Nullable reference type in C#8 when using DTO classes with an ORM

Question

I activated this feature in a project having data transfer object (DTO) classes, as given below:

public class Connection
    {
        public string ServiceUrl { get; set; }
        public string? UserName { get; set; }
        public string? Password { get; set; }
        //... others 
    }

But I get the error:

CS8618: Non-nullable property 'ServiceUrl' is uninitialized. Consider declaring the property as nullable.

This is a DTO class, so I'm not initializing the properties. This will be the responsibility of the code initializing the class to ensure that the properties are non-null.

For example, the caller can do:

var connection = new Connection
{
  ServiceUrl=some_value,
  //...
}

My question: How to handle such errors in DTO classes when C#8's nullability context is enabled?

When you do construction initialization the way you show, the default constructor runs (initializing ServiceUrl to its default value (which, for strings, is null)). Then, the initialize statement runs. If you want it non-nullable, it needs to exit all constructors initialized — Flydog57
POCO doesn't mean it has no constructor or that the properties aren't initialized. It means Plain Old C# Object. Just an object like any other, without inheriting from any special class. I suspect you have a different question. How to create DTOs - Data Transfer Objects — Panagiotis Kanavos
This should be the responsibility of the one initializing the class to ensure that the properties are non-null the class should be in a valid state always. If null isn't allowed, the property should never be null. Perhaps, instead of a basic string you should use a specialized class for the URL that can have a value of None or` Missing`, like the Option class in F#. C# 8 allows you to write such classes and check them with pattern matching — Panagiotis Kanavos
@Panagiotis Kanavos, The class,in my case, has a constraint to be parameter-less Constructor. So I have to use Property Initializer: public string ServiceUrl { get; set; } = default! ;. I hope Roslyn may have in the future a way to handle Late initialization outside the scope of ctor. I was using MayBe<T> (like Option class), but I switched to nullable Reference Type in c#8 for the new code. — M.Hassan
NRTs aren't Maybes, in fact, Maybes are now even more important and easier to use. Provided of course they're build using C# idioms that make them easy to work with pattern matching — Panagiotis Kanavos

M.Hassan M.Hassan · Accepted Answer · 2019-11-30T17:05:52

You can do either of the following:

EF Core suggests initializing to null! with null-forgiving operator

public string ServiceUrl { get; set; } = null! ;
//or
public string ServiceUrl { get; set; } = default! ;

Using backing field:

private string _ServiceUrl;
public string ServiceUrl
{
    set => _ServiceUrl = value;
    get => _ServiceUrl
           ?? throw new InvalidOperationException("Uninitialized property: " + nameof(ServiceUrl));
}

Nullable reference type in C#8 when using DTO classes with an ORM

7 Answers