Providing a default value for an Optional Parameter in C# 4.0

Question

How do I set a default if one of the parameters is a custom type?

public class Vehicle
{
   public string Make {set; get;}
   public int Year {set; get;}
}

public class VehicleFactory
{
   //For vehicle, I need to set default values of Make="BMW", Year=2011
   public string FindStuffAboutVehicle(string customer, Vehicle vehicle)
   {
       //Do stuff
   }
}

Answer 1

You can't, really. However, if you don't need null to mean anything else, you can use:

public string FindStuffAboutVehicle(string customer, Vehicle vehicle = null)
{
    vehicle = vehicle ?? new Vehicle { Make = "BMW", Year = 2011 };
    // Proceed as before 
}

In some cases this is nice, but it does mean you won't catch the situation where a caller accidentally passes null.

It would probably be cleaner to use an overload instead:

public string FindStuffAboutVehicle(string customer, Vehicle vehicle)
{
    ...
}

public string FindStuffAboutVehicle(string customer)
{
    return FindStuffAboutVehicle(customer, 
                                 new Vehicle { Make = "BMW", Year = 2011 });
}

It's also worth reading Eric Lippert's posts about optional parameters and their corner cases.