Question

想象一下，我有以下枚举：

<script>

$("button").click(function(){
    $("#flexMenu").toggle("slow", function() {

});
});

</script>

和#flexMenu { /* display: inline-block;*/ position: fixed; float: left; background: #1f1f1f; margin: 3.9em 0 0 0; padding: .25em; width: 15%; height: 6em; border: 2px solid #fff; z-index: 100; }：

public enum CompassDirection
{
    North,
    East,
    South,
    West
}

如何从字典中优雅地删除Dictionary<string, CompassDirection?>值，或复制到新词典，然后返回var dictionary = new Dictionary<string, CompassDirection?> { { "Foo", CompassDirection.North }, { "Bar", null }, { "Baz", CompassDirection.West } }; （请注意null的通用参数现在不可为空）

我能想到的一个解决方案是：

Dictionary<string, CompassDirection>

没有复制是否有更好的方法？

Answer 1

IMO，只要你说“优雅地删除null”，你就会有高度自以为是的答案。

如果你发现它们很优雅，你可以使用两种方法（1 Linq和1非 - Linq）。可空类型具有HasValue属性，以确定是否为null。如果类型HasValue属性为true，则可以引用Value属性，这样就不必强制转换为CompassDirection。

using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    public static void Main(string[] args)
    {
        var dictionary = new Dictionary<string, CompassDirection?>
        {
            { "Foo", CompassDirection.North },
            { "Bar", null },
            { "Baz", CompassDirection.West }
        };

        Console.WriteLine("New Dictionary 1:");
        PrintDictionary(RemoveNullMethod1(dictionary));

        Console.WriteLine("New Dictionary 2:");     
        PrintDictionary(RemoveNullMethod2(dictionary));
    }

    public static Dictionary<string, CompassDirection> RemoveNullMethod1(Dictionary<string, CompassDirection?> input)
    {
        return input
            .Where(entry => entry.Value.HasValue)
            .ToDictionary(e => e.Key, e => e.Value.Value);
    }

    public static Dictionary<string, CompassDirection> RemoveNullMethod2(Dictionary<string, CompassDirection?> input)
    {
        Dictionary<string, CompassDirection> newDictionary = new Dictionary<string, CompassDirection>();
        foreach(KeyValuePair<string, CompassDirection?> entry in input)
        {
            if (entry.Value.HasValue)
            {
                newDictionary.Add(entry.Key, entry.Value.Value);
            }
        }
        return newDictionary;
    }

    public static void PrintDictionary(Dictionary<string, CompassDirection> input)
    {
        foreach(KeyValuePair<string, CompassDirection> entry in input)
        {
            Console.WriteLine("Key: {0} Value: {1}", entry.Key, entry.Value);
        }
        Console.WriteLine();
    }
}

public enum CompassDirection
{
    North, East, South, West
}

结果：

New Dictionary 1:
Key: Foo Value: North
Key: Baz Value: West

New Dictionary 2:
Key: Foo Value: North
Key: Baz Value: West

Fiddle Demo

Answer 2

我编写了一个通用的扩展方法来解决我的问题：

public static class KeyValuePairHelpers
{
    public static IEnumerable<KeyValuePair<TKey, TValue>> 
        ToValueNonNullableKvps<TKey, TValue>(
            this IEnumerable<KeyValuePair<TKey, TValue?>> input)
            where TValue : struct
    {
        return from kvp in input
               where kvp.Value.HasValue
               select new KeyValuePair(kvp.Key, (TValue)kvp.Value);
    }

用法：

var dictionary = new Dictionary<string, CompassDirection?>
{
    { "Foo", CompassDirection.North },
    { "Bar", null },
    { "Baz", CompassDirection.West }
};
var nonNullableKvps = dictionary.ToValueNullableKvps(); 
// Type of nonNullableKvps is now IEnumerable<KeyValuePair<string, CompassDirection>>

Answer 3

其他答案重建一个全新的字典。如果我们不做那么疯狂的事情怎么样？

var badkeys = dictionary.Where(kvp => !kvp.Value.HasValue).Select(kvp => kvp.Key).ToArray();
foreach(var key in badkeys)
    dictionary.Remove(key);

从Dictionary中删除可空值并重新输入类型

3 个答案: