c＃動態修改列表

Question

正確！謝謝！ 但我不明白如何使用foreach ... 目標是每次修改Num時修改列表。 用Canvas List和工作列表的方式是好的編碼好嗎？

class Program 
{ 
    static void Main(string[] args) 
    { 
     int i_Num = 0; 
     string Str_Num = ""; 
     string[] linkToPLC = {"toto[{0}].test{1}", "tata[{0}].test{1}", "titi[{0}].test{1}"}; 
     List<string> genlnkPLCCanvas = new List<string>(linkToPLC); 
     List<string> genlnkPLCworkingwith = new List<string>(linkToPLC); 

     Console.WriteLine("Insert Num: "); 
     Str_Num = Console.ReadLine(); 
     i_Num = Convert.ToInt32(Str_Num); 
     for (int item = 0; item < genlnkPLCCanvas.Count; item++) 
     { 
      genlnkPLCworkingwith[item] = String.Format(genlnkPLCworkingwith[item], i_Num, 200); 
      Console.WriteLine("with List: result= " + genlnkPLCworkingwith[item]); 
     } 
     //foreach (string item in genlnkPLCCanvas) genlnkPLCworkingwith[item] = String.Format(item, i_Num, 200); 
     Console.ReadKey(); 
    } 
} 

感謝

Answer 1

如果要修改現有的名單，你必須使用for循環，而不是foreach之一：

foreach (var item in list) ... 

應改爲

for (int i = 0; i < list.Count; ++i) { 
    var item = list[i]; // not necessary, but often convenient 
    ... 
    list[i] = ... // modification 
    ... 
} 

例如

for (int i = 0; i < genlnkPLCCanvas.Count; ++i) { 
    var item = genlnkPLCCanvas[i]; 

    genlnkPLCCanvas[i] = string.Format(item, i_StationNum, 200); 
    } 

當測試嘗試創建報告（把所有的邏輯到單個讀取查詢），然後打印出來在一個去：

... 

    var withListReport = genlnkPLC 
    .Select(item => "with List: result = " + string.Format(item, i_StationNum, 200)); 

    var withoutListReport = genlnkPLC 
    .Select(item => "without List: result = " + string.Format(item, i_StationNum, 200)); 

    // now you can do whatever you want with the reports: 
    // - print them to console 
    //  Console.WriteLine(string.Join(Envrironment.NewLine, withListReport)); 
    // - save to file: 
    //  File.WriteAllLines(@"C:\MyFile.txt", withListReport); 
    // - print to, say, WinForm UI: 
    //  MyTextBox.Text = string.Join(Envrironment.NewLine, withListReport); 

    Console.WriteLine(string.Join(Envrironment.NewLine, withListReport)); 
    Console.WriteLine(string.Join(Envrironment.NewLine, withoutListReport)); 

    Console.ReadKey(); 

Answer 2

這是因爲

String.Format(item, i_StationNum, 200) 

不更改列表中的字符串。

您必須將String.Format結果指定給您的項目。

Answer 3

無需重複foreach（第一個，對您的項目沒有任何作用）。試試這個：

foreach (var item in genlnkPLC) 
    Console.WriteLine("with List: result= "+ String.Format(item, i_StationNum, 200)); 

Answer 4

String.Format()返回一個字符串，它不會改變你正在格式化的任何東西。因此，您的第一個foreach (var item in genlnkPLC)會創建立即銷燬的臨時字符串。

foreach (var item in genlnkPLC) 
{ 
    Console.WriteLine("with List = " + String.Format(item, i_StationNum, 200)); 
} 

Answer 5

在您不使用String.Format插入參數到的genlnkPLC成員，這顯然意在作爲格式字符串的聲明

foreach (var item in genlnkPLC) 
    Console.WriteLine("with List: result= "+item); 

。您可以使用

foreach (var item in genlnkPLC) 
    Console.WriteLine("without List result = " + String.Format(item, i_StationNum, 200)); 

代替。

Answer 6

問題是，您無法更改在foreach循環中枚舉的元素的引用。 string是一個不可變對象，所以更改它會替換舊的引用。如果你想改變在列表中的元素，你需要做一個for循環，就像這樣：

for (int item = 0; item < genlnkPLC.Count; item++) 
      genlnkPLC[item]= String.Format(genlnkPLC[item], i_StationNum, 200); 

Answer 7

由於M.Bychenko說：「如果你想修改現有的列表，你必須使用循環而不是foreach：「

並感謝您的報告tipp！

class Program 
{ 
    static void Main(string[] args) 
    { 
     int i_Num = 0; 
     string Str_Num = ""; 
     string[] linkToPLC = {"toto[{0}].test{1}", "tata[{0}].test{1}", "titi[{0}].test{1}"}; 
     List<string> genlnkPLCCanvas = new List<string>(linkToPLC); 
     List<string> genlnkPLCworkingwith = new List<string>(linkToPLC); 

     Console.WriteLine("Insert Num: "); 
     Str_Num = Console.ReadLine(); 
     i_Num = Convert.ToInt32(Str_Num); 
     for (int item = 0; item < genlnkPLCCanvas.Count; item++) 
     { 
      genlnkPLCworkingwith[item] = String.Format(genlnkPLCCanvas[item], i_Num, 200); 
     } 
     var CanvasListReport = genlnkPLCCanvas.Select(item => "Canvas List = " + item); 
     var WorkingListReport = genlnkPLCworkingwith.Select(item => "Working list = " + item);//string.Format(item, i_Num, 200)); 
     // now you can do whatever you want with the reports: 
     // - print them to console 
     //  Console.WriteLine(string.Join(Envrironment.NewLine, withListReport)); 
     // - save to file: File.WriteAllLines(@"C:\MyFile.txt", withListReport); 
     // - print to, say, WinForm UI: 
     //  MyTextBox.Text = string.Join(Envrironment.NewLine, withListReport) 
     Console.WriteLine(string.Join(Environment.NewLine, CanvasListReport)); 
     Console.WriteLine(string.Join(Environment.NewLine, WorkingListReport)); 
     Console.ReadKey(); 
    } 
}