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Why are Nullable Types Misunderstood?

With Visual Studio 2005, Microsoft finally added the ability for Visual Basic and C# to represent nullable value-type variables. Visual Basic (pre .NET) programmers had often asked for this feature. So programmers welcomed it with open arms. But unfortunately, it remains to this day, poorly used and misunderstood. I’d like to take a few minutes of your time to show you some of the good ways to use nullable types, and some of the bad ways to avoid.

In the beginning…

A variable may not have a defined value at all times. For example, a database field might initially be NULL, only to be updated at some later point with an actual value. Another example is when controlling program flow. The code may start off with an unassigned variable and later, in the flow of the code, the code needs to determine if a value has been assigned to the variable.

In the .NET Common Runtime Language (CLR) only reference type variables may have a null value. Value type variables (i.e. structures) cannot be null, but they each have a default value. To allow programmers to represent null value types in their code, an extension exists in the .NET framework called the Nullable structure. There are several good articles online to give you all the background material you could ever need (I’ve included links at the end of this post if you need a refresher). Let’s see what all the trouble is about.

NOTE: I’m going to restrict my examples to C#, but the issues are all applicable to Visual Basic.

A nullable type is declared like this:

Nullable<bool> b = null;

Or this:

bool? b = null;

So right out of the gate, the introduction of the nullable value types is confusing, because you have two ways to declare things. This is by no means the first time in the history of programming languages that this has happened (nor will it be the last). My advice with all .NET languages is to avoid using the CLR syntax and use the C# or VB alias. I use the simpler T? syntax over Nullable<T>.

The nullable type extension includes two read-only properties (orlistat 120 mg price and orlistat 120 mg weight loss) to test for null and retrieve the value:

if (b.HasValue)
{
    var c = b.Value;
}

But, as with the declaration syntax, you can code this more than one way:

if (b != null)
{
    var c = (bool)b;
}

So what is a programmer to do? Well, it’s not this…

DateTime? startDate = null;
if (startDate.ToString() != "")
{
    var convertedValue = Convert.ToDateTime(stringDate);
}

The nullable extension has all the ability in the world to avoid converting things to strings. Unfortunately, I have seen many developers resort to this sort of thinking. It gets even worse when dealing the default values.

Default Values

Often, the use of nullable values includes the use of a default value, if the value is null. Let’s see a few ways you can do this with integers:

int defaultValue = -1;

int? value = null;

// Terrible idea
int convertedTerrible = Convert.ToInt32(value);
if (convertedTerrible == 0)
{
    convertedTerrible = defaultValue;
}

// Bad ideas
int convertedBad1 = value != null ? value.Value : defaultValue;
int convertedBad2 = value.HasValue ? value.Value : defaultValue;
int convertedBad3 = value != null ? Convert.ToInt32(value) : defaultValue;

// Good idea
int convertedGood = value.GetValueOrDefault(defaultValue);

// Better idea
int convertedBetter = value ?? defaultValue;

The ‘terrible’ idea is to use the Convert.ToInt32() method. It is completely unnecessary to ‘convert’ the value. The methods of the Convert class are for conversion from one value type to another, and not for nullable types.

The ‘bad’ ideas rely on the conditional operator to retrieve the value if present, or default value if it is null. orlistat 120 mgs any more. But even if I did, I wouldn’t use it for this purpose.

The ‘good’ idea demonstrates the use of the built-in GetValueOrDefault(). It makes sense that the designers of the Nullable structure would have such a feature, given that it is such a common thing to need. So use it. No need to invent your own ‘creative’ ways to implement the same feature.

And the C# designers included an even ‘better’ idea. A new operator (the ?? or orlistat 180 mg) allows for a concise syntax.

Formatting

Programmers often need to format nullable values, such as dates. Suppose you need a routine that takes a nullable date and formats it into month/day/year format, or an empty string if it is null.

string formattedValue = FormatNullableDate(dateValue);

The following are 4 possible implementations of this sort of function. I have seen all of them used by one programmer or another.

string FormatNullableDateTerrible(DateTime? dateToFormat)
{
    return dateToFormat != null ? DateTime.Parse(Convert.ToString(dateToFormat)).ToString("MM/dd/yyyy") : "";
}

string FormatNullableDateBad(DateTime? dateToFormat)
{
    return dateToFormat != null ? Convert.ToDateTime(dateToFormat).ToString("MM/dd/yyyy") : "";
}

string FormatNullableDateNotAsBad(DateTime? dateToFormat)
{
    return dateToFormat != null ? dateToFormat.Value.ToString("MM/dd/yyyy") : "";
}

string FormatNullableDateGood(DateTime? dateToFormat)
{
    return string.Format("{0:MM/dd/yyyy}", dateToFormat);
}

The ‘terrible’ version parses the value for no good reason. But you can only parse a string, so our good old friend the Convert class makes an appearance. And of course, so does the conditional operator. Yuck.

The ‘bad’ version has figured out that converting to a string and then parsing the string is not necessary. But it still attempts to convert the value from a Nullable date to a ‘regular’ date. And then, the ‘not as bad’ version has come far enough along to understand one can format the Value property directly.

But as is usually the case, the designers already knew this situation would occur often and have given programmers a more elegant solution. The string.Format() method encapsulates the ‘if null return an empty string’ logic. So I humbly suggest you use the ‘good’ version when you need to format nullable dates.

Recommendations

Don’t use the ToString() method on a nullable type in order to test for the presence of a value. Use the HasValue property.

Don’t use the Convert class when retrieving the underlying value. The Value property is already doing all the work for you, just use it.

Don’t use the conditional operator to assign default values. Use the null-coalescing operator (??) instead.

Don’t re-invent the wheel when formatting nullable value types to strings. The string.Format() method has all the functionality built-in.

References

Nullable Types (C# Programming Guide)

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Nullable Value Types (Visual Basic)

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