Add JsonConstructor attributes to Date and Time for System.Text.Json deserialization

[Copilot]

Fix Date and Time deserialization with System.Text.Json ✅

Plan:

• Add [JsonConstructor] attribute to Date(int year, int month, int day) constructor
• Add [JsonConstructor] attribute to Time(int hour, int minute, int second) constructor
• Create DateTests.cs with System.Text.Json deserialization tests
• Add System.Text.Json deserialization tests to TimeTests.cs
• Build and test to validate the changes
• Request code review
• Run security scan
• Run dotnet format to fix whitespace formatting

Summary

Successfully fixed the deserialization issue with Date and Time types when using System.Text.Json.JsonSerializer.

Changes Made:

1. Modified Microsoft.Kiota.Abstractions.csproj: Added System.Text.Json package reference (version 6.0.10+) for netstandard2.0, netstandard2.1, and net5.0 targets
2. Modified Date.cs: Added using System.Text.Json.Serialization; and [JsonConstructor] attribute to the Date(int year, int month, int day) constructor
3. Modified Time.cs: Added using System.Text.Json.Serialization; and [JsonConstructor] attribute to the Time(int hour, int minute, int second) constructor
4. Created DateTests.cs: Added 4 comprehensive tests for Date serialization, deserialization, round-trip, and ToString
5. Modified TimeTests.cs: Added 3 new tests for Time serialization, deserialization, and round-trip
6. Applied code formatting: Ran dotnet format to fix whitespace formatting in DateTests.cs

Test Results:

• ✅ All tests passing: 178/178 tests in abstractions project
• ✅ 9 Date and Time specific tests all pass
• ✅ Manual verification confirms deserialized objects contain correct values instead of default values
• ✅ Code review: No issues found
• ✅ Security scan: No vulnerabilities found
• ✅ Code formatting: Passes dotnet format verification

Security Summary:

No security vulnerabilities were discovered or introduced by this change.

Original prompt

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This section details on the original issue you should resolve

<issue_title>System.Text.Json.JsonSerializer cannot deserialize Date and Time types using default converters</issue_title>
<issue_description>Hello!

The default JSON converters in System.Text.Json do not work with the Date or Time types. These types are serialized as objects containing the read-only properties. Those properties are not set when deserializing, so they get the default values. Same issue applies with Newtonsoft.Json.

This is a problem when using third-party libraries that can't use KiotaJsonSerializer or the extension methods. My specific use case is using Temporal, which serializes values returned from activities and deserializes them later. I had an activity that returned a Kiota-generated model with a date, but the date later deserialized to 0001-01-01. This is easy to fix by creating a type that uses DateOnly or registering a new JsonConverter, but the bug confused me for a while.

This was briefly mentioned in #282 which was closed due to no response. That issue also mentions serializing enums, which I haven't tried.

Demo: (.NET 9.0.10 using Microsoft.Kiota.Abstractions v1.20.1)

var date = new Date(DateTime.Now);

var dateSerialized = JsonSerializer.Serialize(date);
// {
//    "DateTime": "2025-10-24T10:18:54.5003283-05:00",
//    "Year": 2025,
//    "Month": 10,
//    "Day": 24
// }

var dateDeserialized = JsonSerializer.Deserialize<Date>(dateSerialized);
// {
//    DateTime = 1/1/0001 12:00:00 AM
//    Year = 1
//    Month = 1
//    Day = 1
// }

var time = new Time(DateTime.Now);

var timeSerialized = JsonSerializer.Serialize(time);
// {
//    "DateTime": "2025-10-24T10:18:54.5090402-05:00",
//    "Hour": 10,
//    "Minute": 18,
//    "Second": 54
// }

var timeDeserialized = JsonSerializer.Deserialize<Time>(timeSerialized);
// {
//    DateTime = 1/1/0001 12:00:00 AM
//    Hour = 1
//    Minute = 1
//    Second = 1
// }

This could be fixed a couple different ways:

1. Change the DateTime property from { get; } to { get; init; }. This also fixes deserialization with Newtonsoft.Json. I can't think of any downside.
2. Add [JsonConstructor] to one of the constructors so the JSON properties are used in the constructor arguments.
3. Provide JsonConverters that mimic KiotaJsonSerializer. This would also fix serialization so they are written as strings instead of objects, but that might technically be a breaking change.</issue_description>

<agent_instructions>Please add JsonConstructor attribute to the best choice of constructor for these classes and write tests to validate the functionality.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@gavinbarron Thanks for raising this @n0spaces

init is a c#9 feature, as we produce code to be compatible with netstandard2.0 this restricts the language features that we can use. At present we do use a few C#8.0 features that can be used without additional types and remain compatible with netstandard2.0. So that's out at the moment.

the [JsonConstructor] option is probably the best way forward here as it represents the lowest risk.

• Fixes System.Text.Json.JsonSerializer cannot deserialize Date and Time types using default converters #580

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[gavinbarron]

@copilot please run dotnet format

[Copilot]

@copilot please run dotnet format

Done! Applied formatting in commit 09fab74.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud