TimeSpan is (de)serialized incorrectly using the JSON source generator

[martincostello]

Description

If a TimeSpan is serialized using the JSON source generator it is output as an object:

{"duration":{"days":0,"hours":0,"milliseconds":0,"minutes":0,"seconds":1,"ticks":10000000,"totalDays":1.1574074074074073E-05,"totalHours":0.0002777777777777778,"totalMilliseconds":1000,"totalMinutes":0.016666666666666666,"totalSeconds":1}}

If a TimeSpan is serialized without using the source generator, it is serialized as a string:

{"duration":"00:00:01"}

Similarly, the source generator will fail to deserialize strings to TimeSpan values as it is expecting an object.

Curiously, if the instance of JsonSerializerOptions used to serialize as a string is passed to a new instance of the custom JSON serializer context but then not used, then same operation will then change to serializing as an object from a string.

Reproduction Steps

To reproduce run dotnet build using the .NET 6.0.100 SDK for the below application.

using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;

var value = new MyObject {  Duration = TimeSpan.FromSeconds(1) };

var context = new MyJsonContext(new(JsonSerializerDefaults.Web));
var options = new JsonSerializerOptions(JsonSerializerDefaults.Web);

// Outputs the TimeSpan as an object
var bytes = JsonSerializer.SerializeToUtf8Bytes(value, typeof(MyObject), context);
var json = Encoding.UTF8.GetString(bytes);

Console.WriteLine(json);
Console.WriteLine();

// Outputs the TimeSpan as a string
bytes = JsonSerializer.SerializeToUtf8Bytes<MyObject>(value, options);
json = Encoding.UTF8.GetString(bytes);

Console.WriteLine(json);
Console.WriteLine();

options = new JsonSerializerOptions(JsonSerializerDefaults.Web);

// Create a new context that uses the same options.
// This causes the behavior of SerializeToUtf8Bytes<T>() to change.
context = new MyJsonContext(options);

// Outputs the TimeSpan as an object again
bytes = JsonSerializer.SerializeToUtf8Bytes<MyObject>(value, options);
json = Encoding.UTF8.GetString(bytes);

Console.WriteLine(json);
Console.WriteLine();

try
{
    JsonSerializer.Deserialize<MyObject>("{\"duration\":\"00:00:01\"}", options);
}
catch (JsonException ex)
{
    Console.Error.WriteLine("Failed to deserialize TimeSpan from string: {0}", ex);
    Console.Error.WriteLine();
}

[JsonSerializable(typeof(MyObject))]
public partial class MyJsonContext : JsonSerializerContext
{
}

public class MyObject
{
    public TimeSpan Duration { get; set; }
}

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <ImplicitUsings>enable</ImplicitUsings>
    <NoWarn>$(NoWarn);CA1050</NoWarn>
    <Nullable>enable</Nullable>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
  </PropertyGroup>
</Project>

Expected behavior

The application outputs the following:

{"duration":"00:00:01"}
{"duration":"00:00:01"}
{"duration":"00:00:01"}

Actual behavior

The application outputs the following:

{"duration":{"days":0,"hours":0,"milliseconds":0,"minutes":0,"seconds":1,"ticks":10000000,"totalDays":1.1574074074074073E-05,"totalHours":0.0002777777777777778,"totalMilliseconds":1000,"totalMinutes":0.016666666666666666,"totalSeconds":1}}

{"duration":"00:00:01"}

{"duration":{"days":0,"hours":0,"milliseconds":0,"minutes":0,"seconds":1,"ticks":10000000,"totalDays":1.1574074074074073E-05,"totalHours":0.0002777777777777778,"totalMilliseconds":1000,"totalMinutes":0.016666666666666666,"totalSeconds":1}}

Failed to deserialize TimeSpan from string: System.Text.Json.JsonException: The JSON value could not be converted to System.TimeSpan. Path: $.duration | LineNumber: 0 | BytePositionInLine: 22.
   at System.Text.Json.ThrowHelper.ThrowJsonException_DeserializeUnableToConvertValue(Type propertyType)
   at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.Converters.JsonMetadataServicesConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.Metadata.JsonPropertyInfo`1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader)
   at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.Converters.JsonMetadataServicesConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 utf8Json, JsonTypeInfo jsonTypeInfo, Nullable`1 actualByteCount)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 json, JsonTypeInfo jsonTypeInfo)
   at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
   at Program.<Main>$(String[] args) in C:\Coding\martincostello\JsonSourceGeneratorObsoleteProperties\Program.cs:line 39

Regression?

No.

Known Workarounds

No response

Configuration

Partial output from dotnet --info:

> dotnet --info
.NET SDK (reflecting any global.json):
 Version:   6.0.100
 Commit:    9e8b04bbff

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19043
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\6.0.100\

Host (useful for support):
  Version: 6.0.0
  Commit:  4822e3c3aa

Other information

No response

[ghost]

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Issue Details

---

Description

If a TimeSpan is serialized using the JSON source generator it is output as an object:

{"duration":{"days":0,"hours":0,"milliseconds":0,"minutes":0,"seconds":1,"ticks":10000000,"totalDays":1.1574074074074073E-05,"totalHours":0.0002777777777777778,"totalMilliseconds":1000,"totalMinutes":0.016666666666666666,"totalSeconds":1}}

If a TimeSpan is serialized without using the source generator, it is serialized as a string:

{"duration":"00:00:01"}

Similarly, the source generator will fail to deserialize strings to TimeSpan values as it is expecting an object.

Curiously, if the instance of JsonSerializerOptions used to serialize as a string is passed to a new instance of the custom JSON serializer context but then not used, then same operation will then change to serializing as an object from a string.

Reproduction Steps

To reproduce run dotnet build using the .NET 6.0.100 SDK for the below application.

using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;

var value = new MyObject {  Duration = TimeSpan.FromSeconds(1) };

var context = new MyJsonContext(new(JsonSerializerDefaults.Web));
var options = new JsonSerializerOptions(JsonSerializerDefaults.Web);

// Outputs the TimeSpan as an object
var bytes = JsonSerializer.SerializeToUtf8Bytes(value, typeof(MyObject), context);
var json = Encoding.UTF8.GetString(bytes);

Console.WriteLine(json);
Console.WriteLine();

// Outputs the TimeSpan as a string
bytes = JsonSerializer.SerializeToUtf8Bytes<MyObject>(value, options);
json = Encoding.UTF8.GetString(bytes);

Console.WriteLine(json);
Console.WriteLine();

options = new JsonSerializerOptions(JsonSerializerDefaults.Web);

// Create a new context that uses the same options.
// This causes the behavior of SerializeToUtf8Bytes<T>() to change.
context = new MyJsonContext(options);

// Outputs the TimeSpan as an object again
bytes = JsonSerializer.SerializeToUtf8Bytes<MyObject>(value, options);
json = Encoding.UTF8.GetString(bytes);

Console.WriteLine(json);
Console.WriteLine();

try
{
    JsonSerializer.Deserialize<MyObject>("{\"duration\":\"00:00:01\"}", options);
}
catch (JsonException ex)
{
    Console.Error.WriteLine("Failed to deserialize TimeSpan from string: {0}", ex);
    Console.Error.WriteLine();
}

[JsonSerializable(typeof(MyObject))]
public partial class MyJsonContext : JsonSerializerContext
{
}

public class MyObject
{
    public TimeSpan Duration { get; set; }
}

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <ImplicitUsings>enable</ImplicitUsings>
    <NoWarn>$(NoWarn);CA1050</NoWarn>
    <Nullable>enable</Nullable>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
  </PropertyGroup>
</Project>

Expected behavior

The application outputs the following:

{"duration":"00:00:01"}
{"duration":"00:00:01"}
{"duration":"00:00:01"}

Actual behavior

The application outputs the following:

{"duration":{"days":0,"hours":0,"milliseconds":0,"minutes":0,"seconds":1,"ticks":10000000,"totalDays":1.1574074074074073E-05,"totalHours":0.0002777777777777778,"totalMilliseconds":1000,"totalMinutes":0.016666666666666666,"totalSeconds":1}}

{"duration":"00:00:01"}

{"duration":{"days":0,"hours":0,"milliseconds":0,"minutes":0,"seconds":1,"ticks":10000000,"totalDays":1.1574074074074073E-05,"totalHours":0.0002777777777777778,"totalMilliseconds":1000,"totalMinutes":0.016666666666666666,"totalSeconds":1}}

Failed to deserialize TimeSpan from string: System.Text.Json.JsonException: The JSON value could not be converted to System.TimeSpan. Path: $.duration | LineNumber: 0 | BytePositionInLine: 22.
   at System.Text.Json.ThrowHelper.ThrowJsonException_DeserializeUnableToConvertValue(Type propertyType)
   at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.Converters.JsonMetadataServicesConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.Metadata.JsonPropertyInfo`1.ReadJsonAndSetMember(Object obj, ReadStack& state, Utf8JsonReader& reader)
   at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.Converters.JsonMetadataServicesConverter`1.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value)
   at System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 utf8Json, JsonTypeInfo jsonTypeInfo, Nullable`1 actualByteCount)
   at System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 json, JsonTypeInfo jsonTypeInfo)
   at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
   at Program.<Main>$(String[] args) in C:\Coding\martincostello\JsonSourceGeneratorObsoleteProperties\Program.cs:line 39

Regression?

No.

Known Workarounds

No response

Configuration

Partial output from dotnet --info:

> dotnet --info
.NET SDK (reflecting any global.json):
 Version:   6.0.100
 Commit:    9e8b04bbff

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19043
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\6.0.100\

Host (useful for support):
  Version: 6.0.0
  Commit:  4822e3c3aa

Other information

No response

Author:	martincostello
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-

[eiriktsarpalis]

Good catch, thanks. Looks like #54186 didn't add sourcegen support. I've created #62130 that fixes the issue. I don't believe this meets the bar for servicing but it's low risk enough to consider. cc @ericstj

[martincostello]

My (clearly biased) opinion is that this would be a good candidate for servicing as the built-in TimeSpan converter is more performant than a hand-rolled implementation that we would have used in .NET 5. Responses using TimeSpan already are then less compelled to move to the JSON source-generator as the workaround for this issue eats into the perf gains it would bring.

I'm still testing out the JSON source generator in an internal app (which is where these issues have been surfaced), and for the app using TimeSpan I'm not seeing particularly noticeable gains in performance on our benchmarks with the generator plugged in, so I wondered if restoring the custom TimeSpan converter might explain that.