如何按指定键分组List<Dictionary<string,string>>并获取最大值

给定如下C#数据：

List<Dictionary<string,string>> data = new List<Dictionary<string,string>> 
{
    new Dictionary<string,string> { ["HoleName"] = "first", ["Length"] = "20" },
    new Dictionary<string,string> { ["HoleName"] = "second", ["Length"] = "40" },
    new Dictionary<string,string> { ["HoleName"] = "first", ["Length"] = "30" }
};

需要按HoleName字段分组，并提取每组中Length的最大值，最终得到目标格式的列表。

解决方案 #

利用LINQ实现分组与聚合，注意必须将Length字符串转换为数值类型后再比较大小（避免字符串字典序比较导致的错误，比如"100"作为字符串会被判定为小于"90"）：

.NET 6+版本（使用MaxBy简化代码）

var result = data
    .GroupBy(d => d["HoleName"])
    .Select(g => 
        new Dictionary<string, string> 
        {
            ["HoleName"] = g.Key,
            ["Length"] = g.MaxBy(d => int.Parse(d["Length"]))["Length"]
        })
    .ToList();

.NET 6以下版本（使用排序替代MaxBy）

var result = data
    .GroupBy(d => d["HoleName"])
    .Select(g => 
        new Dictionary<string, string> 
        {
            ["HoleName"] = g.Key,
            ["Length"] = g.OrderByDescending(d => int.Parse(d["Length"])).First()["Length"]
        })
    .ToList();

最终输出结果 #

执行上述代码后，result即为目标格式的列表：

List<Dictionary<string,string>> data = new List<Dictionary<string,string>> 
{ 
    new Dictionary<string,string> { ["HoleName"] = "first", ["Length"] = "30" }, 
    new Dictionary<string,string> { ["HoleName"] = "second", ["Length"] = "40" }
};

内容的提问来源于stack exchange，提问作者krishna porwal