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LINQ to JSON – Checking for an Object or Array

Sunday, September 14 2008 - linq, linq-to-json, book, rest

I was writing an event handler for my book that consumes a stream of JSON from a RESTful service. The handler is called when one of several asynchronous events is invoked. All of these events request a steam of either 1 or more products to be returned as JSON. So I wanted this handler to be able to consume the JSON whether it was a single product or an array of products.

The JSON is not wrapped, so a single object will not convert to a JsonArray. What I really wanted was to avoid writing the same LINQ to JSON query just to change out if it was looking for an object or an array. The way I handled this is to parse the raw JSON (a string variable) into a JsonArray object. If the JSON is a single object then when it is cast to a JsonArray it will be null. So at that point I know if it is an array of products or a single product.

Then, in the IF statement if it is a single object I parse the JSON into a JsonObject, create an instance of a JsonArray and add the JsonObject to the JsonArray. If the JSON data can be parsed into a JsonArray, the ELSE section parses it into a JsonArray. So in the end I have a JsonArray of either 1 or more objects. Why is this important? Because the query can now be written to find a list of products without having to handle wether it comes form a JsonObject or against a JsonArray.

C#

private void ParseProducts_AsJson(object sender, DownloadStringCompletedEventArgs e)

{

    string raw = e.Result;

    JsonArray json;

    if (JsonArray.Parse(raw) as JsonArray == null)

        json = new JsonArray {JsonObject.Parse(raw) as JsonObject};

    else

        json = JsonArray.Parse(raw) as JsonArray;

    var query = from product in json

                select new Product

                {

                    ProductId = (int)product["ProductId"],

                    ProductName = (string)product["ProductName"],

                    UnitPrice = (decimal)product["UnitPrice"]

                };

    List<Product> products = query.ToList() as List<Product>;

    lstProducts.DataContext = products;

}

VB

Private Sub ParseProducts_AsJson(ByVal sender As Object, _

        ByVal e As DownloadStringCompletedEventArgs)

    Dim raw As String = e.Result

    Dim json As JsonArray

    If TryCast(JsonArray.Parse(raw), JsonArray) Is Nothing Then

        json = New JsonArray With _

            {TryCast(JsonObject.Parse(raw), JsonObject)}

    Else

        json = TryCast(JsonArray.Parse(raw), JsonArray)

    End If

    Dim query = _

        From product In json _

        Select New Product With 

            {.ProductId = CInt(Fix(product("ProductId"))), _

            .ProductName = CStr(product("ProductName")), _

            .UnitPrice = CDec(product("UnitPrice"))}

    Dim products As List(Of Product) = _

           TryCast(query.ToList(), List(Of Product))

    lstProducts.DataContext = products

End Sub

If there is a better way, I am certainly open to it. I’d love to see some additional methods on these objects to allow a little more variation. But overall I am thrilled to see LINQ to JSON! Now, too bad I can’t create an anonymous type from this LINQ query and bind it to the XAML. Wishes and dream for the next version :)