C# LINQ学习笔记

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<String> htsA = new List<String>() { "Red", "Red", "Black", "White", "White", "Blue" };
    List<String> htsB = new List<String>() { "Red", "Red", "Black", "Black" };

    IEnumerable<String> htsC = htsA.Distinct();
    IEnumerable<String> htsD = htsA.Except(htsB);
    IEnumerable<String> htsE = htsA.Intersect(htsB);
    IEnumerable<String> 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<Company> cmyList = new List<Company>
    {
        new Company {Id=1,ComName="CMA"},
        new Company {Id=2,ComName="CMB"},
    };

    List<Employee> empList = new List<Employee>
    {
        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<int> nl = Enumerable.Range(1, 20).Select(x => x + 2).ToList();
    foreach (int i in nl)
    {
        Console.WriteLine("{0}",i);
    }
}

static public void Repeat()
{
    List<String> hts = new List<String>() { "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<T>。

static public void SequenceEqual()
{
    List<String> htA = new List<String>() { "Red", "Red", "Black", "White", "White", "Blue" };
    List<String> htB = new List<String>() { "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<String> hts = new List<string>() { "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<String> hts = new List<string>() { "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<String> hts = new List<String>() { "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<string> 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<int> sls = ays.OfType<int>().ToList();
    foreach (int s in sls)
    {
        Console.WriteLine(s);
    }

    Console.WriteLine("The new list after convert is as below:");
    List<string> sls_new = ays.OfType<string>().ToList();
    IEnumerable<string> ays_new = sls_new.Cast<string>();
    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<String> htA = new List<String>() { "Red", "Blue" };
    List<String> htB = new List<String>() { "Black", "White" };
    List<string> 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<int> its = new List<int> { 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，请参见官方文档.