Adam Ralph

Guard Clause Abuse

Today sees the release of version 0.10 of the mighty LiteGuard library.

After announcing the release on JabbR, I found myself considering and commenting on the common abuse of guard clauses that I see so very often:

public class Car
{
    private readonly string numberPlate:

    public Car(string numberPlate)
    {
        Guard.AgainstNull("numberPlate", numberPlate);
        
        this.numberPlate = numberPlate;
    }
    
    public string NumberPlate
    {
        get { return this.numberPlate; }
    }
}

(I’m using the British English term “number plate”, known as “license plate” in the US and some other countries.)

The above code should not be using a guard clause and should not raise an ArgumentNullException. It should throw some kind of model* exception, since it is encapsulating a rule which has been identified in the domain of the application. The code is not de-referencing numberPlate so it doesn’t need a guard clause:

public class Car
{
    private readonly string numberPlate:

    public Car(string numberPlate)
    {
        if (numberPlate == null)
        {
            throw new UsedCarDealershipException("The number plate is missing.");
        }
        
        this.numberPlate = numberPlate;
    }
    
    public string NumberPlate
    {
        get { return this.numberPlate; }
    }
}

* Feel free to substitute the word ‘model’ for ‘business’, according to taste .