I'm having trouble deserializing a Json serialized dictionary of objects.

Here is some code to replicate the issue.

using System.Text.Json;

// my dictionary of objects
var dict = new Dictionary<string, object>();
dict.Add("test-date", DateTime.MaxValue);

// serialize the dictionary
var serialized = JsonSerializer.Serialize(dict);        // --> returns serialized = {"test-date":"9999-12-31T23:59:59.9999999"}

// deserialize the dictionary
dict = JsonSerializer.Deserialize<Dictionary<string, object>>(serialized);

DateTime testDate = (DateTime)dict["test-date"];       // --> throws System.InvalidCastException - Specified cast is not valid.

Does anyone know what could be the problem and any possible workarounds for it?

CodePudding user response：

If you look at the JSON after the serialization, the DateTime is contained as a string (there is no data type for DateTimes in JSON). When deserializing to a Dictionary<string, object>, the JsonSerializer does not have any information on how the string that it reads should be interpreted (though a human most likely can see that it is a datetime value). Hence, it uses the generic type JsonElement to store the data. You cannot cast a JsonElement to a DateTime because there are different ways on how to interpret the data.

There are several ways to overcome this:

• If you know that all the values in the dictionary are DateTime values, change the type of the dictionary to Dictionary<string, DateTime>. Then the deserializer knows how to interpret the data.
• If this is not possible, you can try to implement a custom serializer that analyses the JSON and checks whether the value can be converted to a DateTime. However, if there are various types to consider, this might proove to be a tedious task.
• If you only need to check this at a single location, and if you know for sure that the value is a DateTime, you can use the following code to get a DateTime:

var jsonElement = (JsonElement)dict["test-date"];
var dt = jsonElement.GetDateTime();

Also, you could retrieve the value as a string and be more cautious when parsing:

var s = jsonElement.GetString();
if (DateTime.TryParse(s, out var dt2))
    // ...

See this fiddle to test.

CodePudding user response：

Just in one line

DateTime testDate = Convert.ToDateTime(dict["test-date"].ToString()); //9999-12-31 11:59:59 PM

or my usuall advise: Use Newtonsoft.Json and your life will be much easier.

using Newtonsoft.Json;

DateTime testDate = (DateTime) dict["test-date"];