using System;

​

var typeWithString = new TypeWithString("foo");

var typewithSpan = new TypeWithSpan("bar");

​

var working =  System.Text.Json.JsonSerializer.Serialize(typeWithString);

​

Console.WriteLine(working);

​

var json = System.Text.Json.JsonSerializer.Serialize(typewithSpan);

​

Console.WriteLine(json);

​

​

class TypeWithSpan

{ 

    public TypeWithSpan(ReadOnlySpan<char> value)

    {

        FirstLetter = value.Slice(0,1).ToString();

    }

    public string FirstLetter {get; }

}

​

class TypeWithString

{

    public TypeWithString(string value)

    {

        FirstLetter = value.Substring(0, 1);

    }

​

    public string FirstLetter { get; }

}