Object validation with nullable reference types in C#

Question

Let's say I have an configuration or DTO object with use of C# nullable reference object feature:

public class ServiceConfig
    {
        public string ConnectionString { get; set; }
        public string? OptionalComment { get; set; }
    }

With upper code I get compiler warning "CS8618: Non-nullable property 'ConnectionString' must contain a non-null value when exiting constructor. Consider declaring the property as nullable." Because this objects needs parameterless constructor I can add = null! to get rid of the warning. But the problem is that object can still contain property with null value. Because I want to use this in a context of configuration and DTO's, there is an option to validate objects (in deserialization process) before they are passed to "real" objects.

How can I do it? It there a way where I can validate if object is valid by "Nullable references" notation? I see other option also with Data Annotations, but I use of this C# feature is a lot more attractive.

So, in fantasy I would like to have something like this:

public class ServiceConfig
{
    public string ConnectionString { get; set; } = null!;
    public string? OptionalComment { get; set; }
}

public class Deserializer
{
    public static ServiceConfig Deserialize(string data)
    {
        var result = Json.Deserialize<ServiceConfig>(data);
        var isObjectValid = result.IsValid(); // I want method something like this
        if (!isObjectValid) throw new Exception("Deserialization error");
        return result;
    }
}

Answer 1

It seems that doesn't need to declare get setter explicitely, so you can declare the public member by any default value, and let the compiler doing its job for you perfectly. :D

public string ConnectionString = null;

Answer 2

As per the docs @ https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references

The compiler doesn't add any null checks or other runtime constructs in a nullable context. At runtime, a nullable reference and a non-nullable reference are equivalent.

So, when a DTO is deserialized at runtime, you may not have any additional constructs to denote whether a field is Nullable reference type or non-nullable reference type at atleast at the time of writing this answer.