C#-linq实战003-查询-Where

Where语句检查符合条件的元素，并返回新的迭代器

1：基于谓词筛选值序列

IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)

1.1：foreach解析：

            //IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)//foreach解析：//source:TSource->stringList<string> fruits15 = new List<string> { "apple", "passionfruit", "banana", "mango","orange", "blueberry", "grape", "strawberry" };//predicate:TSource->itemConsole.WriteLine("基于谓词筛选值序列foreach解析：");foreach (string item in fruits15){if (item.StartsWith("a"))//predicate:bool->item.StartsWith("a"){Console.Write(item + ",");}}

1.2：原理图：执行方式：延迟流式执行

1.3：linq写法(查询语句)：

            //2:linq写法(查询语句)：var fruits16 = from f in fruits15.AsEnumerable()where f.StartsWith("a")select f;Console.WriteLine("基于谓词筛选值序列linq写法：");foreach (string item in fruits16){Console.WriteLine(item);}

1.4：查询方法写法：

            //3：查询方法写法：var fruits17 = fruits15.AsEnumerable().Where(f=> f.StartsWith("a"));Console.WriteLine("基于谓词筛选值序列查询方法写法：");foreach (string item in fruits17){Console.WriteLine(item);}

2：基于谓词筛选值序列。将在谓词函数的逻辑中使用每个元素的索引。

IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)

2.1：foreach解析：

            //IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, int, bool> predicate)//foreach解析：//source:TSource->stringList<string> fruits25 = new List<string> { "apple", "passionfruit", "banana", "mango","orange", "blueberry", "grape", "strawberry" };int index = 0;//predicate:TSource->item//predicate:int->indexConsole.WriteLine("基于谓词筛选值序列foreach解析：");foreach (string item in fruits25){if (item.StartsWith("a"))//predicate:bool->item.StartsWith("a"){Console.Write(item + ",");}index++;}

2.2:原理解析：：执行方式：延迟流式执行

2.2：linq写法(查询语句)：没有

2.3：查询方法写法：

            //3：查询方法写法：var fruits27 = fruits25.AsEnumerable().Where((f, index25) => f.StartsWith("a") && index25 > 0);Console.WriteLine("基于谓词筛选值序列带索引查询方法写法：");foreach (string item in fruits27){Console.WriteLine(item);}

3：多层元素（类）的Where查询：

这里以搜索赛车手为例：

      /// <summary>/// 赛车手/// </summary>[Serializable]public class Racer : IComparable<Racer>, IFormattable{/// <summary>/// 名字/// </summary>public string FirstName { get; set; }/// <summary>/// 赛车手姓氏/// </summary>public string LastName { get; set; }/// <summary>/// 赛车手国家/// </summary>public string Country { get; set; }/// <summary>/// 夺冠次数/// </summary>public int Wins { get; set; }public int Starts { get; set; }/// <summary>/// 赛车手夺冠用的车名称/// </summary>public IEnumerable<string> Cars { get; private set; }/// <summary>/// 赛车手夺冠时的年份/// </summary>public IEnumerable<int> Years { get; private set; }public Racer(string firstName, string lastName, string country, int starts, int wins, IEnumerable<int> years, IEnumerable<string> cars){this.FirstName = firstName;this.LastName = lastName;this.Country = country;this.Starts = starts;this.Wins = wins;this.Years = new List<int>(years);this.Cars = new List<string>(cars);}public Racer(string firstName, string lastName, string country, int starts, int wins) : this(firstName, lastName, country, starts, wins, null, null){}public int CompareTo(Racer other){if (other == null) return -1;//升序//return string.Compare(this.LastName, other.LastName);//降序return string.Compare(other.LastName, this.LastName);}public string ToString(string format, IFormatProvider formatProvider){switch (format){case null:case "N":return ToString();case "F":return FirstName;case "L":return LastName;case "C":return Country;case "S":return Starts.ToString();case "W":return Wins.ToString();case "A":return String.Format("{0} {1}, {2}; starts: {3}, wins: {4}",FirstName, LastName, Country, Starts, Wins);default:throw new FormatException(String.Format("Format {0} not supported", format));}}public string ToString(string format){return ToString(format, null);}public override string ToString(){return String.Format("{0} {1}", FirstName, LastName);}}public static class Formula1{private static List<Racer> racers;/// <summary>/// 返回一组赛车手，包含了1950-2011年之间的所有一级方程式冠军/// </summary>/// <returns></returns>public static IList<Racer> GetChampions(){if (racers == null){racers = new List<Racer>(40);racers.Add(new Racer("Nino", "Farina", "Italy", 33, 5, new int[] { 1950 }, new string[] { "Alfa Romeo" }));racers.Add(new Racer("Alberto", "Ascari", "Italy", 32, 10, new int[] { 1952, 1953 }, new string[] { "Ferrari" }));racers.Add(new Racer("Juan Manuel", "Fangio", "Argentina", 51, 24, new int[] { 1951, 1954, 1955, 1956, 1957 }, new string[] { "Alfa Romeo", "Maserati", "Mercedes", "Ferrari" }));racers.Add(new Racer("Mike", "Hawthorn", "UK", 45, 3, new int[] { 1958 }, new string[] { "Ferrari" }));racers.Add(new Racer("Phil", "Hill", "USA", 48, 3, new int[] { 1961 }, new string[] { "Ferrari" }));racers.Add(new Racer("John", "Surtees", "UK", 111, 6, new int[] { 1964 }, new string[] { "Ferrari" }));racers.Add(new Racer("Jim", "Clark", "UK", 72, 25, new int[] { 1963, 1965 }, new string[] { "Lotus" }));racers.Add(new Racer("Jack", "Brabham", "Australia", 125, 14, new int[] { 1959, 1960, 1966 }, new string[] { "Cooper", "Brabham" }));racers.Add(new Racer("Denny", "Hulme", "New Zealand", 112, 8, new int[] { 1967 }, new string[] { "Brabham" }));racers.Add(new Racer("Graham", "Hill", "UK", 176, 14, new int[] { 1962, 1968 }, new string[] { "BRM", "Lotus" }));racers.Add(new Racer("Jochen", "Rindt", "Austria", 60, 6, new int[] { 1970 }, new string[] { "Lotus" }));racers.Add(new Racer("Jackie", "Stewart", "UK", 99, 27, new int[] { 1969, 1971, 1973 }, new string[] { "Matra", "Tyrrell" }));racers.Add(new Racer("Emerson", "Fittipaldi", "Brazil", 143, 14, new int[] { 1972, 1974 }, new string[] { "Lotus", "McLaren" }));racers.Add(new Racer("James", "Hunt", "UK", 91, 10, new int[] { 1976 }, new string[] { "McLaren" }));racers.Add(new Racer("Mario", "Andretti", "USA", 128, 12, new int[] { 1978 }, new string[] { "Lotus" }));racers.Add(new Racer("Jody", "Scheckter", "South Africa", 112, 10, new int[] { 1979 }, new string[] { "Ferrari" }));racers.Add(new Racer("Alan", "Jones", "Australia", 115, 12, new int[] { 1980 }, new string[] { "Williams" }));racers.Add(new Racer("Keke", "Rosberg", "Finland", 114, 5, new int[] { 1982 }, new string[] { "Williams" }));racers.Add(new Racer("Niki", "Lauda", "Austria", 173, 25, new int[] { 1975, 1977, 1984 }, new string[] { "Ferrari", "McLaren" }));racers.Add(new Racer("Nelson", "Piquet", "Brazil", 204, 23, new int[] { 1981, 1983, 1987 }, new string[] { "Brabham", "Williams" }));racers.Add(new Racer("Ayrton", "Senna", "Brazil", 161, 41, new int[] { 1988, 1990, 1991 }, new string[] { "McLaren" }));racers.Add(new Racer("Nigel", "Mansell", "UK", 187, 31, new int[] { 1992 }, new string[] { "Williams" }));racers.Add(new Racer("Alain", "Prost", "France", 197, 51, new int[] { 1985, 1986, 1989, 1993 }, new string[] { "McLaren", "Williams" }));racers.Add(new Racer("Damon", "Hill", "UK", 114, 22, new int[] { 1996 }, new string[] { "Williams" }));racers.Add(new Racer("Jacques", "Villeneuve", "Canada", 165, 11, new int[] { 1997 }, new string[] { "Williams" }));racers.Add(new Racer("Mika", "Hakkinen", "Finland", 160, 20, new int[] { 1998, 1999 }, new string[] { "McLaren" }));racers.Add(new Racer("Michael", "Schumacher", "Germany", 287, 91, new int[] { 1994, 1995, 2000, 2001, 2002, 2003, 2004 }, new string[] { "Benetton", "Ferrari" }));racers.Add(new Racer("Fernando", "Alonso", "Spain", 177, 27, new int[] { 2005, 2006 }, new string[] { "Renault" }));racers.Add(new Racer("Kimi", "Räikkönen", "Finland", 148, 17, new int[] { 2007 }, new string[] { "Ferrari" }));racers.Add(new Racer("Lewis", "Hamilton", "UK", 90, 17, new int[] { 2008 }, new string[] { "McLaren" }));racers.Add(new Racer("Jenson", "Button", "UK", 208, 12, new int[] { 2009 }, new string[] { "Brawn GP" }));racers.Add(new Racer("Sebastian", "Vettel", "Germany", 81, 21, new int[] { 2010, 2011 }, new string[] { "Red Bull Racing" }));}return racers;}
}

示例：这里以1.1的方法变体查询

找出至少赢得15场比赛的巴西和奥地利赛车手

3.1：foreach解析：

            //IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)//foreach解析：//source:TSource->stringvar gc1 = Formula1.GetChampions();//predicate:TSource->itemforeach (var r in gc1){//bool的判断使用lambda表达式//predicate:bool->r.Wins >= 15 && (r.Country == "Brazil" || r.Country == "Austria")if (r.Wins >= 15 && (r.Country == "Brazil" || r.Country == "Austria")){Console.WriteLine("2.1.1:{0:A}", r);}}

3.2：linq写法(查询语句)：

            //2.1:找出至少赢得15场比赛的巴西和奥地利赛车手//2.1.1：语法糖var racers = from r in Formula1.GetChampions()where r.Wins >= 15 && (r.Country == "Brazil" || r.Country == "Austria")select r;foreach (Racer r in racers){Console.WriteLine("2.1.1:{0:A}", r);}

3.3：查询方法写法：

            //写法2：2.1.3var racers3 = Formula1.GetChampions().Where(r => r.Wins >= 15 && (r.Country == "Brazil" || r.Country == "Austria")).Select(r => r);//与写法2一样，select方法可以舍弃var racers35 = Formula1.GetChampions().Where(r => r.Wins >= 15 && (r.Country == "Brazil" || r.Country == "Austria"));foreach (Racer r in racers3){Console.WriteLine("2.1.3:{0:A}", r);}

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C#-linq实战003-查询-Where

1：基于谓词筛选值序列

1.1：foreach解析：

1.2：原理图：执行方式：延迟流式执行

1.3：linq写法(查询语句)：

1.4：查询方法写法：

2：基于谓词筛选值序列。将在谓词函数的逻辑中使用每个元素的索引。

2.1：foreach解析：

2.2:原理解析：：执行方式：延迟流式执行

2.2：linq写法(查询语句)：没有

2.3：查询方法写法：

3：多层元素（类）的Where查询：

3.1：foreach解析：

3.2：linq写法(查询语句)：

3.3：查询方法写法：

C#-linq实战003-查询-Where相关推荐

最新文章

热门文章

C#-linq实战003-查询-Where

1：基于谓词筛选值序列

1.1：foreach解析：

1.2：原理图：执行方式：延迟流式执行

1.3：linq写法(查询语句)：

1.4：查询方法写法：

2：基于谓词筛选值序列。 将在谓词函数的逻辑中使用每个元素的索引。

2.1：foreach解析：

2.2:原理解析：：执行方式：延迟流式执行

2.2：linq写法(查询语句)：没有

2.3：查询方法写法：

3：多层元素（类）的Where查询：

3.1：foreach解析：

3.2：linq写法(查询语句)：

3.3：查询方法写法：

C#-linq实战003-查询-Where相关推荐

最新文章

热门文章

2：基于谓词筛选值序列。将在谓词函数的逻辑中使用每个元素的索引。