Why can't Point and Rectangle be used as optional parameters?

问题

I'm trying to pass an optional argument to a geometry function, called offset, which may or may not be specified, but C# doesn't allow me to do any of the following. Is there a way to accomplish this?

• Null as default

  Error: A value of type '' cannot be used as a default parameter because there are no standard conversions to type 'System.Drawing.Point'

  public void LayoutRelative(.... Point offset = null) {}
  

• Empty as default

  Error: Default parameter value for 'offset' must be a compile-time constant

  public void LayoutRelative(.... Point offset = Point.Empty) {}
  

回答1:

If your default value doesn't require any special initialization, you don't need to use a nullable type or create different overloads. You can use the default keyword:

public void LayoutRelative(.... Point offset = default(Point)) {}

If you want to use a nullable type instead:

public void LayoutRelative(.... Point? offset = null)
{
    if (offset.HasValue)
    {
        DoSomethingWith(offset.Value);
    }
}

来源：https://stackoverflow.com/questions/12235653/why-cant-point-and-rectangle-be-used-as-optional-parameters