C# in Depth

132 CHAPTER 4 Saying nothing with nullable typesslightly ambivalent way. You see, null is a theoretically valid value in a Hashtable, butin my experience most uses of Hashtable never use null values, which makes it perfectlyacceptable to have code that assumes that a null value means a missing key.One common scenario is to have each value of the Hashtable as a list: the first timean item is added for a particular key, a new list is created and the item added to it.Thereafter, adding another item for the same key involves adding the item to theexisting list. Here’s the code in C# 1:ArrayList list = hash[key];if (list==null){list = new ArrayList();hash[key] = list;}list.Add(newItem);Hopefully you’d use variable names more specific to your situation, but I’m sure youget the idea and may well have used the pattern yourself. 5 With nullable types, this patterncan be extended to value types—and in fact, it’s safer with value types, because ifthe natural result type is a value type, then a null value could only be returned as a failurecase. Nullable types add that extra Boolean piece of information in a nice genericway with language support—so why not use them?To demonstrate this pattern in practice and in a context other than dictionarylookups, I’ll use the classic example of the TryXXX pattern—parsing an integer. Theimplementation of the TryParse method in listing 4.5 shows the version of the patternusing an output parameter, but then we see the use of the version using nullabletypes in the main part at the bottom.Listing 4.5An alternative implementation of the TryXXX patternstatic int? TryParse (string data){int ret;Classic call withif (int.TryParse(data, out ret)) output parameter{return ret;}else{return null;}}...Nullableint? parsed = TryParse("Not valid"); callif (parsed != null){Console.WriteLine ("Parsed to {0}", parsed.Value);}5Wouldn’t it be great if Hashtable and Dictionary could take a delegate to call whenever anew value was required due to looking up a missing key? Situations like this would be a lot simpler.Licensed to Rhona Hadida