UnitType.cs

public sealed class UnitType
{
    public static readonly UnitType Default = new UnitType();
    
    private UnitType() {}
}


public abstract class CommandHandler<TMessage> 
    : ICommandHandler<TMessage, UnitType>
    where TMessage : ICommand
{
    public UnitType Handle(TMessage message)
    {
        HandleCore(message);

        return UnitType.Default;
    }

    protected abstract void HandleCore(TMessage message);
}