Nullability Mismatch Between Property Getter and Setter?

I've written a class that implements IDataParameter in C# with .NET 8. I get CS8767 on the Setter for this code:

public class TestDataParameter: IDataParameter
{
   ...

   public string ParameterName
   {
      get; set;
   } = "";
   ...
}

Making the return type string? leads to CS8766 on the Getter.

public class TestDataParameter: IDataParameter
{
   ...

   public string? ParameterName
   {
      get; set;
   } = "";
   ...
}

This is extremely annoying. The interface declares the property as string. I don't understand why it would need different nullabilities for the getter and the setter and how this should be implemented. IDataParameter declares string ParameterName { get; set; } so implementing the property non-nullable should be right. Does anyone have any idea why this happens and how to deal with it correctly? Thanks, Alex.

Solution

If you want to remove the warning you can follow the approach which the interfaces uses itself - use the AllowNullAttribute:

public class TestDataParameter : IDataParameter
{
    // ...
    [AllowNull]
    public string ParameterName { get; set; } = "";
    // ...
}

You might want though to switch from the auto-property implementation to one which will maintain the contract (i.e. you can pass null but null is never returned as for example OdbcParameter does):

private string? _parameterName = "";

[AllowNull]
public string ParameterName
{
    get => _parameterName ?? string.Empty;
    set => _parameterName = value;
}