
  Console.WriteLine(x);
//选出满足条件的元素

int[]      ints    = ...{ 1, 2, 3, 4, 5, 6 };
decimal?[] values  = ...{ 1, null, 2, null, 3, 4 };
int      sum1     = ints.Sum();
decimal? sum2     = values.Sum();
Console.WriteLine(sum1);   // 21
Console.WriteLine(sum2);   // 10
//求和，可以应用在 int, int?, long, long?, decimal, decimal?, double, or double?. 这些类型上，返回值与上述类型一样

Doctors doctors = new Doctors();
var query = from doc in doctors
where doc.Name == "Mary"
select doc;
List<Doctor> chicago = query.ToList();
//把查询结果转换成List集合

int[] ints = ...{1, 2, 3, 4, 5, 6};
var even    = ints.Where( x => x % 2 == 0);
foreach(var x in even)      // 2, 4, 6
  Console.WriteLine(x);
//返回满足条件的元素集合