Querying and Filtering Array of JObjects with Linq

Question

I suppose this is another entry in my series of questions, but I'm stuck again. This time, I'm having trouble working with a JArray of JObjects and determining the Property.Value type for each element in the JArray...

My code is here: https://dotnetfiddle.net/bRcSAQ

The difference between my previous questions and this question is that my outer Linq query gets both JObject and JArray tokens, so that's why I have an if (jo is JObject) at Line 40 and a if (jo is JArray) at Line 48.

Once I know I have a JArray of <JObjects>, I have code that looks like this (Lines 48-):

    if (jo is JArray)
    {
        var items = jo.Children<JObject>();                     
        // return a JObject object
    }

When I use a debugger and look at items, I see that it contains 3 JObject objects--one for Item_3A1, Item_3A2, and Item3A3. But I need to know the JTokenType for each JProperty.Value because I am interested only in Property values of type JTokenType.String.

So I tried:

// doesn't work :(
var items = jo.Children<JObject>()
              .Where(p => p.Value.Type == JTokenType.String);

The compiler red-lines the Value property with the error CS0119 'JToken.Value<T>(object)' is a method, which is not valid in the given context.

I realize that "p" in the Linq express is not a JProperty. I guess it's a JObject. And I don't know how to cast "p" so that I can examine the type of JProperty object it represents.

Ultimately, I need the code for JArray processing (starting at Line 48) to add an return a JObject that contains an JSON array composed only of JProperty objects of type JTokenType.String. This means that given the sample JSON, it first should return a JObject holding these JSON properties:

{ ""Item_3A1"": ""Desc_3A1"" },
{ ""Item_3A2"": ""Desc_3A2"" },
{ ""Item_3A3"": ""Desc_3A3"" }

On the next iteration, it should return a JObject holding these JSON properties (notice that the nested Array3B1 properties are omitted because Array3B1 is not a JProperty with a Value type of JTokenType.String):

{ ""Item_3B1"": ""Desc_3B1"" },
{ ""Item_3B2"": ""Desc_3B2"" },

The third iteration would contain:

{ ""Item_3B11"": ""Desc_3B11"" },
{ ""Item_3B12"": ""Desc_3B12"" },
{ ""Item_3B13"": ""Desc_3B13"" }

And the fourth (final) iteration would contain:

{ ""Item_3C1"": ""Desc_3C1"" },
{ ""Item_3C2"": ""Desc_3C2"" },
{ ""Item_3C3"": ""Desc_3C3"" }

This might be my final hurdle in this "series".

Sincere thanks to anyone who can and will help--and a special thanks again to users "Brian Rogers" and "dbc" for their truly amazing JSON.NET/Linq knowledge.

Yes, but that did not help. The key was to use SelectMany() and tack a Where() to the end of that resultset. — Jazimov
– Jazimov, Commented Jul 30, 2016 at 14:20

dbc · Accepted Answer · 2016-07-30 15:23:39Z

4

This produces the output you require:

var root = (JContainer)JToken.Parse(json);
var query = root.Descendants()
    .Where(jt => (jt.Type == JTokenType.Object) || (jt.Type == JTokenType.Array))
    .Select(jo =>
        {
            if (jo is JObject)
            {
                if (jo.Parent != null && jo.Parent.Type == JTokenType.Array)
                    return null;
                // No help needed in this section               
                // populate and return a JObject for the List<JObject> result 
                // next line appears for compilation purposes only--I actually want a populated JObject to be returned
                return new JObject();
            }

            if (jo is JArray)
            {
                var items = jo.Children<JObject>().SelectMany(o => o.Properties()).Where(p => p.Value.Type == JTokenType.String);
                return new JObject(items);
            }
            return null;
        })
    .Where(jo => jo != null)
    .ToList();

Here I use SelectMany() to flatten the nested enumeration of properties of the enumeration of child objects of jo to a single enumeration of all properties of child objects. o => o.Properties() is a lambda expression mapping the JObject o to its collection of properties, and p => p.Value.Type == JTokenType.String is another lambda mapping a property p (generated by the previous SelectMany clause) to a true/false value indicating whether the property has a string value. Both o and p are lambda input parameters that are implicitly typed.

Also, in the // No help needed in this section section, objects whose parents are arrays are skipped, since they will get collected by the (jo is JArray) clause.

Note that if different children of the jo array happen to have identical property names, the JObject constructor may throw a duplicated key exception.

Forked fiddle.

edited Jul 30, 2016 at 15:23

answered Jul 30, 2016 at 13:25

dbc

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4 Comments

Jazimov Over a year ago

Awesome, dbc. So is it correct to say that the o refers to a JObject (from Children()) and the p refers to a JProperty (from o.Properties())? If yes, that was my "missing linq". :)

Jazimov Over a year ago

In your forked fiddle results (after running), I see empty query items. How can I skip them? Would I add an any() modifier to the Linq expression? If so, where?

dbc Over a year ago

@Jazimov - OK, looks like you did need help in the No help needed section. I'd recommend skipping all objects whose parents are arrays since they will get collected in the subsequent if (jo is JArray) clause.

Jazimov Over a year ago

Well, in all fairness, I was not running the code that's in the No help needed section and it does in fact skip as needed (it also processes other non-array JSON but wasn't listed because it wasn't relevant to the question in this post). Apologies for the follow up question. :) May I ask for one final comment from you on my first comment? Then I'm on my way. Thank you again!

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