C# - default parameter values from previous parameter

Question

namespace HelloConsole
{
    public class BOX
    {
        double height, length, breadth;

        public BOX()
        {

        }
        // here, I wish to pass 'h' to remaining parameters if not passed
        // FOLLOWING Gives compilation error.
        public BOX (double h, double l = h, double b = h)
        {
            Console.WriteLine ("Constructor with default parameters");
            height = h;
            length = l;
            breadth = b;
        }
    }
}

// 
// BOX a = new BOX(); // default constructor. all okay here.
// BOX b = new BOX(10,20,30); // all parameter passed. all okay here.

// BOX c = new BOX(10);
// Here, I want = length=10, breadth=10,height=10;

// BOX d = new BOX(10,20);
// Here, I want = length=10, breadth=20,height=10;

Question is : 'To achieve above, Is 'constructor overloading' (as follows) is the only option?

public BOX(double h)
{
    height = length = breadth = h;
}

public BOX(double h, double l)
{
    height = breadth = h;
    length = l;
}

pascalh · Accepted Answer · 2014-08-23 10:16:42Z

4

You can create multiple constructors calling each other:

public BOX(double h) : this(h, h, h) { }
public BOX(double h, double l) : this(h, l, h) { }
public BOX(double h, double l, double b)
{
    height = h;
    length = l;
    breadth = b;
}

Another solution is to use default values of null:

public BOX(double h, double? l = null, double? b = null)
{
    height = h;
    breadth = b ?? h;
    length = l ?? h;
}

This last approach also lets you call it with height and breadth only:

var box = new BOX(10, b: 15); // same as new BOX(10, 10, 15);

answered Aug 23, 2014 at 10:16

pascalh

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2 Comments

JSQuareD Over a year ago

Can you assign a double? to a double without a cast? I was pretty sure you couldn't, but I might be wrong.

pascalh Over a year ago

@JSQuareD According to ideone, it works (and I think that the null coalescing is treated as a special case here).

rene · Accepted Answer · 2014-08-23 11:23:14Z

Constructor overloading is the nicest/cleanest solution. If you only want to have just one constructor you could use a params array. This only works if the parameters are of the same type.

public clas BOX 
{
    public BOX (params double[] d)
    {     
        switch(d.Length) 
        {
            case 3:
                height = d[0];
                length = d[1];
                breadth = d[2];
                break;
            case 2:
                height = d[0];
                breadth = d[1];
                length = d[0];
                break;
            case 1:
                height = d[0];
                breadth = d[0];
                length = d[0];
                break;
             case 0:
                // no parameters
                break;
             default:
                // we rather not throw 
                // http://msdn.microsoft.com/en-us/library/ms229060(v=vs.110).aspx
                // throw new ArgumentException();
                Debug.Assert(d.Length<4, "too many arguments");
                break;
          }   
    }
}

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