Non-nullable property must contain a non-null value when exiting constructor. Consider declaring the property as nullable

Question

I have a simple class like this.

public class Greeting
{
    public string From { get; set; }
    public string To { get; set; } 
    public string Message { get; set; }
}

Strangely I get the following warning.

Severity Code Description Project File Line Suppression State Warning CS8618 Non-nullable property 'From' must contain a non-null value when exiting constructor. Consider declaring the property as nullable. MxWork.Elsa2Wf.Tuts.BasicActivities
D:\work\MxWork\Elsa2.0WfLearning\MxWork.Elsa2.0Wf.Tuts\src \MxWork.Elsa2Wf.Tuts.BasicActivities\Messages\Greeting.cs 5 Active

I am baffled at these messages. I got them from all the three properties, and this has suddenly appeared.

Can some one please suggest how this can be mitigated?

Answer 1

The compiler is warning you that the default assignment of your string property (which is null) doesn't match its stated type (which is non-null string).

This is emitted when nullable reference types are switched on, which changes all reference types to be non-null, unless stated otherwise with a ?.

For example, your code could be changed to

public class Greeting
{
    public string? From { get; set; }
    public string? To { get; set; } 
    public string? Message { get; set; }
}

to declare the properties as nullable strings, or you could give the properties defaults in-line or in the constructor:

public class Greeting
{
    public string From { get; set; } = string.Empty;
    public string To { get; set; } = string.Empty;
    public string Message { get; set; } = string.Empty;
}

if you wish to retain the properties' types as non-null.