Runtime: IgnoreNullValues does not work with non-nullable value types

[findyoucef]

Setting JsonSerializerOptions property of IgnoreNullValues to true does not work.

When attempting to deserialize a DateTime value that is null in the json string it throws the exception below. I thought setting the option to IgnoreNullValues should ignore the property, no?

converted to System.DateTime. Path: $.value[0].deletedDateTime
InvalidOperationException: Cannot get the value of a token type 'Null' as a string.

Thanks
Youcef

[ViktorHofer]

cc @ahsonkhan, @steveharter

[ahsonkhan]

Here's a sample repro (looks like it can repro with any value type - not just DateTime, like int32, but works fine with reference types like string):

```C#
[Fact]
public static void IgnoreNullValuesBug()
{
var options = new JsonSerializerOptions { IgnoreNullValues = true };
Temp result = JsonSerializer.Deserialize("{\"Foo\": null}", options);
Console.WriteLine(JsonSerializer.Serialize(result, options));
}

public class Temp
{
public DateTime Foo { get; set; }
}

```text
System.Text.Json.Tests.Utf8JsonReaderTests.IgnoreNullValuesBug [FAIL]
        System.Text.Json.JsonException : The JSON value could not be converted to System.DateTime. Path: $.Foo | LineNumber: 0 | BytePositionInLine: 12.
        ---- System.InvalidOperationException : Cannot get the value of a token type 'Null' as a string.
        Stack Trace:
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\ThrowHelper.Serialization.cs(178,0): at System.Text.Json.ThrowHelper.ReThrowWithPath(ReadStack& readStack, Utf8JsonReader& reader, Exception ex)
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializer.Read.cs(132,0): at System.Text.Json.JsonSerializer.ReadCore(JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& readStack)
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializer.Read.Helpers.cs(22,0): at System.Text.Json.JsonSerializer.ReadCore(Type returnType, JsonSerializerOptions options, Utf8JsonReader& reader)
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializer.Read.String.cs(74,0): at System.Text.Json.JsonSerializer.ParseCore(String json, Type returnType, JsonSerializerOptions options)
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializer.Read.String.cs(31,0): at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
          E:\GitHub\Fork\corefx\src\System.Text.Json\tests\Utf8JsonReaderTests.cs(19,0): at System.Text.Json.Tests.Utf8JsonReaderTests.IgnoreNullValuesBug()
          ----- Inner Stack Trace -----
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\Reader\Utf8JsonReader.TryGet.cs(820,0): at System.Text.Json.Utf8JsonReader.TryGetDateTime(DateTime& value)
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\Reader\Utf8JsonReader.TryGet.cs(396,0): at System.Text.Json.Utf8JsonReader.GetDateTime()
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\Serialization\Converters\JsonValueConverterDateTime.cs(11,0): at System.Text.Json.Serialization.Converters.JsonConverterDateTime.Read(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options)
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\Serialization\JsonPropertyInfoNotNullable.cs(25,0): at System.Text.Json.JsonPropertyInfoNotNullable`4.OnRead(JsonTokenType tokenType, ReadStack& state, Utf8JsonReader& reader)
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\Serialization\JsonPropertyInfo.cs(353,0): at System.Text.Json.JsonPropertyInfo.Read(JsonTokenType tokenType, ReadStack& state, Utf8JsonReader& reader)
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializer.Read.HandleNull.cs(62,0): at System.Text.Json.JsonSerializer.HandleNull(Utf8JsonReader& reader, ReadStack& state)
          E:\GitHub\Fork\corefx\src\System.Text.Json\src\System\Text\Json\Serialization\JsonSerializer.Read.cs(117,0): at System.Text.Json.JsonSerializer.ReadCore(JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& readStack)

Note that serialization (i.e. writing) works as expected (where null values are getting ignored). The issue is specific to deserialization (reading).

[steveharter]

Work-arounds:

• Use a nullable type (e.g. Nullable<DateTime>)
• Create a custom converter for that type that can handle null

[steveharter]

There is a question here on correctness and validation. Since the value can never be null anyway, should IgnoreNullValues actually ignore "invalid" JSON? i.e. there can be reasons to want an exception in this case.

I compared against Json.NET (using NullValueHandling = NullValueHandling.Ignore) and it does ignore non-nullable values on deserialization, so perhaps compat is reason enough.

[steveharter]

Some options assuming IgnoreNullValues=true.
1) Keep existing semantics; the workaround is to use a nullable type such as DateTime? (or a custom converter if that is not desired). IgnoreNullValues=true means "ignore null only when null is a valid value".
2) Keep existing semantics but add an attribute that can be applied per property -- such as a new JsonIgnoreNullValueAttribute that will always ignore nulls. This feature was originally discussed but cut. It is also possible to extend the existing JsonIgnoreAttribute.
2) Make the change to ignore "null" on non-nullable structs. This could potentially break users that depend on the existing exception for validation reasons (since a non-nullable struct can't be null). IgnoreNullValues=true means "ignore null even when null is not a valid value".

4) ...

[ahsonkhan]

@rynowak, @JamesNK - what do you think? To me, option 3 makes the most sense. When I see the IgnoreNullValues option from an end-user perspective, I assume it would work for both serialization and deserialization (even on value types that can't be null).

Although, I can see an argument to be made about option 1, but that requires the user to change their object model or workaround it with their own converter (maybe that's OK).

Given this is effectively by-design, moving to 5.0 for now.

[steveharter]

For the "ignore null" feature we discussed the following benefits:

• Trimming the JSON during serialization, primarily for performance including subsequent deserialization since the JSON being deserialized is not littered with nulls.
• Avoiding erasing default values during deserialization -- for example a constructor may initialize some values to a non-null values and may not want them overwritten with a null during deserialization. Also helps performance a bit since no setter needs to be called. We also discussed a "default value" feature that is a superset of this, but that was cut.

We never discussed whether this feature can be used to disable validation by removing null values from non-nullable structs.

[rynowak]

Option 3 makes the most sense. I don't think there's anything terrible about the JSON.NET behaviour.

[layomia]

Punting this to future, as we don't want to loosen this behavior before designing upcoming validation features including required properties and (de)serialization callbacks.

[chris-rogala]

Today I ran into an issue with this with the a DateTime property. Basically, my object is something like:

public class SomeDTO
{
    public DateTime SomeAwesomeDate { get; set; }
}

When the body looks like this:

{
  "someAwesomeDate": null
}

And the method looks like this:

public Task<IActionResult> Post([FromBody] SomeDTO model)

The model is always null. I try the same thing with a double, and it reacts the same way with a double. Keep in mind, this is for a more complex contract and even if all the other properties are set, it will set the model from the body to null. This is using .NET Core 3.1. Thoughts?