C#: Avoiding Bugs caused by not Overriding ToString

Question

I find the following bug occurring far too often in my code and wondered if anyone knows some good strategies to avoid it.

Imagine a class like this:

public class Quote
{
   public decimal InterestRate { get; set; }
}

At some point I create a string that utilises the interest rate, like this:

public string PrintQuote(Quote quote)
{
    return "The interest rate is " + quote.InterestRate;
}

Now imagine at a later date I refactored the InterestRate property from a decimal to its own class:

public class Quote
{
    public InterestRate InterestRate { get; set; }
}

... but say that I forgot to override the ToString method in the InterestRate class. Unless I carefully looked for every usage of the InterestRate property I would probably never notice that at some point it is being converted to a string. The compiler would certainly not pick this up. My only chance of saviour is through an integration test.

The next time I call my PrintQuote method, I would get a string like this:

"The interest rate is Business.Finance.InterestRate".

Ouch. How can this be avoided?

Answer 1

By creating an override of ToString in the IntrestRate class.

Answer 2

The way to prevent this kind of problem is to have a unit test for absolutely all your class members, which therefore includes your PrintQuote(Quote quote) method:

[TestMethod]
public void PrintQuoteTest()
{
    quote = new Quote();
    quote.InterestRate = 0.05M;
    Assert.AreEqual(
        "The interest rate is 0.05",
        PrintQuote(quote));
}

In this case, unless you defined a implicit conversion between your new InterestRate class and System.Decimal, this unit test would actually no longer compile. But that would definitely be a signal! And if you did define an implicit conversion between your InterestRate class and System.Decimal, but forgot to override the ToString method, then this unit test would compile, but would (correctly) fail at the Assert.AreEqual() line.

The need for having a unit test for absolutely every class member cannot be overstated.