Querying JSON with SelectToken

For some time I have been using an UnmanagedExports wrapper around JsonTextReader and JsonTextWriter from Json.NET which was quite handy but had limitations.

Today I have added the ability to Querying JSON with SelectToken which so far is pretty damn awesome!

Consider this JSON data for input:

    "solutions": [
            "departureTime": "08:24",
            "rego": "ABC123",
            "departureTime": "09:24",
            "rego": "ZZZ321",

I can do things like this:

  i=-1 ! remember zero based arrays in .NET
    vehicleRego = Json.SelectToken('solutions[' & i & '].rego')
    IF vehicleRego = ''
      ! We are done with the solutions, see ya!
    Debug.Message('Vehicle Rego: ' & vehicleRego)

Of course at the moment it considers everything a string on the .NET side and relies on Clarion data conversions which might bite me at some point but I image you could do something like Json.SelectToken('solutions[0].ChildSomething[22].MoneyField', EQUATE_TYPE:DECIMAL) and add a bit of handling on the other end to deal with different data types or whatever.

Anyway, I thought is was pretty cool and worth a share. With this I can react to incoming Json data in a very dynamic way without lots of pain. Yay!

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Hi Brahn.

Looks good. I’m a bit confused that your links go to the Json.NET site, but your code here is in Clarion? Are you developing a Clarion class as well?

I noticed that your check if there are elements s[i].rego left, is whether the value it returns is blank. Wouldn’t that cause a problem when the actual value is a blank string? Because Clarion doesn’t have a native NULL value, maybe you can add a property to your Json class to indicate that the SelectToken could not find the value, maybe a property called Error that returns an error when an operation failed?

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Correct. I create a .NET class that “exports” methods that I can call from Clarion. If you look for unmanaged or dllexport here you should get the idea. You can use them from the Clarion side by creating a LIB from the DLL or ,my preference, use LoadLibrary.

Yeah, in my particular use case this was not going to be a concern. Maybe something like this would work:

CASE Json.SelectTokenResult('solutions[' & i & '].rego') 
  Stop('SelectToken response was NULL' )
  Stop('SelectToken says: ' & Json.GetLastError())
  Stop('SelectToken found: ' & Json.GetCurrentValue())