使用LINQ处理C＃字典

Question

如何使用LINQ Lambda Expression/Statemene Expression编写“//使用Foreach显示”循环实现？

我想简化我的开发并尽可能避免嵌套的foreach循环。我想在第二个foreach语句中包含更多的逻辑，并且我想使用Lambda/Statement表达式。

internal class Program 
{ 
    internal class Country 
    { 
     public string CountryName { get; set; } 
     public int CountryCode { get; set; } 
    } 
    static void Main(string[] args) 
    { 
     List<Country> countries = new List<Country>() 
     { 
      new Country{CountryName = "India", CountryCode=1}, 
      new Country{CountryName = "Andaman Nicobar", CountryCode=1}, 
      new Country{CountryName = "United States of America", CountryCode=2}, 
      new Country{CountryName = "Alaska", CountryCode=2}, 
      new Country{CountryName = "Hawaii", CountryCode=2}, 
      new Country{CountryName = "United Kingdom", CountryCode=3}, 
      new Country{CountryName = "Australia", CountryCode=4} 
     }; 

     Dictionary<int, List<Country>> countriesDictionary = new Dictionary<int, List<Country>>(); 
     foreach (Country country in countries) 
     { 
      if (!countriesDictionary.ContainsKey(country.CountryCode)) 
       countriesDictionary.Add(country.CountryCode, new List<Country>()); 
      countriesDictionary[country.CountryCode].Add(country);     
     } 

     // Display using Foreach 

     foreach (int key in countriesDictionary.Keys) 
     {        
      List<Country> dCountries = countriesDictionary[key]; 

      foreach (Country country in dCountries) 
      { 
       if (country.CountryCode.Equals(key)) 
       { 
        Console.WriteLine(country.CountryName); 
       }     
      } 
      Console.WriteLine();    
     } 
     Console.ReadLine(); 
    } 

请建议。

Answer 1

如果你想通过代码对国家进行分组，那么你不需要两个字典。使用Enumerable.GroupBy

foreach(var codeGroup in countries.GroupBy(c => c.CountryCode)) 
{ 
    foreach(var country in codeGroup) 
     Console.WriteLine(country.CountryName); 

    Console.WriteLine(); 
} 

或者只是使用您的countriesDictionary（它已经国家的代码分组）：

foreach(var kvp in countriesDictionary) 
{ 
    foreach(var country in kvp.Value) 
     Console.WriteLine(country.CountryName); 

    Console.WriteLine(); 
} 

Answer 2

这是另一种选择：

countriesDictionary.ToList().ForEach 
(
    pair => 
    { 
     pair.Value.ForEach(country => Console.WriteLine(country.CountryName)); 
     Console.WriteLine(); 
    } 
); 

---

此外，这一个基于 Romoku's Answer （a nswer移除）：

var countriesDictionary = countries.ToLookup(x => x.CountryCode, x => x); 

var dCountries = new List<Country>(); 

foreach(var countryGroup in countriesDictionary) 
{ 
    foreach(var country in countryGroup) 
    { 
     Console.WriteLine(country.CountryName); 
    } 
    Console.WriteLine(); 
} 

Answer 3

或者只是使它很容易...前面已经提到，你已经通过国家代码分组...

 foreach (var country in countriesDictionary.SelectMany(pair => pair.Value)) 
     { 
      Console.WriteLine(country.CountryName); 
     } 

Answer 4

我会用一个扩展做它。

static class CountryExtension 
    { 
     public static void WriteCountriesGroupyCountryCode(this IEnumerable<Country> list) 
     { 
      int currentCountry=int.MinValue; 
      list.OrderBy(c => c.CountryCode).ThenBy(c=>c.CountryName).ToList().ForEach(c => 
      { 
       if (currentCountry == int.MinValue) 
       { 
        currentCountry = c.CountryCode; 
       } 
       else if (currentCountry != c.CountryCode) 
       { 
        Console.WriteLine(); 
        currentCountry = c.CountryCode; 
       } 
       Console.WriteLine(c.CountryName); 
      }); 
     } 
    } 

Answer 5

的一种方法是这样的，避免与的GroupBy第一的foreach，刚刚1的foreach逻辑打印每个国家名称与指定的代码：

Dictionary<int, List<Country>> countriesDictionary = countries.GroupBy(g => g.CountryCode).ToDictionary(k => k.Key, k => k.ToList()); 

foreach (int key in countriesDictionary.Keys) 
{ 
     Console.WriteLine("****Countries with code {0}****",key); 
     int count = 0; 
     while (count < countriesDictionary[key].Count) 
     { 
      Console.WriteLine(countriesDictionary[key][count].CountryName); 
      count++; 
     } 
     Console.WriteLine(); 
} 

Console.ReadLine(); 

Answer 6

虽然你可能有足够的答案现在，这个LINQ会将你的字典压缩到一个国家名称列表中，允许一个简单的foreach显示它们。

List<string> countryNames = countriesDictionary.SelectMany(
     pair=>pair.Value.Where(
      country=>country.CountryCode == pair.Key 
     ).Select(x=>x.CountryName)).ToList(); 

    foreach (var name in countryNames) 
     Console.WriteLine(name); 

但是你的字典设置的方式，关键要始终匹配值的国家代码，是否正确？