Deserialized JSON object not null for wrong type

September 1, 2022

If i have the following c# record:

public record MyRecord(string EndPoint, string Id);

and i have some json that i attempt to deserialize to my type:

var myType= JsonSerializer.Deserialize<MyRecord>(
            jsonStr,
            new JsonSerializerOptions
            {
                PropertyNameCaseInsensitive = true
            }
        );

      if(myType is null)
      {
         ...
      }

where jsonStr is not of type MyRecord but some other type, the resulting myType rather then being null, is simply an instance of MyRecord but with empty properties. How can i check that the returned type is invalid if its not null ?

>Solution :

JsonSerializer does not make assumptions about the lack of contents of a json string, provided that it is at least in the same format as the target structure (e.g. "{ }").

As a result, you need to check one of your properties for validity as well as the object itself:

var jsonStr = "{}";

var myType = JsonSerializer.Deserialize<MyRecord>(
        jsonStr,
        new JsonSerializerOptions
        {
            PropertyNameCaseInsensitive = true
        }
    );

if (!string.IsNullOrEmpty(myType?.Id))
{
    var x = myType;
}

Note that the behavior is identical for classes as well:

public class MyClass
{
    public string EndPoint { get; set; }
    public string Id { get; set; }
}

var myType2 = JsonSerializer.Deserialize<MyClass>(
        jsonStr,
        new JsonSerializerOptions
        {
            PropertyNameCaseInsensitive = true
        }
    );

if (!string.IsNullOrEmpty(myType2?.Id))
{
    var x = myType;
}