Use of "var" type in variable declaration_问答_开发者

Our internal audit suggests us to use explicit variable type declaration instead of using the keyword var. They argue that using of var "may lead to unexpected results in some cases".

I am not aware of any difference between explicit type declaration and using of var once the code is compiled to MSIL.

The auditor is a respe开发者_StackOverflowcted professional so I cannot simply refuse such a suggestion.

How about this...

double GetTheNumber()
{
    // get the important number from somewhere
}

And then elsewhere...

var theNumber = GetTheNumber();
DoSomethingImportant(theNumber / 5);

And then, at some point in the future, somebody notices that GetTheNumber only ever returns whole numbers so refactors it to return int rather than double.

Bang! No compiler errors and you start seeing unexpected results, because what was previously floating-point arithmetic has now become integer arithmetic without anybody noticing.

Having said that, this sort of thing should be caught by your unit tests etc, but it's still a potential gotcha.

I tend to follow this scheme:

var myObject = new MyObject(); // OK as the type is clear

var myObject = otherObject.SomeMethod(); // Bad as the return type is not clear

If the return type of SomeMethod ever changes then this code will still compile. In the best case you get compile errors further along, but in the worst case (depending on how myObject is used) you might not. What you will probably get in that case is run-time errors which could be very hard to track down.

Some cases could really lead to unexpected results. I'm a var fan myself, but this could go wrong:

var myDouble = 2;
var myHalf = 1 / myDouble;

Obviously this is a mistake and not an "unexpected result". But it is a gotcha...

var is not a dynamic type, it is simply syntactic sugar. The only exception to this is with Anonymous types. From the Microsoft Docs

In many cases the use of var is optional and is just a syntactic convenience. However, when a variable is initialized with an anonymous type you must declare the variable as var if you need to access the properties of the object at a later point.

There is no difference once compiled to IL unless you have explicitly defined the type as different to the one which would be implied (although I can't think of why you would). The compiler will not let you change the type of a variable declared with var at any point.

From the Microsoft documentation (again)

An implicitly typed local variable is strongly typed just as if you had declared the type yourself, but the compiler determines the type

In some cases var can impeed readability. More Microsoft docs state:

The use of var does have at least the potential to make your code more difficult to understand for other developers. For that reason, the C# documentation generally uses var only when it is required.

In the non-generic world you might get different behavior when using var instead of the type whenever an implicit conversion would occur, e.g. within a foreach loop.

In the example below, an implicit conversion from object to XmlNode takes place (the non-generic IEnumerator interface only returns object). If you simply replace the explicit declaration of the loop variable with the var keyword, this implicit conversion no longer takes place:

using System;
using System.Xml;

class Program
{
    static void Foo(object o)
    {
        Console.WriteLine("object overload");
    }

    static void Foo(XmlNode node)
    {
        Console.WriteLine("XmlNode overload");
    }

    static void Main(string[] args)
    {
        XmlDocument doc = new XmlDocument();
        doc.LoadXml("<root><child/></root>");

        foreach (XmlNode node in doc.DocumentElement.ChildNodes)
        {
            Foo(node);
        }

        foreach (var node in doc.DocumentElement.ChildNodes)
        {
            // oops! node is now of type object!
            Foo(node);
        }
    }
}

The result is that this code actually produces different outputs depending on whether you used var or an explicit type. With var the Foo(object) overload will be executed, otherwise the Foo(XmlNode) overload will be. The output of the above program therefore is:

XmlNode overload
object overload

Note that this behavior is perfectly according to the C# language specification. The only problem is that var infers a different type (object) than you would expect and that this inference is not obvious from looking at the code.

I did not add the IL to keep it short. But if you want you can have a look with ildasm to see that the compiler actually generates different IL instructions for the two foreach loops.

It's an odd claim that using var should never be used because it "may lead to unexpected results in some cases", because there are subtleties in the C# language far more complex than the use of var.

One of these is the implementation details of anonymous methods which can lead to the R# warning "Access to modified closure" and behaviour that is very much not what you might expect from looking at the code. Unlike var which can be explained in a couple of sentences, this behaviour takes three long blog posts which include the output of a disassembler to explain fully:

The implementation of anonymous methods in C# and its consequences (part 1)
The implementation of anonymous methods in C# and its consequences (part 2)
The implementation of anonymous methods in C# and its consequences (part 3)

Does this mean that you also shouldn't use anonymous methods (i.e. delegates, lambdas) and the libraries that rely on them such as Linq or ParallelFX just because in certain odd circumstances the behaviour might not be what you expect?

Of course not.

It means that you need to understand the language you're writing in, know its limitations and edge cases, and test that things work as you expect them to. Excluding language features on the basis that they "may lead to unexpected results in some cases" would mean that you were left with very few language features to use.

If they really want to argue the toss, ask them to demonstrate that a number of your bugs can be directly attributed to use of var and that explicit type declaration would have prevented them. I doubt you'll hear back from them soon.

They argue that using of var "may lead to unexpected results in some cases".to unexpected results in some cases".

If unexpected is, "I don't know how to read the code and figure out what it is doing," then yes, it may lead to unexpected results. The compiler has to know what type to make the variable based on the code written around the variable.

The var keyword is a compile time feature. The compiler will put in the appropriate type for the declaration. This is why you can't do things like:

var my_variable = null
or
var my_variable;

The var keyword is great because, you have to define less information in the code itself. The compiler figures out what it is supposed to do for you. It's almost like always programming to an interface when you use it (where the interface methods and properties are defined by what you use within the declaration space of the variable defined by var). If the type of a variable needs to change(within reason of course), you don't need to worry about changing the variable declaration, the compiler handles this for you. This may sound like a trivial matter, but what happens if you have to change the return value in a function, and that function is used all throughout the program. If you didn't use var, then you have to find and replace every place that variable is called. With the var keyword, you don't need to worry about that.

When coming up with guidelines, as an auditor has to do, it is probably better to err on the side of fool safe, that is white listing good practices / black listing bad practices as opposed to telling people to simply be sensible and do the right thing based on an assessment of the situation at hand.

If you just say "don't use var anywhere in code", you get rid of a lot of ambiguity in the coding guidelines. This should make code look & feel more standardized without having to solve the question of when to do this and when to do that.

I personally love var. I use it for all local variables. All the time. If the resulting type is not clear, then this is not an issue with var, but an issue with the (naming of) methods used to initialize a variable...

I follow a simple principle when it comes to using the var keyword. If you know the type beforehand, don't use var. In most cases, I use var with linq as I might want to return an anonymous type.

var best using when you have obviously declaration

ArrayList<Entity> en = new ArrayList<Enity>()

complicates readability

var en = new ArrayList<Entity>()

Lazy, clear code, i like it

I use var only where it is clear what type the variable is, or where it is no need to know the type at all (e.g. GetPerson() should return Person, Person_Class, etc.).

I do not use var for primitive types, enum, and string. I also do not use it for value type, because value type will be copied by assignment so the type of variable should be declared explicitly.

About your auditor comments, I would say that adding more lines of code as we have been doing everyday also "lead to unexpected results in some cases". This argument validity has already proven by those bugs we created, therefore I would suggest freezing the code base forever to prevent that.

using var is lazy code if you know what the type is going to be. Its just easier and cleaner to read. When looking at lots and lots of code, easier and cleaner is always better

There is absolutely no difference in the IL output for a variable declaration using var and one explicitly specified (you can prove this using reflector). I generally only use var for long nested generic types, foreach loops and anonymous types, as I like to have everything explicitly specified. Others may have different preferences.

var is just a shorthand notation of using the explicit type declaration.

You can only use var in certain circumstances; You'll have to initialize the variable at declaration time when using var. You cannot assign a variable that is of another type afterwards to the variable.

It seems to me that many people tend to confuse the 'var' keyword with the 'Variant' datatype in VB6 .

The "only" benefit that i see towards using explicit variable declaration, is with well choosen typenames you state the intent of your piece of code much clearer (which is more important than anything else imo). The var keyword's benefit really is what Pieter said.

I also think that you will run into trouble if you declare your doubles without the D on the end. when you compile the release version, your compiler will likely strip off the double and make them a float to save space since it will not consider your precision.

var will compile to the same thing as the Static Type that could be specified. It just removes the need to be explicit with that Type in your code. It is not a dynamic type and does not/can not change at runtime. I find it very useful to use in foreach loops.

foreach(var item in items)
{
item.name = ______;
}

When working with Enumerations some times a specific type is unknown of time consuming to lookup. The use of var instead of the Static Type will yeald the same result. I have also found that the use of var lends it self to easier refactoring. When a Enumeration of a different type is used the foreach will not need to be updated.

Use of var might hide logical programming errors, that otherwise you would have got warning from the compiler or the IDE. See this example:

float distX = innerDiagramRect.Size.Width / (numObjInWidth + 1);

Here, all the types in the calculation are int, and you get a warning about possible loss of fraction because you pick up the result in a float variable.

Using var:

var distX = innerDiagramRect.Size.Width / (numObjInWidth + 1);

Here you get no warning because the type of distX is compiled as int. If you intended to use float values, this is a logical error that is hidden to you, and hard to spot in executing unless it triggers a divide by zero exception in a later calculation if the result of this initial calculation is <1.