如何在 C# 中使用 LINQ 过滤嵌套字典？

Question

如何在 C# 中使用 LINQ 过滤嵌套字典？

这是我开始使用的字典：

Dictionary<string, Dictionary<string, string>> countriesByContinentDictionary = new Dictionary<string, Dictionary<string, string>>
{
    {
        "Europe", 
        new Dictionary<string, string> {
            { "0", "France" },
            { "1", "Germany"},
            { "2", "Switzerland" }
        } 
    },
    {
        "Asia",
        new Dictionary<string, string>
        {
            {"zero", "India" },
            {"1", "Pakistan" },
            {"2", "Russia" }
        }
    },
    {
        "Africa",
        new Dictionary<string, string>
        {
            { "0", "South Africa" },
            { "1", "Nigeria" },
            { "2", "Lybia"}
        }
    }
};

我正在尝试过滤到具有字母 r 的国家/地区。 这就是我试图让它看起来像过滤后的样子：

Dictionary<string, Dictionary<string, string>> countriesByContinentDictionary = new Dictionary<string, Dictionary<string, string>>
{
    {
        "Europe", 
        new Dictionary<string, string> {
            { "0", "France" },
            { "1", "Germany"},
            { "2", "Switzerland" }
        } 
    },
    {
        "Asia",
        new Dictionary<string, string>
        {
        }
    },
    {
        "Africa",
        new Dictionary<string, string>
        {
            { "0", "South Africa" },
            { "1", "Nigeria" }
        }
    }
};

这是我目前的代码：

public static Dictionary<string, Dictionary<string, string>> GetRecordsContainingSubStringWithLambda(Dictionary<string, Dictionary<string, string>> originalNestedDictionary, string lambdaSubString)
{
    var filteredNestedDictionary = Enumerable.Range(0, originalNestedDictionary.Count)
        .Select(
                row => Enumerable.Range(0, originalNestedDictionary.ElementAt(row).Value.Count)
                .Select(col => originalNestedDictionary.ElementAt(row).Value.ElementAt(col))
                .Where(keyValuePair => keyValuePair.Value != null && keyValuePair.Value.Contains(lambdaSubString))
                .ToDictionary<string, string>(keyValuePair => keyValuePair.Key, keyValuePair => keyValuePair.Value)
        ).ToDictionary<string, Dictionary<string, string>>(keyValuePair => keyValuePair.Key, keyValuePair => keyValuePair.Value);
    return filteredNestedDictionary;
}

但我收到以下错误：

error CS0029: Cannot implicitly convert type 'System.Collections.Generic.Dictionary<System.Collections.Generic.Dictionary<string, string>, string>' to 'System.Collections.Generic.Dictionary<string, System.Collections.Generic.Dictionary<string, string>>'

Answer 1

注意Dictionary<TKey, TValue>直接实现了IEnumerable<KeyValuePair<TKey, TValue>>，所以我们不需要计算索引。相反，使用Select 转换外部字典中的每个项目，并使用Where 过滤掉与过滤器不匹配的内部项目，如下所示：

public static Dictionary<string, Dictionary<string, string>> GetRecordsContainingSubStringWithLambda(Dictionary<string, Dictionary<string, string>> originalNestedDictionary, string lambdaSubString)
{
    return originalNestedDictionary.Select(outerPair => new KeyValuePair<string, Dictionary<string, string>>(
        outerPair.Key,
        outerPair.Value
            .Where(innerPair => innerPair.Value != null && innerPair.Value.Contains(lambdaSubString))
            .ToDictionary(innerPair => innerPair.Key, innerPair => innerPair.Value)
    )).ToDictionary(outerPair => outerPair.Key, outerPair => outerPair.Value);
}