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. These new kind of messages that it throws pulls down my confidence. I got them from all the three properties. And this has suddenly appeared.

Can some one please suggest how this can be mitigated.

Update

These days I have seen using default! like so, and its working.

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

Also you may put a question mark symbol(?) to indicate that the type is nullable, if you feel appropriate as follows.

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

Answer 1

If you don't want this, you can disable this by deleting the below line from the csproj file or setting it as disable. By default value is disable.

<Nullable>enable</Nullable>

Here is the official documentation.

Answer 2

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.

Answer 3

You can annotate a property directly as non-nullable.

public string Property{ get; set; } = null!;

And it will give a warning if user tries to set the Property as null

Answer 4

For Entity Framework Working with nullable reference types:

public class NullableReferenceTypesContext : DbContext {
    public DbSet<Customer> Customers => Set<Customer>();
    public DbSet<Order> Orders => Set<Order>();
}