调用JObject.Parse（）时处理null

Question

In the below code, I'm getting an exception due the value with in JObject.Parse() resulting in a null, which of course can't be parsed.

JObject result = JObject.Parse(responseData["Result"].ToString());

To try and work around this I tried the following so that a null check was done before calling parse.

var resultString = string.IsNullOrEmpty(responseData["Result"].ToString()) ? "" : responseData["Result"].ToString();
JObject result = JObject.Parse(resultString);

This still causes an exception though, I feel like I'm just overlooking something very small here. What can I do to address a possible null value here.

Answer 1

This has nothing to do with JObject.Parse - I suspect that will handle a null by just returning a null, or empty, JObject.

Most likely is that you;re missing that responseData["Result"] is null, and you're trying to call ToString() on it. This will cause a NullReferenceException

Fix it by checking for null:

var resultString = (responseData["Result"] == null || string.IsNullOrEmpty(responseData["Result"].ToString())) 
    ? "" 
    : responseData["Result"].ToString();

Answer 2

JObject result = JObject.Parse(responseData["Result"].ToString()); could be throwing an exception for several reasons:

• If responseData itself is null, then trying to access responseData["Result"] will throw an exception.
• If responseData["Result"] is null (eg if there is no key called "Result" present in responseData ) then trying to access responseData["Result"].ToString() will throw an exception.
• If responseData["Result"].ToString() is null (or empty), then JObject.Parse() will throw because null is not a valid JSON string.

Merely checking string.IsNullOrEmpty(responseData["Result"].ToString()) is not good enough to fix it because it only addresses the last of these three cases. Most likely the exception is being thrown before the ToString() call, and you are only checking whether the result of ToString() is null. By that point it is too late.

When you run into situations like this where you have a complex line of code and it is throwing an exception somewhere within it, it helps to break it down and do null checks at each step. That way you can easily step through with a debugger and see exactly what is going on.

JObject result = null;
if (responseData != null)
{
    object obj = responseData["Result"];
    if (obj != null)
    {
        string str = obj.ToString();
        if (!string.IsNullOrEmpty(str))
        {
            result = JObject.Parse(str);
        }
    }
}

Having said all that, I want to point out that you seem to be making your code more complex than it needs to be in the first place. You didn't say in your question what responseData is, but based on how you're using it I'm guessing it is a JObject . If that is true, you shouldn't need to convert responseData["result"] to string only to then re-parse it back into a JObject . Instead all you need to do is cast it:

JObject result = responseData != null ? responseData["Result"] as JObject : null;