c# - Default implementation in interface is not seen by the compiler?

Question

Here is a my code inside a c# project that targets .NET Core 3.0 (so I should be in C# 8.0) with Visual Studio 2019 (16.3.9)

public interface IJsonAble
{
    public string ToJson() => System.Text.Json.JsonSerializer.Serialize(this);
}

public class SumRequest : IJsonAble
{
    public int X { get; set; }

    public int Y { get; set; }

    public void Tmp()
    {
        new SumRequest().ToJson(); //compile error
    }
}

The compile error is:

CS1061 'SumRequest' does not contain a definition for 'ToJson' and no accessible extension method 'ToJson' accepting a first argument of type 'SumRequest' could be found (are you missing a using directive or an assembly reference?)

Can someone shed some light on this behavior ?

Answer 1

Methods are only available on the interface, not the class. So you can do this instead:

IJsonAble request = new SumRequest()
var result = request.ToJson();

Or:

((IJsonAble)new SumRequest()).ToJson();

The reason for this is it allows you to add to the interface without worrying about the downstream consequences. For example, the ToJson method may already exist in the SumRequest class, which would you expect to be called?