如何在 C# 中使用 LINQ 过滤嵌套字典？

Posted 2023-04-12

【中文标题】如何在 C# 中使用 LINQ 过滤嵌套字典？

【英文标题】：How can I filter a nested dictionary using LINQ in C#?

【发布时间】：2021-06-22 18:49:13

【问题描述】：

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>>'

【参考方案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);

【讨论】：

感谢您的回答。这行得通。我通过将innerPair.Key 替换为innerPair.Value 进行了一项更改，以检查该值是否为null。如果innerPair.Value 是null，我会收到错误消息。

@j.ruiz 啊，是的，那是我的错字。我会编辑答案:)