“Why is not allowed to assign
null to a DateTime?” Again and again, this question keeps showing up on StackOverflow and similar sites. Different phrasing, maybe a different type (“Why type “int” is never equal to ‘null’?”), but the same question, in essence. Which is only natural, considering that probably thousands of developers join the field every year.
The short answer is: because DateTime/int/float/whatever is a value type, and value types can never be
null. Only reference types can be
That’s great but…why???
I mean, at some point, the C# designers had to make that call. They deliberately decided that
null can’t be assigned to a value type. Is there something intrinsic to value types that prevent them from being
null? Maybe it has something to do with the stack and the heap?
Well, I think before we try to answer those questions, it’d be helpful to get a step back and think about other question.
What exactly is ‘null’?
You probably think of
null as a way to represent a missing piece of information. Something that is unknown, absent or irrelevant. Maybe a piece of information that isn’t where it’s supposed to be yet, but it might be in the future.
In the previous post about reference and value types, we learned that a variable of a reference type contains a reference that points to an instance of that type. But when
null is assigned to the variable, what does the variable contain? Nothing?
Not exactly. Even though
null basically means nothing, this “nothing” has to be represented in some way in the machine’s memory.
In C#, the
null keyword represents a null reference, which is a reference that doesn’t point to any object.
In practice, the
null value is represented as all zero bits.
Of course, this is just an implementation detail. It could be implemented in different ways, so this isn’t really relevant for us. What you need to understand is, you need something to represent nothing. You need some kind of special value that means “this value is missing” or “this value is unknown”.
Once you understand this, it becomes easier to see what the problem is with value types.
How would you represent null for a value type?
To understand why would be complicated to represent
null using a value type, you must bear in mind that value types usually have a range of possible values that they’re able to represent.
It’s easier to get this with an example. So, consider the byte value type.
This type can represent one byte of possible values. Since 2^8 = 256, it can represent 256 values. It is also unsigned, meaning it can only represent values greater than or equals to zero. Thus, the range of possible values that the byte type can represent is 0 to 255.
00000000 => 0 00000001 => 1 00000010 => 2 . . . . . . 11111111 => 255
Now comes the tricky part. If the C# language designers wanted the byte type to be nullable, they’d have to pick a value from the range and elect it as the
null value. Let’s imagine they picked zero as the
null value for
byte. If that was the case, we wouldn’t be able to use zero as a valid value never again!
That’s the whole point: for value types to be nullable, it is necessary to sacrifice one of the possible values of the range to represent the
Jon Skeet, author of C# In Depth, puts it better than me:
You’ve got to be able to store the values 0-255 in that variable; otherwise it’s useless for reading arbitrary binary data. With the 256 normal values and one
nullvalue, you’d have to cope with a total of 257 values, and there’s no way of squeezing that many values into a single byte.
The designers could’ve decided that every value type would have an extra flag bit somewhere determining whether a value was
nullor contained real data, but the memory usage implications are horrible, not to mention the fact that you’d have to check the flag every time you wanted the use the value.
So, there is nothing in principle that prevents value types from being
null. It’d just be so complicated that the C# designers considered that the benefits of doing so don’t outweigh the costs.
But I really want/need a nullable value type. What can I do?
You now hopefully understand the complications involved in representing
null for a value type, and why the C# developers decided against it. But, what if you really need to do this? One common scenario is when you’re fetching data from a relational database. What should you do?
Well, thankfully, there’s an easy solution. Even though “normal” value types can’t be
null, C# 2 introduced the Nullable
I’ll talk about nullable types in the next article. As a bonus, you’ll learn a bit about another nice feature called generics.
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