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· LINQ 概览
原始访问形式
SqlConnection conn = new SqlConnection("server=(local);database=AdventureWorks;uid=sa;pwd=password01!");
SqlCommand comm = new SqlCommand();
comm.Connection = conn;
comm.CommandType = CommandType.Text;
comm.CommandText = "SELECT * FROM HumanResources.Department";
DataSet ds = new DataSet();
SqlDataAdapter da = new SqlDataAdapter(comm);
da.Fill(ds);
this.GridView1.DataSource = ds.Tables[0];
this.GridView1.DataBind();
Linq:
PubsDataContext dc = new PubsDataContext();
var result = from item in dc.authors
select item;
this.GridView1.DataSource = result;
this.GridView1.DataBind();
string[] cities = {"Shanghai","Beijing","Dalian",
"Chengdu"};
IEnumerable<string> places = from city in cities
where city.Length > 5
orderby city descending
select city;
List<string > list = places .ToList<string>();
DataSet ds = new DataSet();
DataTable dt = new DataTable();
dt.Columns.Add("City", typeof(string));
for (int i = 0 ;i<list.Count ;i++)
{
DataRow dr = dt.NewRow();
dr["City"] = list[i];
dt.Rows.Add(dr);
}
ds.Tables.Add(dt);
this.GridView1.DataSource = ds;
this.GridView1.DataBind();
· 访问数组
string[] cities = {"Shanghai","Beijing","Dalian",
"Chengdu"};
IEnumerable<string> places = from city in cities
where city.Length > 5
orderby city descending
select city;
this.GridView1.DataSource = places;
this.GridView1.DataBind();
· 访问集合
List<string> list = new List<string>
{
"ShangHai", "Beijing","Dalian", "Chengdu","shenyang"
};
IEnumerable<string> places = from city in list
where city.Length > 5
orderby city descending
select city;
this.GridView1.DataSource = places;
this.GridView1.DataBind();
Demo2:Linq to 集合
public class Location
{
public string Country { get; set; }
public string City { get; set; }
public double DistanceFromBeijing { get; set; }
public Location()
{
}
}
code
List<Location> list = new List<Location>
{
new Location {City="Shenyang",Country="China",DistanceFromBeijing=800},
new Location{City="Shanghai",Country="China",DistanceFromBeijing=1000},
new Location{City="London",Country ="UK",DistanceFromBeijing=8000}
};
IEnumerable<Location> places = from item in list
select item;
this.GridView1.DataSource = places;
this.GridView1.DataBind();
前台
<asp:GridView ID="GridView1" runat="server" CellPadding="4" ForeColor="#333333"
GridLines="None" AutoGenerateColumns="false">
<Columns>
<asp:BoundField HeaderText="City" DataField="City" />
<asp:BoundField HeaderText="Country" DataField="Country" />
<asp:BoundField HeaderText="Dist" DataField="DistanceFromBeijing" />
</Columns>
</asp:GridView>
· 查询投影
·查询投影(Select)
·不返回所有数据列/属性
·修改或者转化查询的数据
·利用编译器对"匿名类型"的支持查询数据列/属性
·生成匿名类型('a')
List<City> cities = City.GetLocations();
var places = from city in cities
where city.DistanceFromBeijing > 3000
select new
{
Name = city.Name,
Country = city.Country,
DistanceFromBeijing = city.DistanceFromBeijing * 1.61
};
GridView1.DataSource = places;
GridView1.DataBind();
· 使用lambda表达式
·查询语法是一个方便的声明性代码缩写，您可以手动编写它：
·IEnumerable expr = names
.Where(s=>s.Length==5)
.OrderBy(s=>s)
.Select(s=>s.ToUpper());
· 查询操作符
调用
·普通的方式来调用扩展方法
·IEnumerable<string> query = Enumerable.Where(names,s => s.Length < 6);
· C#语言允许我们使用如下的方式来调用扩展方法:
·IEnumerable<string> query = names.Where(s=> s.Length < 6);
Demo:利用Lambda表达式
List<Location> list = new List<Location>
{
new Location {Name="Shenyang",Country="China",DistanceFromBeijing=800},
new Location{Name="Shanghai",Country="China",DistanceFromBeijing=1000},
new Location{Name="London",Country ="UK",DistanceFromBeijing=8000}
};
var places = list.Where(city => city.DistanceFromBeijing < 1500)
.Select(city => new { city.Name, city.Country, DistanceFromBeijing = city.DistanceFromBeijing });
//IEnumerable<Location> places = from item in list
// select item;
this.GridView1.DataSource = places;
this.GridView1.DataBind();
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Demo：
List<City> list = new List<City>
{
new City {Name="Shenyang",Country="China",DistanceFromBeijing=800},
new City{Name="Shanghai",Country="China",DistanceFromBeijing=1000},
new City{Name="London",Country ="UK",DistanceFromBeijing=8000}
};
//IEnumerable<Location> places = from item in list
// select item;
//1.
//var places = list.Where(city => city.DistanceFromBeijing < 1500)
// .Select(city => new { city.Name, city.Country, DistanceFromBeijing = city.DistanceFromBeijing });
//2.利用匿名方法
//var places = list.Where(delegate(City city) { return (city.DistanceFromBeijing < 1500); })
// .Select(city => new { city.Name, city.Country, DistanceFromBeijing = city.DistanceFromBeijing });
//3.
var places = list.Where(new Func<City, bool>(Predict))
.Select(city => new { city.Name, city.Country, DistanceFromBeijing = city.DistanceFromBeijing });
this.GridView1.DataSource = places;
this.GridView1.DataBind();
public bool Predict(City city)
{
return city.DistanceFromBeijing < 1500;
}
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