Debugging JSON.NET

Question

I'm using JSON.NET to deserialize some JSON returned from a web service. Unfortunately, i'm getting the following err:

Cannot deserialize JSON array into type 'System.Collections.Generic.Dictionary`2[System.String,System.String]'.

but i have no idea how to debug it. is there a way to have it tell me what the offending JSON is? or even what it's trying to deserialize it to? unfortunately this is part of a larger object hierarchy, so it's not just a simple piece of JSON. i've tried stepping through the code, but there's miles of it and little documentation. If it could at least tell me the offending piece, then i can see if maybe i cocked up my object definition or something.

score 30 · Answer 1 · edited Sep 03 '13 at 13:00

If you want to deserialize what you're converting from json into C# classes there's something built into json.net for this. Just create a settings object and pass it into your deserializeobject call:

if (!string.IsNullOrEmpty(json))
{
   var settings = new JsonSerializerSettings
       {
           Error = (sender, args) =>
                   {
                       if (System.Diagnostics.Debugger.IsAttached)
                       {
                            System.Diagnostics.Debugger.Break();
                       }
                   }
       };

    result = JsonConvert.DeserializeObject<T>(json, settings);
}

Then when there are errors in the deserialization process (like a mismapped C# class -> json object) it'll break in the debugger. Inspect the args object and you'll see a property called CurrentObject. This is the C# class that the parser is trying to deserialize the JSON into (and will provide details about what the error might be).

Hope that helps.

How can I use the above code in vb.net for the same problem? — PeterCo, May 15 '16 at 19:13
Perfect! - I had to turn on `MissingMemberHandling` detailed in http://stackoverflow.com/a/21032130 to get this to work. — bendecko, Jul 31 '16 at 16:38
Good answer. In my case, this didn't give any errors. The cause was that one of my properties wasn't 'public'. Doh! — JsAndDotNet, Apr 27 '18 at 13:26

score 6 · Accepted Answer · answered Oct 18 '11 at 23:24

6

Figured out where to look after walking through tons of code. line 1118ish JsonSerializerInternalReader is:

          SetPropertyValue(property, reader, newObject);

and if you put a breakpoint there and watch "property" you can see what property is getting serialized. since it does it in order of the json string you can watch that and see the last one to successfully get set. then you at least know where the serializer is failing.

it would be helpful, however, if JSON.net raised at least the property name in the error, but until that happens, this seems to be the next best thing.

answered Oct 18 '11 at 23:24

bryan costanich

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Use the source Luke! and contribute it back ;-) – poupou Oct 18 '11 at 23:55
yeah, i might just have to do that. – bryan costanich Oct 18 '11 at 23:57
This is incredibly useful! I can literally debug it property after property, thank you for pointing me into the right direction @bryancostanich! – Nek0pi Oct 18 '21 at 08:42

score 0 · Answer 3 · answered Oct 18 '11 at 23:15

0

Based on the information you've provided, I would start by looking for an object that has a property of type Dictionary. That's probably your problem class.

answered Oct 18 '11 at 23:15

Justin Rusbatch

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I'm curious, how many classes are being serialized in one object? – Justin Rusbatch Oct 18 '11 at 23:25
not into one object. into a hierarchy of objets. i'm consuming ARCGis web services hosted by USGS.gov. ARCGis has some services that return hundreds of objects in a hierarchy. they also do some very screwy stuff, such as abstract type resolution based on parameters. :( – bryan costanich Oct 18 '11 at 23:32

Debugging JSON.NET

3 Answers3