If a function uses "this", does invoking the function from a child class include the properties of the child class?

Question

I create a class that includes a function to serialize itself to a JSON string using the "this" keyword, this class is then inherited by a child class.

public class Parent
{
    int a { get; set; }

    public ToJsonString()
    {
        return JsonSerializer.Serialize(this);
    }
}

public class Child : Parent
{
    int b { get; set; }
}

If an instance of the child class then invokes the ToJsonString() function, will the returned string include the properties from the child class, or only those from the parent class?

Child instance = new Child();
string serialized = instance.ToJsonString();

Answer 1

This can easily be tested see (code below).

The original version of ToJsonString has a Parent parameter and only serialises the Parent properties. This can be remedied by overriding ToJsonString in Child with an identical implementation:

using System.Text.Json;


Console.WriteLine($"Using parent version of 'ToJsonString': {new Child().BaseToJsonString()}");
Console.WriteLine($"Using child version of 'ToJsonString': {new Child().ToJsonString()}");
Console.WriteLine($"Using better version of 'ToJsonString': {new Child().BetterToJsonString()}");


public class Parent
{
    public int A { get; set; } = 1;

    public string BaseToJsonString()
    {
        return JsonSerializer.Serialize(this);
    }
    public virtual string ToJsonString()
    {
        return JsonSerializer.Serialize(this);
    }
    public string BetterToJsonString()
    {
        return JsonSerializer.Serialize(this, GetType());
    }
}

public class Child : Parent
{
    public int B { get; set; } = 2;

    public override string ToJsonString()
    {
        return JsonSerializer.Serialize(this);
    }
}

This yields:

Using parent version of 'ToJsonString': {"A":1}
Using child version of 'ToJsonString': {"B":2,"A":1}
Using better version of 'ToJsonString': {"B":2,"A":1}

Edit:

As Petrusion correctly points out, another - much simpler - solution would be to force the Serialize method to use the actual runtime type (see BetterToJsonStringmethod above).