C#8 nullable and non-nullable reference types - nullable output only if input is nullable

Question

I have a bunch of extension methods that convert my entities to DTOs like the following.

public static LocationDto? ToDto(this Location? @this)
{
    if (@this == null) return null;

    return new LocationDto
            {
                Id = @this.Id,
                Text = @this.Name,
                ParentId = @this?.Parent?.Id,
            };
}

The issue here is that if you pass not nullable entity still receive a nullable and you cannot define a public static LocationDto ToDto(this Location @this) because they would be compiled to the same method.
Also, I don't like to use ! for the time I am calling it. So the following is not my answer.

Location entity = AMethod();
LocationDto dto = entity.ToDto()!;

Is there an attribute or syntax to tell the compiler how this method behaves? Somehting like:

public static [NullableOnlyIfInputIsNull] LocationDto? ToDto(this Location? @this)

Answer 1

The attribute you are asking for is NotNullIfNotNullAttribute

The attribute accepts the name of the parameter you use to infer nullity.

In your case this would look like:

using System.Diagnostics.CodeAnalysis;

// ...

[return:NotNullIfNotNull("this")]
public static LocationDto? ToDto(this Location? @this)
{
    // Your code here
}