What is the exact use of var keyword?

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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, e.g.:-

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, e.g.:-

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, e.g.:-

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.

Using var `you don't have to figure out the type` seems like a very bad reason to use it... you're supposed to know what the type is, var is just a shortcut to avoid typing it — Developer, Mar 20 '12 at 10:08
Did you just copy the question from @Henrik's duplicate comment? — Bali C, Mar 20 '12 at 10:08
@Bali C: Yes, and if you look at his previous questions, you'll notice a rather disturbing pattern... — BoltClock, Mar 20 '12 at 10:10
@BoltClock Is there anyway of flagging a user rather than comment if they are doing weird stuff like copying other questions etc..? Just curious — Burkely91, Jun 14 '15 at 14:46

score 2 · Answer 1 · answered Mar 20 '12 at 10:08

Personally I find the circumstances you describe far from questionable, since there is no point in repeating yourself unless you specifically want the static type of a variable to be different than the static type of the expression used to initialize the variable. For example:

IEnumerable<int> foo = new List<int>(); // It's IEnumerable on purpose

Furthermore, there are absolutely no type safety concerns with var. The point is not that the variable can be of "any" type. It is of a very specific type, but you simply do not care to spell that type out.

I think you've hit the nail on the head here. +1 – Connell Mar 20 '12 at 10:13 — Connell, Mar 20 '12 at 10:13

score 0 · Answer 2 · answered Mar 20 '12 at 10:08

0

I'm only using it as a place holder until I'm sure which datatypes I'm using.

Sure this is a short answer but I think it's pretty close that when you should use the var keyword.

answered Mar 20 '12 at 10:08

CodingBarfield

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score 0 · Answer 3 · answered Mar 20 '12 at 10:09

the var keyword is used as shorthand in the language, but isn't a .NET type. The compiler must know the type of the variable to use the var keyword - so it is type-safe.

I personally only use it if the type name is also used in the assignment and the name is possible too long to duplicate in the code.

var dictionary = new Dictionary<string, string>();

It is also used for anonymous types (but still, the compiler must know the signature of the anonymous type).

var fred = new { Age = 23, Name = "Fred" };

This method is used commonly in the select clause of LINQ queries.

score 0 · Answer 4 · answered Mar 20 '12 at 10:10

Just an "abstraction" or "syntax sugar" to be able to write a code without specifying first the type (this is no your first cases)

In second case: LINQ queries, instead, to rapresent some unknown, dynamic, not concrete, if you wish, type.

could be:

var results = from item in someList
              where item != 3
              select item; //item a class instance

could be

 var results = from item in someList
              where item != 3
              select item.ItemName; //string property of that class

could be

 var results = from item in someList
          where item != 3
          select new {item.ItemName, item.ID}; //unknown type dynamically generated, that conains the string and integer, like result

score 0 · Answer 5 · answered Mar 20 '12 at 10:10

As far as I know var remains strong typed. The compiler calculated what the proper type should be. In fact it has no real meaning.

It is only a trick to reduce the number of manipulations. For instance when you change a type in one class, this can result in a cascade of modification. But its only a way to migrate work from the programmer to the compiler.

For instance your Linq query will result in a type IEnumerable<TA>. When you change some class so the result will be IEnumerable<TB> there is no need to change this part of the code.

What is the exact use of var keyword?

5 Answers5