c# - 如何在列表和字典之间进行左连接？

Question

我正在尝试将列表与字典相交，这非常有效:

    public static IDictionary<string, string> GetValues(IReadOnlyList<string> keys, IHeaderDictionary headers)
    {
        return keys.Intersect(headers.Keys)
            .Select(k => new KeyValuePair<string, string>(k, headers[k]))
            .ToDictionary(p => p.Key, p => p.Value);
    }

上述方法的用法是这样的:

    [TestMethod]
    public void GetValues_returns_dictionary_of_header_values()
    {
        var headers = new List<string> { { "trackingId" }, { "SourceParty" }, { "DestinationParty" } };
        var trackingIdValue = "thisismytrackingid";
        var sourcePartyValue = "thisismysourceparty";
        var destinationPartyValue = "thisismydestinationparty";
        var requestHeaders = new HeaderDictionary
        {
            {"trackingId", new Microsoft.Extensions.Primitives.StringValues(trackingIdValue) },
            {"SourceParty", new Microsoft.Extensions.Primitives.StringValues(sourcePartyValue) },
            {"DestinationParty", new Microsoft.Extensions.Primitives.StringValues(destinationPartyValue) },
            {"randomHeader", new Microsoft.Extensions.Primitives.StringValues("dontcare") }
        };
        var headerValues = HeaderOperators.GetValues(headers, requestHeaders);
        Assert.IsTrue(headerValues.ContainsKey("trackingId"));
        Assert.IsTrue(headerValues.ContainsKey("SourceParty"));
        Assert.IsTrue(headerValues.ContainsKey("DestinationParty"));
        Assert.IsTrue(headerValues.Count == headers.Count);
    }

但是，我想做一个left join，而不是intersect，例如，如果我在字典中搜索一个不存在的值，它仍然会返回带有一些默认 value 的键。

例如，如果我们输入oneMoreKey:

        var headers = new List<string> { { "trackingId" }, { "SourceParty" }, { "DestinationParty" }, {"oneMoreKey"} };

那么我希望这个结果是这样的:

        var headerValues = HeaderOperators.GetValues(headers, requestHeaders, "myDefaultValue");

headerValues 是:

 {"trackingId", "thisismytrackingid"}
 {"SourceParty", "thisismysourceparty"}
 {"DestinationParty", "thisismydestinationparty"}
 {"oneMoreKey", "myDefaultValue"}

如果交集不存在，我如何向交集添加默认值？

Answer 1

基于这个例子:Left join on two Lists and maintain one property from the right with Linq你也可以像这样用 GroupJoin 解决它:

public static IDictionary<string, string> GetValues(IReadOnlyList<string> keys, IHeaderDictionary headers, string defaultValue)
{
    return keys.GroupJoin(headers, key => key, header => header.Key, (key, header) => new { key, header })
        .SelectMany(x => x.header.DefaultIfEmpty(), (x, header) => new { x.key, header.Value })
        .Select(x => new KeyValuePair<string, string>(x.key, x.Value))
        .ToDictionary(p => p.Key, p => p.Value ?? defaultValue);
}