Support (de)serializing types that implement `IDictionary` and `IDictionary<string, TValue>` without hint of implemented type

[layomia]

e.g. Given

public class StringToObjectIDictionaryWrapper : IDictionary<string, object>
{
    private readonly IDictionary<string, object> _dict;

    public StringToObjectIDictionaryWrapper(IDictionary<string, object> dict)
    {
        _dict = dict;
    }

    // Implementation for IDictionary<string, object>
    ...
}

After dotnet/corefx#38512 is merged, this will work:

[Fact]
public static void ImplementsIDictionaryOfObject()
{
    var input = new StringToObjectIDictionaryWrapper(new Dictionary<string, object>
    {
        { "Name", "David" },
        { "Age", 32 }
    });

    string json = JsonSerializer.ToString(input, typeof(IDictionary<string, object>)); // Providing implemented type
    Assert.Equal(@"{""Name"":""David"",""Age"":32}", json);

    IDictionary<string, object> obj = JsonSerializer.Parse<IDictionary<string, object>>(json); // Parsing to implemented type
    Assert.Equal(2, obj.Count);
    Assert.Equal("David", ((JsonElement)obj["Name"]).GetString());
    Assert.Equal(32, ((JsonElement)obj["Age"]).GetInt32());
}

This doesn't work

[Fact]
public static void ImplementsIDictionaryOfObject()
{
    var input = new StringToObjectIDictionaryWrapper(new Dictionary<string, object>
    {
        { "Name", "David" },
        { "Age", 32 }
    });

    // This throws NotSupportedException
    string json = JsonSerializer.ToString(input); // Not providing implemented type
    Assert.Equal(@"{""Name"":""David"",""Age"":32}", json);

    // (Assuming json == @"{""Name"":""David"",""Age"":32}") This throws NotSupportedException
    StringToObjectIDictionaryWrapper obj = JsonSerializer.Parse<StringToObjectIDictionaryWrapper>(json); // Parsing to actual type
    Assert.Equal(2, obj.Count);
    Assert.Equal("David", ((JsonElement)obj["Name"]).GetString());
    Assert.Equal(32, ((JsonElement)obj["Age"]).GetInt32());
}

[ahsonkhan]

@steveharter, can you please take a look and see if this issue is something we should be fixing, or is by design.

[steveharter]

{ "Name", "David" } is not a valid way to represent a dictionary entry, @layomia we don't support this currently, do we?

Only name:value pairs like this { "Name" :"David" } and perhaps { "Key":"Name", "Value":"David" }

[layomia]

{ "Name", "David" } is not a valid way to represent a dictionary entry, @layomia we don't support this currently, do we?

Only name:value pairs like this { "Name" :"David" } and perhaps { "Key":"Name", "Value":"David" }

@steveharter this code is creating a new instance of the StringToObjectIDictionaryWrapper type which implements IDictionary<string, object>.

var input = new StringToObjectIDictionaryWrapper(new Dictionary<string, object>
{
    { "Name", "David" },
    { "Age", 32 }
});

The question is should we be able to parse the json string @"{""Name"":""David"",""Age"":32}" to a StringToObjectIDictionaryWrapper type (and roundtrip i.e. serialize back to the json string) since it implements IDictionary<string, object>?

Same question would apply to types that implement any of the interfaces we support - ICollection, IList<T>, etc.

[steveharter]

I see. Yes we should deserialize "David" and 32 both into a JsonElement, like we do for other "object" cases in dictionaries.