使用Json.NET测试嵌套键? [英] Testing for nested keys with Json.NET?

问题描述

我想过滤像这样的json编码字符串的反应流:

I want to filter a reactive stream of json encoded strings like this:

{ "Key1" : {"key2":"value"},
  "key3" : "other values"
}

我想过滤具有key2值的项目，例如:

I want to filter for items that have some value for key2, like this:

IDisposable valueQuery = globalEventStream
    .Select(e => JObject.Parse(e.EventArgs.Data))
    .Where(e => e["key1"]["key2"] != null)

但这给了我错误

Cannot access child value on Newtonsoft.Json.Linq.JValue

我能找到解决此问题的唯一方法是执行以下操作:

The only way I can find to get around this is to do the following:

IDisposable deathDisposable = globalEventStream
    .Select(e => JObject.Parse(e.EventArgs.Data))
    .Where(e => e["key1"] != null).Select(e => e["key1"])
    .Where(e => e["key2"] != null).Select(e => e["key2"])

是否有一种方法可以通过单个Where语句来过滤嵌套键?

Is there a way to filter for nested keys with a single Where statement?

推荐答案

您可以使用 JToken.SelectTokens() 在LINQ上运行嵌套查询到JSON对象:

You can use JToken.SelectTokens() to run nested queries on LINQ to JSON objects:

var valueQuery = globalEventStream
    .Select(e => JObject.Parse(e.EventArgs.Data))
    .Where(o => o.SelectTokens("Key1.key2").Any());

示例小提琴.

JSONPath的规范是此处，并且支持以下查询语法:

The specification for JSONPath is here and supports the following query syntax:

XPath   JSONPath    Description
/       $            the root object/element
.       @           the current object/element
/       . or []     child operator
..      n/a         parent operator
//      ..          recursive descent. JSONPath borrows this syntax from E4X.
*       *           wildcard. All objects/elements regardless their names.
@       n/a         attribute access. JSON structures don't have attributes.
[]      []          subscript operator. XPath uses it to iterate over element collections and for predicates. In Javascript and JSON it is the native array operator.
|       [,]         Union operator in XPath results in a combination of node sets. JSONPath allows alternate names or array indices as a set.
n/a     [start:end:step]    array slice operator borrowed from ES4.
[]      ?()         applies a filter (script) expression.
n/a     ()          script expression, using the underlying script engine.
()      n/a         grouping in Xpath 

例如，用于以下JSON:

E.g., for the following JSON:

{ "Key1" : [{"NoKey2":"value"}, {"key2":"value"}],  "key3" : "other values" }

您可以使用通配符*来测试具有"key2"属性的数组中是否存在任何项目，如下所示:

You could use the wildcard operator * to test for the presence of any item in the array with a "key2" property as follows:

var valueQuery = globalEventStream
    .Select(e => JObject.Parse(e.EventArgs.Data))
    .Where(o => o.SelectTokens("Key1[*].key2").Any());

在 Enumerable.Any() 中进行测试以查看SelectTokens()查询发现了不止零个结果-即"Key1"数组中至少有一个具有"key2"属性的条目.它比做 Enumerable.Count() 效率更高，因为它会立即停止评估作为一项退回.

Here Enumerable.Any() tests to see whether the SelectTokens() query found more than zero results -- i.e., is there at least one entry in the "Key1" array with a "key2" property. It's more efficient than doing Enumerable.Count() since it stops the evaluation as soon as one item is returned.

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