Language-Ext Unions - Target runtime doesn't support default interface implementation

Question

I'm trying to user the Union type in Language-Ext and am getting error: "Target runtime doesn't support default interface implementation" for the method name and "The type or namespace name 'InconsistentFirstName' could not be found"

My code looks like this:

[Union]
public interface Inconsistency
{
    public Inconsistency InconsistentFirstName(string localValue, string contactCentreValue);
    public Inconsistency InconsistentSurname(string localValue, string contactCentreValue);

    public string GetFieldName(Inconsistency inconsistency)
        => inconsistency switch
        {
            InconsistentFirstName n => "Name",
            InconsistenctSurname s => "Surname",
            _ => ""
        };
}

Answer 1

GetFieldName needs to be added to a separate class file. There is a partial class that is automatically code generated called InconsistencyCon, so what I was trying to achieve can be done with the following code:

[Union]
public abstract partial class Inconsistency
{
    public abstract Inconsistency InconsistentFirstName(string localValue, string remoteValue); 
    public abstract Inconsistency InconsistentSurname(string localValue, string remoteValue);
}

public static partial class InconsistencyCon
{
     public static string FieldName(this Inconsistency inconsistency) =>
         inconsistency switch
         {
             InconsistentFirstName n => "Name",
             InconsistentSurname s   => "Surname",
             _  => ""
         };

     public static string Test(Inconsistency inconsistency)
        {
            if (inconsistency is InconsistentFirstName n)
            {
                var a = n.LocalValue;
                var b = n.FieldName;
                // etc
            }
        }
}