LINQ筆記：SelectMany的使用方式
不太常使用到LINQ的SelectMany，每次使用到SelectMany都要回去翻網路文章或是微軟文件確認語法，乾脆簡單記錄在這裡一勞永逸，沒啥技術性請參考參考即可。
LINQ SelectMany的使用方式
兩個資料類別的程式碼如下：
//部門
class OrmDepartment
{
public int iDepartment { get; set; }
public string cDepartmentName { get; set; }
public System.Collections.Generic.List<OrmEmployee> oEmployee { get; set; }
}
//員工
class OrmEmployee
{
public int iEmployee { get; set; }
public string cEmployeeName { get; set; }
}
主要使用方式如下：
static void Main(string[] args)
{
var oList = new System.Collections.Generic.List<OrmDepartment>()
{
new OrmDepartment()
{
iDepartment = 1, cDepartmentName = "行銷部", oEmployee = new System.Collections.Generic.List<OrmEmployee>()
{
new OrmEmployee { iEmployee = 1000, cEmployeeName = "王小明" },
new OrmEmployee { iEmployee = 2000, cEmployeeName = "李小華" },
}
},
new OrmDepartment()
{
iDepartment = 2, cDepartmentName = "研發部", oEmployee = new System.Collections.Generic.List<OrmEmployee>()
{
new OrmEmployee { iEmployee = 3000, cEmployeeName = "張大頭" },
new OrmEmployee { iEmployee = 4000, cEmployeeName = "林小英" },
}
},
};
//列舉出所有的員工編號與姓名
var oSelectMany_1 = oList.SelectMany(x => x.oEmployee).Select(x => $"ID={x.iEmployee}；Name={x.cEmployeeName}");
foreach (var item in oSelectMany_1)
{ Console.WriteLine(item); }
//列舉出所有的員工編號與姓名，以及所屬部門（拍扁）
var oSelectMany_2 = oList.SelectMany(x => x.oEmployee, (x, y) => new { x.iDepartment, x.cDepartmentName, y.iEmployee, y.cEmployeeName });
foreach (var item in oSelectMany_2)
{ Console.WriteLine(item); }
ReadKey();
}
輸出結果如下：
ID=1000；Name=王小明
ID=2000；Name=李小華
ID=3000；Name=張大頭
ID=4000；Name=林小英
{ iDepartment = 1, cDepartmentName = 行銷部, iEmployee = 1000, cEmployeeName = 王小明 }
{ iDepartment = 1, cDepartmentName = 行銷部, iEmployee = 2000, cEmployeeName = 李小華 }
{ iDepartment = 2, cDepartmentName = 研發部, iEmployee = 3000, cEmployeeName = 張大頭 }
{ iDepartment = 2, cDepartmentName = 研發部, iEmployee = 4000, cEmployeeName = 林小英 }
進行笛卡爾乘積運算（Cartesian product）
有些情境下我們會需要進行TableA與TableB進行交叉連接（CrossJoin）進行乘積運算，例如：要列出總共的排列組合可能性，這時候我們也可以使用SelectMany來進行運算，雖然說以這個案例來進行舉例並不太恰當，但是就將就一下湊或著看看吧。
//其實這就是oSelectMany_1的另外一種寫法，透過一次性的指令直接把職員挑出來成為IEnumerable>string>列表
var oEmployeeList = oList.SelectMany(x => x.oEmployee, (x, y) => y.cEmployeeName );
//利用SelectMany進行交互聯集
var oSelectMany_3 = oList.SelectMany(x => oEmployeeList, (x, y) => new { cName = y, iDeaprtment = x.iDepartment, cDepartmentName = x.cDepartmentName });
foreach (var item in oSelectMany_3)
{ Console.WriteLine(item); }
輸出結果如下，可以看到透過SelectMany進行A、B兩個資料表進行交叉連接（CrossJoin）運算，把所有的排列組合都列出來了：
{ cName = 王小明, iDeaprtment = 1, cDepartmentName = 行銷部 }
{ cName = 李小華, iDeaprtment = 1, cDepartmentName = 行銷部 }
{ cName = 張大頭, iDeaprtment = 1, cDepartmentName = 行銷部 }
{ cName = 林小英, iDeaprtment = 1, cDepartmentName = 行銷部 }
{ cName = 王小明, iDeaprtment = 2, cDepartmentName = 研發部 }
{ cName = 李小華, iDeaprtment = 2, cDepartmentName = 研發部 }
{ cName = 張大頭, iDeaprtment = 2, cDepartmentName = 研發部 }
{ cName = 林小英, iDeaprtment = 2, cDepartmentName = 研發部 }