Use of var keyword in C#
After discussion with colleagues regarding the use of the 'var' keyword in C# 3 I wondered what people's opinions were on the appropriate uses of type inference via var?
For example I rather lazily used var in questionable circumstances, eg:-
foreach(var item in someList) { // ... } // Type of 'item' not clear.
var something = someObject.SomeProperty; // Type of 'something' not clear.
var something = someMethod(); // Type of 'something' not clear.
More legitimate uses of var are as follows:-
var l = new List<string>(); // Obvious what l will be.
var s = new SomeClass(); // Obvious what s will be.
Interestingly LINQ seems to be a bit of a grey area, eg:-
var results = from r in dataContext.SomeTable
select r; // Not *entirely clear* what results will be here.
It's clear what results will be in that it will be a type which implements IEnumerable, however it isn't entirely obvious in the same way a var declaring a new object is.
It's even worse when it comes to LINQ to objects, eg:-
var results = from item in someList
where item != 3
select item;
This is no better than the equivilent foreach(var item in someList) { // ... } equivilent.
There is a real concern about type safety here - for example if we were to place the results of that query into an overloaded method that accepted IEnumerable<int> and IEnumerable<double> the caller might inadvertently pass in the wrong type.
var does maintain strong typing but the question is really whether it's dangerous for the type to not be immediately apparent on definition, something which is magnified when overloads mean compiler errors might not be issued when you unintentionally pass the wrong type to a method.
I still think var can make code more readable in some cases. If I have a Customer class with an Orders property, and I want to assign that to a variable, I will just do this:
var orders = cust.Orders;
I don't care if Customer.Orders is IEnumerable<Order> , ObservableCollection<Order> or BindingList<Order> - all I want is to keep that list in memory to iterate over it or get its count or something later on.
Contrast the above declaration with:
ObservableCollection<Order> orders = cust.Orders;
To me, the type name is just noise. And if I go back and decide to change the type of the Customer.Orders down the track (say from ObservableCollection<Order> to IList<Order> ) then I need to change that declaration too - something I wouldn't have to do if I'd used var in the first place.
I use var extensively. There has been criticism that this diminishes the readability of the code, but no argument to support that claim.
Admittedly, it may mean that it's not clear what type we are dealing with. So what? This is actually the point of a decoupled design. When dealing with interfaces, you are emphatically not interested in the type a variable has. var takes this much further, true, but I think that the argument remains the same from a readability point of view: The programmer shouldn't actually be interested in the type of the variable but rather in what a variable does. This is why Microsoft also calls type inference “duck typing.”
So, what does a variable do when I declare it using var ? Easy, it does whatever IntelliSense tells me it does. Any reasoning about C# that ignores the IDE falls short of reality. In practice, every C# code is programmed in an IDE that supports IntelliSense.
If I am using a var declared variable and get confused what the variable is there for, there's something fundamentally wrong with my code. var is not the cause, it only makes the symptoms visible. Don't blame the messenger.
Now, the C# team has released a coding guideline stating that var should only be used to capture the result of a LINQ statement that creates an anonymous type (because here, we have no real alternative to var ). Well, screw that. As long as the C# team doesn't give me a sound argument for this guideline, I am going to ignore it because in my professional and personal opinion, it's pure baloney. (Sorry; I've got no link to the guideline in question.)
Actually, there are some (superficially) good explanations on why you shouldn't use var but I still believe they are largely wrong. Take the example of “searchabililty”: the author claims that var makes it hard to search for places where MyType is used. Right. So do interfaces. Actually, why would I want to know where the class is used? I might be more interested in where it is instantiated and this will still be searchable because somewhere its constructor has to be invoked (even if this is done indirectly, the type name has to be mentioned somewhere).
Var, in my opinion, in C# is a good thingtm. Any variable so typed is still strongly typed, but it gets its type from the right-hand side of the assignment where it is defined. Because the type information is available on the right-hand side, in most cases, it's unnecessary and overly verbose to also have to enter it on the left-hand side. I think this significantly increases readability without decreasing type safety.
From my perspective, using good naming conventions for variables and methods is more important from a readability perspective than explicit type information. If I need the type information, I can always hover over the variable (in VS) and get it. Generally, though, explicit type information shouldn't be necessary to the reader. For the developer, in VS you still get Intellisense, regardless of how the variable is declared. Having said all of that, there may still be cases where it does make sense to explicitly declare the type -- perhaps you have a method that returns a List<T> , but you want to treat it as an IEnumerable<T> in your method. To ensure that you are using the interface, declaring the variable of the interface type can make this explicit. Or, perhaps, you want to declare a variable without an initial value -- because it immediately gets a value based on some condition. In that case you need the type. If the type information is useful or necessary, go ahead and use it. I feel, though, that typically it isn't necessary and the code is easier to read without it in most cases.
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