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Correct or best way to declare variable in C# ASP.NET

In the following code, which option (A or B), are the best way to declare a variable which will be assigned in the following if statement.
Alternatively, is there a better way than A or B.
(A) List<Dollars> amount;
(B) var amount = new List<Dollars>();
if (vortex is null)
{
amount = this.seven.Map<List<Dollars>>(twelve);
}
else
{
amount = rendered;
}

The answer lies in the post of Microsoft docs Implicitly Typed Local Variables:
The var keyword can also be useful when the specific type of the
variable is tedious to type on the keyboard, or is obvious, or does
not add to the readability of the code.
Look at the line -
The var keyword can also be useful when the specific type of the variable is tedious to type on the keyboard, or is obvious, or does
not add to the readability of the code.
One example where var is helpful in this manner is with nested generic types such as those used with group operations. In the following query, the type of the query variable is IEnumerable>. As long as you and others who must maintain your code understand this, there is no problem with using implicit typing for convenience and brevity.
The disadvantage here is, 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.
Your answer is, no. There is no
best way to declare a variable
There is: what type of way suits your needs the most.

Is using var actually slow? If so, why?

I am learning C# and .NET, and I frequently use the keyword var in my code. I got the idea from Eric Lippert and I like how it increases my code's maintainability.
I am wondering, though... much has been written in blogs about slow heap-located refs, yet I am not observing this myself. Is this actually slow? I am referring to slow compile times due to type inferencing.

You state:
I am referring to slow time for compile due to type 'inferencing'
This does not slow down the compiler. The compiler already has to know the result type of the expression, in order to check compatibility (direct or indirect) of the assignment. In some ways using this already-known type removes a few things (the potential to have to check for inheritance, interfaces and conversion operators, for example).
It also doesn't slow down the runtime; they are fully static compiled like regular c# variables (which they are).
In short... it doesn't.

'var' in C# is not a VARIANT like you're used to in VB. var is simply syntactic sugar the compiler lets you use to shorthand the type. The compiler figures out the type of the right-hand side of the expression and sets your variable to that type. It has no performance impact at all - just the same as if you'd typed the full type expression:
var x = new X();
exactly the same as
X x = new X();
This seems like a trivial example, and it is. This really shines when the expression is much more complex or even 'unexpressable' (like anonymous types) and enumerables.

Var is replaced at compile time with your actual variable type. Are you thinking of dynamic?

A "variant" is typeless, so access to state (or internal state conversion) always must go through two steps: (1) Determine the "real" internal type, and (2) Extract the relevant state from that "real" internal type.
You do not have that two-step process when you start with a typed object.
True, a "variant" thus has this additional overhead. The appropriate use is in those cases where you want the convenience of any-type for code simplicity, like is done with most scripting languages, or very high-level APIs. In those cases, the "variant" overhead is often not significant (since you are working at a high-level API anyway).
If you're talking about "var", though, then that is merely a convenience way for you to say, "Compiler, put the proper type here" because you don't want to do that work, and the compiler should be able to figure it out. In that case, "var" doesn't represent a (runtime) "variant", but rather a mere source-code specification syntax.

The compiler infers from the constructor the type.
var myString = "123"; is no different from string myString = "123";
Also, generally speaking, reference types live on the heap and value types live on the stack, regardless if they're declared using var.

What is the purpose of 'var'? [duplicate]

This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
What's the point of the var keyword?
I'm not asking how it works. I am not asking if it affects performance. I already know those answers.
I want to know what inspired the MS C# team to add it to the language in the first place. You don't add frivolous things to a language. There must have been a noteworthy problem it solved. What was/is that problem?
The closest example I've seen to "the problem it solves" is when using anonymous types, like this:
var linqResult = from element in SomeCollection s
elect new { element.A, element.B } 
The irony about this usage is that style and coding-standards guides (such as provided by Microsoft) advise the coder to avoid using 'var' when the resulting type is not obvious. In other words, the (presumably) intended purpose of 'var' is in conflict with the coding-standard guidelines.
If I were writing coding-standards, and was trying to prevent overuse of 'var', I'd be somewhat inclined to say "use 'var' only in response to anonymous types." But that brings the question full-circle: what was/is the purpose of having added 'var' to the language?

Anonymous Types was the big one, but also reducing repetition within methods:
Dictionary<MyCustomType, List<MyOtherCustomType>> dict = new Dictionary<MyCustomType, List<MyOtherCustomType>>();

Exactly! Anonymous types. How in the world would you keep a reference to a created anonymous type if there was no var and still have intellisense along with it?
In other words: When there was no var, there wouldn't be any anonymous types in the usable sense we have now with compile time support. There would be just too many runtime errors with anonymous types.
The added sugar (of complexity when overused) is that you can use var with even non-anonymous variables. Sure it works but it's a bad habit.

As far as I know the var keyword was made pretty much to make LINQ easierby enabling the use of anonymous types

From MSDN:
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. This
is a common scenario in LINQ query
expressions.

Use of "var" type in variable declaration

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 respected 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.

Why is C# statically typed?

I am a PHP web programmer who is trying to learn C#.
I would like to know why C# requires me to specify the data type when creating a variable.
Class classInstance = new Class();
Why do we need to know the data type before a class instance?

As others have said, C# is static/strongly-typed. But I take your question more to be "Why would you want C# to be static/strongly-typed like this? What advantages does this have over dynamic languages?"
With that in mind, there are lots of good reasons:
Stability Certain kinds of errors are now caught automatically by the compiler, before the code ever makes it anywhere close to production.
Readability/Maintainability You are now providing more information about how the code is supposed to work to future developers who read it. You add information that a specific variable is intended to hold a certain kind of value, and that helps programmers reason about what the purpose of that variable is.
This is probably why, for example, Microsoft's style guidelines recommended that VB6 programmers put a type prefix with variable names, but that VB.Net programmers do not.
Performance This is the weakest reason, but late-binding/duck typing can be slower. In the end, a variable refers to memory that is structured in some specific way. Without strong types, the program will have to do extra type verification or conversion behind the scenes at runtime as you use memory that is structured one way physically as if it were structured in another way logically.
I hesitate to include this point, because ultimately you often have to do those conversions in a strongly typed language as well. It's just that the strongly typed language leaves the exact timing and extent of the conversion to the programmer, and does no extra work unless it needs to be done. It also allows the programmer to force a more advantageous data type. But these really are attributes of the programmer, rather than the platform.
That would itself be a weak reason to omit the point, except that a good dynamic language will often make better choices than the programmer. This means a dynamic language can help many programmers write faster programs. Still, for good programmers, strongly-typed languages have the potential to be faster.
Better Dev Tools If your IDE knows what type a variable is expected to be, it can give you additional help about what kinds of things that variable can do. This is much harder for the IDE to do if it has to infer the type for you. And if you get more help with the minutia of an API from the IDE, then you as a developer will be able to get your head around a larger, richer API, and get there faster.
Or perhaps you were just wondering why you have to specify the class name twice for the same variable on the same line? The answer is two-fold:
Often you don't. In C# 3.0 and later you can use the var keyword instead of the type name in many cases. Variables created this way are still statically typed, but the type is now inferred for you by the compiler.
Thanks to inheritance and interfaces sometimes the type on the left-hand side doesn't match the type on the right hand side.

It's simply how the language was designed. C# is a C-style language and follows in the pattern of having types on the left.
In C# 3.0 and up you can kind of get around this in many cases with local type inference.
var variable = new SomeClass();
But at the same time you could also argue that you are still declaring a type on the LHS. Just that you want the compiler to pick it for you.
EDIT
Please read this in the context of the users original question
why do we need [class name] before a variable name?
I wanted to comment on several other answers in this thread. A lot of people are giving "C# is statically type" as an answer. While the statement is true (C# is statically typed), it is almost completely unrelated to the question. Static typing does not necessitate a type name being to the left of the variable name. Sure it can help but that is a language designer choice not a necessary feature of static typed languages.
These is easily provable by considering other statically typed languages such as F#. Types in F# appear on the right of a variable name and very often can be altogether ommitted. There are also several counter examples. PowerShell for instance is extremely dynamic and puts all of its type, if included, on the left.

One of the main reasons is that you can specify different types as long as the type on the left hand side of the assignment is a parent type of the type on the left (or an interface that is implemented on that type).
For example given the following types:
class Foo { }
class Bar : Foo { }
interface IBaz { }
class Baz : IBaz { }
C# allows you to do this:
Foo f = new Bar();
IBaz b = new Baz();
Yes, in most cases the compiler could infer the type of the variable from the assignment (like with the var keyword) but it doesn't for the reason I have shown above.
Edit: As a point of order - while C# is strongly-typed the important distinction (as far as this discussion is concerned) is that it is in fact also a statically-typed language. In other words the C# compiler does static type checking at compilation time.

C# is a statically-typed, strongly-typed language like C or C++. In these languages all variables must be declared to be of a specific type.

Ultimately because Anders Hejlsberg said so...

You need [class name] in front because there are many situations in which the first [class name] is different from the second, like:
IMyCoolInterface obj = new MyInterfaceImplementer();
MyBaseType obj2 = new MySubTypeOfBaseType();
etc. You can also use the word 'var' if you don't want to specify the type explicitely.

Why do we need to know the data type
before a class instance?
You don't! Read from right to left. You create the variable and then you store it in a type safe variable so you know what type that variable is for later use.
Consider the following snippet, it would be a nightmare to debug if you didn't receive the errors until runtime.
void FunctionCalledVeryUnfrequently()
{
ClassA a = new ClassA();
ClassB b = new ClassB();
ClassA a2 = new ClassB(); //COMPILER ERROR(thank god)
//100 lines of code
DoStuffWithA(a);
DoStuffWithA(b); //COMPILER ERROR(thank god)
DoStuffWithA(a2);
}
When you'r thinking you can replace the new Class() with a number or a string and the syntax will make much more sense. The following example might be a bit verbose but might help to understand why it's designed the way it is.
string s = "abc";
string s2 = new string(new char[]{'a', 'b', 'c'});
//Does exactly the same thing
DoStuffWithAString("abc");
DoStuffWithAString(new string(new char[]{'a', 'b', 'c'}));
//Does exactly the same thing

C#, as others have pointed out, is a strongly, statically-typed language.
By stating up front what the type you're intending to create is, you'll receive compile-time warnings when you try to assign an illegal value. By stating up front what type of parameters you accept in methods, you receive those same compile-time warnings when you accidentally pass nonsense into a method that isn't expecting it. It removes the overhead of some paranoia on your behalf.
Finally, and rather nicely, C# (and many other languages) doesn't have the same ridiculous, "convert anything to anything, even when it doesn't make sense" mentality that PHP does, which quite frankly can trip you up more times than it helps.

c# is a strongly-typed language, like c++ or java. Therefore it needs to know the type of the variable. you can fudge it a bit in c# 3.0 via the var keyword. That lets the compiler infer the type.

That's the difference between a strongly typed and weakly typed language. C# (and C, C++, Java, most more powerful languages) are strongly typed so you must declare the variable type.

When we define variables to hold data we have to specify the type of data that those variables will hold. The compiler then checks that what we are doing with the data makes sense to it, i.e. follows the rules. We can't for example store text in a number - the compiler will not allow it.
int a = "fred"; // Not allowed. Cannot implicitly convert 'string' to 'int'
The variable a is of type int, and assigning it the value "fred" which is a text string breaks the rules- the compiler is unable to do any kind of conversion of this string.

In C# 3.0, you can use the 'var' keyword - this uses static type inference to work out what the type of the variable is at compile time
var foo = new ClassName();
variable 'foo' will be of type 'ClassName' from then on.

One things that hasn't been mentioned is that C# is a CLS (Common Language Specification) compliant language. This is a set of rules that a .NET language has to adhere to in order to be interopable with other .NET languages.
So really C# is just keeping to these rules. To quote this MSDN article:
The CLS helps enhance and ensure
language interoperability by defining
a set of features that developers can
rely on to be available in a wide
variety of languages. The CLS also
establishes requirements for CLS
compliance; these help you determine
whether your managed code conforms to
the CLS and to what extent a given
tool supports the development of
managed code that uses CLS features.
If your component uses only CLS
features in the API that it exposes to
other code (including derived
classes), the component is guaranteed
to be accessible from any programming
language that supports the CLS.
Components that adhere to the CLS
rules and use only the features
included in the CLS are said to be
CLS-compliant components
Part of the CLS is the CTS the Common Type System.
If that's not enough acronyms for you then there's a tonne more in .NET such as CLI, ILasm/MSIL, CLR, BCL, FCL,

Because C# is a strongly typed language

Static typing also allows the compiler to make better optimizations, and skip certain steps. Take overloading for example, where you have multiple methods or operators with the same name differing only by their arguments. With a dynamic language, the runtime would need to grade each version in order to determine which is the best match. With a static language like this, the final code simply points directly to the appropriate overload.
Static typing also aids in code maintenance and refactoring. My favorite example being the Rename feature of many higher-end IDEs. Thanks to static typing, the IDE can find with certainty every occurrence of the identifier in your code, and leave unrelated identifiers with the same name intact.
I didn't notice if it were mentioned yet or not, but C# 4.0 introduces dynamic checking VIA the dynamic keyword. Though I'm sure you'd want to avoid it when it's not necessary.

Why C# requires me to specify the data type when creating a variable.
Why do we need to know the data type before a class instance?
I think one thing that most answers haven't referenced is the fact that C# was originally meant and designed as "managed", "safe" language among other things and a lot of those goals are arrived at via static / compile time verifiability. Knowing the variable datatype explicitly makes this problem MUCH easier to solve. Meaning that one can make several automated assessments (C# compiler, not JIT) about possible errors / undesirable behavior without ever allowing execution.
That verifiability as a side effect also gives you better readability, dev tools, stability etc. because if an automated algorithm can understand better what the code will do when it actually runs, so can you :)

Statically typed means that Compiler can perform some sort of checks at Compile time not at run time. Every variable is of particular or strong type in Static type. C# is strongly definitely strongly typed.