.net Dictionary.FirstOrDefault() 如何判断是否找到了结果

发布于03月25日

我有(或想要)这样的代码:

IDictionary<string,int> dict = new Dictionary<string,int>();
// ... Add some stuff to the dictionary.

// Try to find an entry by value (if multiple, don't care which one).
var entry = dict.FirstOrDefault(e => e.Value == 1);
if ( entry != null ) { 
   // ^^^ above gives a compile error:
   // Operator '!=' cannot be applied to operands of type 'System.Collections.Generic.KeyValuePair<string,int>' and '<null>'
}

我还试着像这样改变冒犯的台词:

if ( entry != default(KeyValuePair<string,int>) )

但这也会产生一个编译错误:

Operator '!=' cannot be applied to operands of type 'System.Collections.Generic.KeyValuePair<string,int>' and 'System.Collections.Generic.KeyValuePair<string,int>'

这是怎么回事？

推荐答案

Jon的答案将适用于Dictionary<string, int>，因为字典中不能有空键值.然而，它不适用于Dictionary<int, string>，因为它没有空键值..."失败"模式将以0键结束.

有两种 Select :

写一个TryFirstOrDefault方法，如下所示:

public static bool TryFirstOrDefault<T>(this IEnumerable<T> source, out T value)
{
    value = default(T);
    using (var iterator = source.GetEnumerator())
    {
        if (iterator.MoveNext())
        {
            value = iterator.Current;
            return true;
        }
        return false;
    }
}

或者，投影到可为空的类型:

var entry = dict.Where(e => e.Value == 1)
                .Select(e => (KeyValuePair<string,int>?) e)
                .FirstOrDefault();

if (entry != null)
{
    // Use entry.Value, which is the KeyValuePair<string,int>
}