为什么Enumerable可重复在这里？

Question

我试图根据下面的代码在排序前获取枚举中的单个值。该值显示两次。

为什么？什么是最好的解决方案？

干杯，
Berryl

private static IEnumerable<Currency> _commonCurrencies; 

    public static IEnumerable<Currency> GetCommonCurrencies() { 
     return _commonCurrencies ?? 
       (_commonCurrencies 
       = Currency.GetCachedCurrencies() 
         .Concat(new[] {Currency.Get(CurrencyIsoCode.KWD)}) 
         .Where(x => !x.AlphabeticCode.StartsWith("X", StringComparison.InvariantCultureIgnoreCase)) 
         .OrderBy(x => x.AlphabeticCode)); 
    } 

输出

... 
JPY 
KWD 
KWD // repeats? 
... 

EDIT

Currency.GetCurrencies（）被拨打电话到一个线程安全高速缓存的实施方案，其封装了一个字典并返回其值：

public ICollection<TValue> Values { get { return _inner.Values; } } 

我没有这种类型。将代码更改为'slap'ToArray（）修复了问题，以便concat值只显示一次，并且是可接受的解决方案。不知道为什么它显示了两次而没有强制枚举，虽然线程似乎在根..虽然线程似乎在根..

Answer 1

就像@CodeInChaos说，这可能是因为Lazy Evaluation如果GetCachedCurrencies不包含元素。

我把一个小程序放在一起，我只得到'KWD'的一个实例。

enum CurrencyIsoCode 
{ 
    USD, 
    KWD, 
    JPY, 
    XCD, 
    TVD 
} 

class Currency 
{ 
    public Currency() : this(false) { } 
    public Currency(bool inner) { } 

    public static IEnumerable<Currency> GetCachedCurrencies() 
    { 
     return new[] { 
      new Currency() { currencyCode = CurrencyIsoCode.USD }, 
      new Currency() { currencyCode = CurrencyIsoCode.JPY }, 
      new Currency() { currencyCode = CurrencyIsoCode.XCD } 
      }; 
    } 

    public CurrencyIsoCode currencyCode { set; get; } 

    public string AlphabeticCode 
    { 
     get { return currencyCode.ToString(); } 
    } 

    public static Currency Get(CurrencyIsoCode isOCode) 
    { 
     return new Currency() { currencyCode = isOCode }; 
    } 
} 

class Program 
{ 
    private static IEnumerable<Currency> _commonCurrencies = null; 

    static void Main(string[] args) 
    { 
     var currencies = GetCommonCurrencies(); 

     foreach (var curr in currencies) 
      Console.WriteLine(curr.AlphabeticCode); 

     Console.Read(); 
    } 


    public static IEnumerable<Currency> GetCommonCurrencies() 
    { 
     return _commonCurrencies ?? 
      (_commonCurrencies 
      = Currency.GetCachedCurrencies() 
        .Concat(new[] {Currency.Get(CurrencyIsoCode.KWD)}) 
        .Where(x => !x.AlphabeticCode.StartsWith("X", StringComparison.InvariantCultureIgnoreCase)) 
        .OrderBy(x => x.AlphabeticCode)); 
    }  
}