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Validate instance of class in contructor before creation

Validate instance of class in contructor before creation

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ragingben	8 Oct, 2008 - 02:52 AM Post #1
New D.I.C Head Joined: 7 Oct, 2008 Posts: 30 My Contributions	Ok heres a question that I hope some of you C# gurus will be able to answer pretyy easily. I want to create a new instance of a class with 1 argument (custom enumeration) in in the constructor. The constructor then checks to see if this instance is legal by seeing if the argument is within an array of legal instances. If it is not, I don't want the instance of the class to be created. I know this would easy to do in the code for the main program by checking the validity before creating the new instance, but I don't wan't it to be checked there! Any ideas? CODE public class Stepper : Axis { /// <summary> /// Create a new stepper motor for a specified axis /// </summary> /// <param name="axis"></param> public Stepper(EStepperAxis proposedAxis) { Boolean axisAvailable = false; // look at all the elements in static array to see if axis is available for (Int32 index = 0; index < Stepper.availableAxis.Length; index++ ) { // if found then update boolean if (Stepper.availableAxis[index] == proposedAxis) { axisAvailable = true; } } // if the axis is avaiable if (axisAvailable == true) { // assign axis this.axis = proposedAxis; // remove from list Stepper.availableAxis[(Int32)proposedAxis] = null; // display message Console.WriteLine("Axis {0} has been assigned to {1}", proposedAxis, this); } else { // display message Console.WriteLine("Could not create a new axis {0}: Axis not available.", proposedAxis); // THIS IS WHERE IT SHOULD ABORT CREATING INSTANCE!!! } }
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baavgai

8 Oct, 2008 - 04:49 AM

Post #2

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My Contributions

One way would be to place an exception in the constructor:

csharp


class Widget {
	private string name;
	private int grams;
	public Widget(string name, int grams) {
		if (grams < 1) {
			throw new System.ArgumentException("Grams cannot be less than 1.");
		}
		this.name = name;
		this.grams = grams;
	}

Another would be to place an "IsValid" flag in the object.

You could also use something called a factory pattern, where another object provides instances for you and no other object can. Though you implied you didn't want that.

A variation on the factory is to allow the object itself to handle the creation on instances external to the constructor. This allows for some extra failover behavior, including just returning null. Here's an example of that:

csharp


class Widget {
	private string name;
	private int grams;
	// private constructor
	// no one other than me can create an instance
	private Widget() { }
	public string Name { get { return this.name; } }
	public int Grams { get { return this.grams; } }
	public override string ToString() {
		return "Widget: " + this.name + ", " + this.grams + "g";
	}

	// one of the few times I'd ever use static
	public static Widget GetNew(string name, int grams) {
		// we don't have to throw errors
		// just send back a null object.
		if (grams < 1) { return null; }

		Widget obj = new Widget();
		obj.name = name;
		obj.grams = grams;
		return obj;
	}
}