Get JSON.Net to throw Exception if object not of expected type in Generic method

Question

I created a generic deserialization method that I pass a json string into. T is the type that I want returned.

    private static T Deserialize<T>(string input)
    {
        try
        {
            var result = JsonConvert.DeserializeObject<T>(input);
            return result;
        }
        catch (JsonSerializationException ex)
        {
            var errorResult = JsonConvert.DeserializeObject<Dictionary<string, string>>(input);

            var errorMessage = errorResult["error"];

            throw new ApplicationException(errorMessage, ex);
        }
    }

So to call the method we choose

Deserialize<MyObject>(jsonString)

Which successfully returns an object of Type MyObject.

However, the json passed in can sometimes contain an error message (error : message) if things have gone wrong.

What I wanted to do in this case is cause a JsonSerializationException and read the error message.

However, no such error is thrown and the MyObject is retured but with all null and zero values.

Error json

"{\"error\":\"Error message"}"

Other json

"[{\"path\":\"\",\"id\":2000,\"name\":\"Name1\"},{\"path\":\"\",\"id\":2001,\"name\":\"Name2\"},{\"path\":\"\",\"id\":2002,\"name\":\"Name3\"}]"

The other json could be in pretty much any format - but the point is that the required Type is passed into the method and that is what should get returned or an exception should be thrown.

wingyip · Accepted Answer · 2016-11-03 21:28:56Z

2

There no JsonSerializationException because the Json is well formated but no property match with the properties of the type T. So the result is a new T object.

I would say that the following code should work :

//First, convert to JToken 
var o = JsonConvert.DeserializeObject<JToken>(input);
//Then check if the property error exists
if(!(o is JArray) && o["error"] != null);
{
     throw new Exception(...)      
}
//Finally convert the object
return o.ToObject<T>();

edited Nov 3, 2016 at 21:28

wingyip

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answered Nov 2, 2016 at 16:05

asfez

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3 Comments

wingyip Over a year ago

This does not work unfortunately. I am getting a "Error reading JObject from JsonReader. Current JsonReader item is not an object: StartArray. Path '', line 1, position 1."

wingyip Over a year ago

Added json examples to the original post

asfez Over a year ago

I corrected the solution to take into account that your object is an array.

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