check property values before deserializing JSON into C# Objects

Question

I have the following json being returned by a third party api

{
    'category':'Cat1',
    'date': '12-12-2021'
    'amount' 23.23
}

which is being deserialized to following C# class

public class Quote{
    public string category {get;set;}
    public DateTime date {get;set;}
    public decimal amount {get;set;}
}

but sometimes we get None as value, eg:

{
    'category': None,
    'date': None
    'amount' None
}

and the application errors out while deserializing, actually it errors out while parsing the json before even attempting to serialize.

 JsonConvert.DeserializeObject<Quote>(json, settings);<--exception here

'Error parsing NaN value. Path 'category', line X, position XX.'

We tried replacing None with null using json.Replace("None", "null");, but then we get an exception when it tries to set date & amount, which is a DateTime and decimal type respectively, to null.

Is there a way to check the incoming value, and whenever we get None, we set the property with default(Type) value either globally in JsonSerializerSettings or using individually using custom attribute?

So what you are actually saying is you are getting invalid JSON - your first example is actually also invalid - well, they both are really — Mark Schultheiss, Dec 02 '21 at 17:17
first JSON is fine....I didn't copy paste the actual json............the problem is the `None` which is equivalent to C# `null` or `default(Type)` where a type cannot be `null`. — j4rey, Dec 02 '21 at 19:54
You could look into custom deserialize that checks for "None" and makes it null instead? https://www.newtonsoft.com/json/help/html/CustomJsonConverterGeneric.htm — Nikki9696, Dec 03 '21 at 00:11

score 1 · Answer 1 · answered Dec 03 '21 at 13:21

This is really about validation - which you should do before consuming anything from an external source.

This is invalid JSON:

{
    'category':'Cat1',
    'date': '12-12-2021'
    'amount' 23.23
}

{
    'category': Done,
    'date': Done
    'amount' Done
}

It should be this format

Double quotes not single
no missing ":" between the name/value pair
No missing comma between except the last one should have no comma after it

Example of allowed:

{
    "category":'Cat1",
    "date": "12-12-2021",
    "amount": 23.23
}

Example of allowed:

{
    "category": "None",
    "date": "None",
    "amount": "None"
}

You COULD use a valid null value set of pairs in the JSON string:

{
    "category": null,
    "date": null,
    "amount": null
}

Here is an example c# class with nullable value types:

public class Quote
{
    public string category {get;set;}
    public DateTime? date {get;set;}
    public decimal? amount {get;set;}
}

Now as for the actual validation and what gets replaced with what, that could be on the method accepting the values or in the setter for Quote. Rather than repeat a lot of code here I reference other questions with answers for that since that is a architectural and design issue:

Put validation in the setter: C# add validation on a setter method

Now if you want to validate before/outside the setter, you can do something like this very rough outline: How to handle exceptions is up to you.

public class TestQuote
{
    // these COULD be nullable types (see above) or some default
    public string Category { get; private set; }
    public DateTime? QuoteDate { get; private set; }
    public decimal? Amount { get; private set; }

    private NumberStyles style;

    public void SetCategory(string value)
    {
         //... your logic, throw exception if validation failed
         Category = value;
    }

    public void SetAmount(string value)
    {
         //... your logic, throw exception if validation failed
         // Now how you parse and default value is up to you re:
         // https://learn.microsoft.com/en-us/dotnet/api/system.decimal.parse?view=net-6.0
         style = NumberStyles.Number | NumberStyles.AllowCurrencySymbol;
         Amount = Decimal.Parse(value, style, CultureInfo.InvariantCulture);
    }
    
    public void SetQuoteDate(string value)
    {
         //... your logic, throw exception if validation failed 
         // OR change to some default DateTime
         // HOW you parse this and what default is up to you ref:
         // https://learn.microsoft.com/en-us/dotnet/api/system.datetime.parse?view=net-6.0
         QuoteDate = DateTime.Parse(value);
    }
}

Now how you call that is up to you since you provided no method that accesses the source of the JSON.

check property values before deserializing JSON into C# Objects

1 Answers1