LINQ，语言集成查询；
LINQ TO SQL，同EF,NHibernate一样，也是一种ORM框架；
1. 入门应用示例：
static public void LinqBasic(){ var colors = new[] { "Red", "Orange", "Yellow", "Black", "Purple", "Grenn", "White", "Blue", "Hello World !" }; var query = from color in colors orderby color ascending, color.Length descending where color == "Red" || color.StartsWith("P") && color.Length >= 3 || !color.Any() select color; foreach (var color in query) { Console.WriteLine(color); }} 2. 限定符运算：
Any() //判断是否有元素满足某个条件 All()//判断是否所有的数据都满足某个条件 Contains//判断是否包含某个元素；3. Set集合的用法：
主要测试了几中常见方法：Distinct(),Except(),Intersect()及Union()
static public void Set(){ List htsA = new List () { "Red", "Red", "Black", "White", "White", "Blue" }; List htsB = new List () { "Red", "Red", "Black", "Black" }; IEnumerable htsC = htsA.Distinct(); IEnumerable htsD = htsA.Except(htsB); IEnumerable htsE = htsA.Intersect(htsB); IEnumerable htsF = htsA.Union(htsB); foreach (String ht in htsA) { Console.WriteLine(ht); }} 4. Skip()/SkipWhile() 及 Take()/TakeWhile()
Skip(n):跳过n元素开始读取;
SkipWhile(): 从第一个条件为false的地方开始读取元素，遇到第一个true条件则停止读取;
Take(n): 从第一个元素开始,读取n个元素
TakeWhile(); 从第一个条件为true的地方开始读取元素，遇到第一个false条件则停止读取;
static public void Skip(){ var colors = new[] { "Red", "Orange", "Yellow", "Black", "Purple", "Grenn", "White", "Blue", "Hello World !" }; var res = colors.SkipWhile(n => n != "Black"); foreach (var color in res) //foreach (var color in colors.Skip(5)) { Console.WriteLine(color); }}static public void Take(){ var colors = new[] { "Red", "Orange", "Yellow", "Black", "Purple", "Grenn", "White", "Blue", "Hello World !" }; var res = colors.TakeWhile(n => n != "Black"); foreach (var color in res) //foreach (var color in colors.Take(5)) { Console.WriteLine(color); }} 5. Select/SelectMany用法：
SelectMany：将嵌套列表合并为一个列表:
static public void Select(){ var mat = new[] { new[] { 11,12,13,14}, new[] { 21,22,23,24}, new[] { 31,32,33,34}, new[] { 41,42,43,44} }; var query_01 = mat.Select(n => n.ToList()); var query_02 = mat.SelectMany(n => n.ToList()); foreach (var val in query_01) { Console.WriteLine(val); } foreach (var val in query_02) { Console.WriteLine(val); }} 6. Join/GroupJoin用法：
Join(param1,param2,param3,param4)： param1:要联接的第一个集合；param2:第一个集合中与第二个集合的关联列;param3:第二个集合中与第一个集合的关联列;param4:返回列的集合(可以对返回列进行重命名)
Group join:适合返回有级别的列表
static public void Join(){ List cmyList = new List { new Company {Id=1,ComName="CMA"}, new Company {Id=2,ComName="CMB"}, }; List empList = new List { new Employee {CompanyId=2,EmpName="Jam",}, new Employee {CompanyId=1,EmpName="Ke"}, new Employee {CompanyId=1,EmpName="Lin"}, new Employee {CompanyId=2,EmpName="SyEn"}, new Employee {CompanyId=3,EmpName="Kate"} }; var matJoin11 = cmyList.Join(empList, m => m.Id, n => n.CompanyId, (m, n) => new {CompanyName=m.ComName,EmployeeName=n.EmpName}).OrderBy(p => p.EmployeeName); var matJoin12 = from cmy in cmyList join emp in empList on cmy.Id equals emp.CompanyId select new { CompanyName = cmy.ComName, EmployeeName = emp.EmpName }; foreach (var val in matJoin11) { Console.WriteLine(val); } foreach (var val in matJoin12) { Console.WriteLine(val); } var matGJoin21 = cmyList.GroupJoin(empList, m => m.Id, n => n.CompanyId, (m, o) => new { CompanyName = m.ComName,EmployeeName=o.Select(n => n.EmpName)}); var matGJoin22 = (from cmy in cmyList join emp in empList on cmy.Id equals emp.CompanyId into nt select new { CompanyName = cmy.ComName, EmployeeName = nt }).ToList(); foreach (var val in matGJoin21) { Console.WriteLine(val.CompanyName+":"); foreach(var emp in val.EmployeeName) { Console.WriteLine("--"+emp); } } foreach (var val in matGJoin22) { Console.WriteLine(val.CompanyName + ":"); foreach (var emp in val.EmployeeName) { Console.WriteLine("--" + emp.EmpName); } }} 运行结果：
7. Range()/Repeat()/SequenceEqual()
Range(param1,param2) param1:第一个值，默认第二个值为2，依次为3,4,5...; param2:需要生成的元素个数
Repeat()列表复制
SequenceEqual() 列表是否相等(元素个数，每个位置元素内容)的判断
static public void Range(){ //Range(param1,param2) param1:第一个值，默认第二个值为2，依次为3,4,5...; param2:需要生成的元素个数 List nl = Enumerable.Range(1, 20).Select(x => x + 2).ToList(); foreach (int i in nl) { Console.WriteLine("{0}",i); }}static public void Repeat(){ List hts = new List () { "Red", "Red", "Black", "White", "White", "Blue" }; var nht = Enumerable.Repeat(hts,10).ToList().SelectMany(n => n.ToList()); foreach (var v in nht) { Console.WriteLine(v); }}//Enumerable.Empty:返回具有指定类型参数的空 IEnumerable 。static public void SequenceEqual(){ List htA = new List () { "Red", "Red", "Black", "White", "White", "Blue" }; List htB = new List () { "Red", "Red", "Black", "White", "White", "Blue" }; bool b = htA.SequenceEqual(htB); Console.WriteLine(b);} 8. 列表元素操作：First()/FirstOrDefault()/Last()/ElementAt()/Single()
static public void ElementOperation(){ List hts = new List () { "Red", "Orange", "Yellow", "Black", "Purple", "Grenn", "White", "Blue"}; Console.WriteLine(hts.First()); Console.WriteLine(hts.FirstOrDefault(color => color.Length>10)); Console.WriteLine(hts.Last()); Console.WriteLine(hts.ElementAt(5)); //Console.WriteLine(hts.Single(color => color.Length>5));}static public void ElementOperation(){ List hts = new List () { "Red", "Orange", "Yellow", "Black", "Purple", "Grenn", "White", "Blue"}; Console.WriteLine(hts.First()); Console.WriteLine(hts.FirstOrDefault(color => color.Length>10)); Console.WriteLine(hts.Last()); Console.WriteLine(hts.ElementAt(5)); //Console.WriteLine(hts.Single(color => color.Length>5));} 9. GroupBy用法：
static public void GroupBy(){ List hts = new List () { "Red", "Red", "Black", "White", "White", "Blue" }; var query = from ht in hts group ht by ht.Substring(0, 1); foreach (var gp in query) { Console.WriteLine("Colors start with "+gp.Key); foreach (var dt in gp) { Console.WriteLine(dt); } }} 10. Union用法：
static public void Union(){ var matA = new[] { new[] { 11,12,13,14}, new[] { 21,22,23,24} }; var matB = new[] { new[] { 31,32,33,34}, new[] { 41,42,43,44} }; var matJoin = matA.Union(matB); var v = matJoin.SelectMany(n => n.ToList()); foreach (var val in v) { Console.WriteLine(val); }} 11. 类型转换：
Cast():将IEnumerable的元素强制转换为指定的IEnumerable类型;
AsEnumerable():将其他类型转为IEnumerable<T>; OfType():根据指定类型筛选IEnumerable的元素。 static public void TypeConvert(){ var colors = new[] { "Red", "Black", "Green", "Purple", "White", "Blue" }; List hts = colors.AsEnumerable().ToList(); foreach (string s in hts) { Console.WriteLine(s); } Console.WriteLine("The int value in the list is as below:"); ArrayList ays = new ArrayList() { "Red", "Black", "Green", 100, "White", "Blue" }; List sls = ays.OfType ().ToList(); foreach (int s in sls) { Console.WriteLine(s); } Console.WriteLine("The new list after convert is as below:"); List sls_new = ays.OfType ().ToList(); IEnumerable ays_new = sls_new.Cast (); foreach (string s in ays_new) { Console.WriteLine(s); }} 12.序列合并：
把两个序列合并为一个：
static public void Concat(){ List htA = new List () { "Red", "Blue" }; List htB = new List () { "Black", "White" }; List hts = htA.Select(x => x).Concat(htB.Select(y => y)).ToList(); foreach (string ht in hts) { Console.WriteLine(ht); }} 13. 聚合运算：
static public void Aggregate(){ List its = new List { 9, 3, 4, 7, 6, 1, 8 }; double avg = its.Average(); int max = its.Max(); int min = its.Min(); int cut = its.Count(); int sum = its.Sum(); Console.WriteLine("Average is {0};", avg); Console.WriteLine("Max is {0};", max); Console.WriteLine("Min is {0};", min); Console.WriteLine("Count is {0};", cut); Console.WriteLine("Sum is {0};", sum);}
至此，关于Linq的基本用法几乎都在这里了，对于更多详细说明，请参见.