Testable virtual

FormattingService.cs

public class FormattingService
{
    public string Format(string message) 
    {
        return Now + ": " + message;
    }
    
    protected virtual DateTime Now => DateTime.Now;
}

gistfile1.txt

internal class TestableFormattingService : FormattingService
{
  private readonly DateTime _currentDate;

  public TestableFormattingService(DateTime currentDate)
  {
    _currentDate = currentDate;
  }
    
  protected override DateTime Now => _currentDate;
}