Investigate ways to extend deserialization support for arbitrary collection types

[hughbe]

//var s = JsonSerializer.Deserialize<ConcurrentBag<int>>("[1,2,3]");
//var s = JsonConvert.DeserializeObject<ConcurrentBag<int>>("[1,2,3]");
Console.WriteLine($"Count: {s.Count}");
var t = new List<int>(s);
Console.WriteLine(t[0]);
Console.WriteLine(t[1]);
Console.WriteLine(t[2]);

Output with JsonConvert:

Count: 3
3
2
1

Output with JsonSerializer:

Unhandled exception. System.NotSupportedException: The collection type 'System.Collections.Concurrent.ConcurrentBag`1[System.Int32]' is abstract, an interface, or is read only, and could not be instantiated and populated. Path: $ | LineNumber: 0 | BytePositionInLine: 1.
 ---> System.NotSupportedException: The collection type 'System.Collections.Concurrent.ConcurrentBag`1[System.Int32]' is abstract, an interface, or is read only, and could not be instantiated and populated.
   --- End of inner exception stack trace ---
   at System.Text.Json.ThrowHelper.ThrowNotSupportedException(ReadStack& state, Utf8JsonReader& reader, NotSupportedException ex)
   at System.Text.Json.ThrowHelper.ThrowNotSupportedException_CannotPopulateCollection(Type type, Utf8JsonReader& reader, ReadStack& state)
   at System.Text.Json.Serialization.Converters.IEnumerableOfTConverter`2.CreateCollection(Utf8JsonReader& reader, ReadStack& state, JsonSerializerOptions options)
   at System.Text.Json.Serialization.Converters.IEnumerableDefaultConverter`2.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, TCollection& 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.ReadCore[TValue](JsonConverter jsonConverter, Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadCore[TValue](Utf8JsonReader& reader, Type returnType, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, Type returnType, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
   at t.Program.Main(String[] args) in /Users/hugh/Documents/GitHub/t/Program.cs:line 13

[eiriktsarpalis]

There are many collection types that STJ can't deserialize out of the box, but we simply can't add converter for every type in BCL. Instead, we should be looking at potential extensibility mechanisms that let users specify deserialization constructors for arbitrary collection types (including ones that are not in BCL).

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