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Is there a way to define a class in a method in C#? [duplicate]

One of the common programming best practices is "define variables as close to where they are used as possible".
I use structs frequently to create code thats almost self documenting in places. However, C# forces me to define the struct outside the method. This breaks the aforementioned best practice - its basically creating an unwanted global variable type for the entire class.
Is it possible to define a local struct inside a method, just like a local variable, and if not, could you give me a window into the reasons the C# designers decided to prevent this?
Use Case
I'm converting part of a spreadsheet into C# code. I'd like to use local structs within the method to store temporary information in an organized manner, without having to resort to hundreds of separate variables that are global in scope.
Update 2016-August: C# 7.0 may have this feature!
As of 2016-Aug, apparently, this will be a feature in C# 7.0.
So the C# compiler team agreed - wow!
Update 2020-July: Now supported by C# and C++
C++ has always fully supported this. And it's fantastic.
C# 7.0 now has value tuples for a lightweight data structure with named fields. See answer from Ghost4Man.

I believe it's not permitted to define named types within a method. As to why, I'll have to speculate. If a type is not going to be used outside, then its existence probably cannot be justified.
You can however define anonymous type variables within a method. It will somewhat resembles structures. A compromise.
public void SomeMethod ()
{
var anonymousTypeVar = new { x = 5, y = 10 };
}

It is a little late but this is my solution for lists - using anonymous vars as the structs inside of methods:
var list = new[] { new { sn = "a1", sd = "b1" } }.ToList(); // declaring structure
list.Clear(); // clearing dummy element
list.Add(new { sn="a", sd="b"}); // adding real element
foreach (var leaf in list) if (leaf.sn == "a") break; // using it
Anonymous elements (sn and sd) are somehow read only.

Since C# 7.0, you can use value tuples if you want a lightweight data structure with named fields. They can be used not only locally inside methods, but also in parameters, returns, properties, fields, etc. You can use local functions to somewhat emulate struct methods.
var book = (id: 65, pageCount: 535); // Initialization A
(int id, int pageCount) book2 = (44, 100); // Initialization B
Console.WriteLine($"Book {book.id} has {book.pageCount} pages.");
(int id, int pageCount) = book; // Deconstruction into variables
Console.WriteLine($"Book {id} has {pageCount} pages.");
Here book is of type System.ValueTuple<int, int> (a generic struct).

You could do something like this using anonymous types. MSDN examples below:
var v = new { Amount = 108, Message = "Hello" };
or
var productQuery =
from prod in products
select new { prod.Color, prod.Price };
foreach (var v in productQuery)
{
Console.WriteLine("Color={0}, Price={1}", v.Color, v.Price);
}

Nowadays, you could also use a named tuple: https://learn.microsoft.com/en-us/dotnet/csharp/tuples

No, this is not possible. If you are using .net 4.0, you could use Tuple<T1, ..., Tn> to replicate such a thing.
I don't see the reason why you would need such a struct - just use variables with speaking names and this shouldn't be any problem at all. In combination with explicit declaration using the class names there is very little space for ambiguity.

You can define an anonymous type within your method and use it. The anonymous type will be readonly, so it gets you the immutability that is desired of structs. It will not explicitly be a struct, but it will be fully defined and contained within your method.
var myLocalType = new
{
SomeValue = "Foo",
SomeId = 14
};

it's not a struct, but mayme a var can help you out here?
var person = new {Name= "John", City = "London"};
it's strong typed so it will be compile time checked

You can create a dynamic type in c# 4.0 to accomplish this task, but its not exactly what you are looking for.
However I believe that the maximum of defining variables as close to where they are used is meant to mean where a variable is introduced into program flow not where the type is declared. I believe that most types have some ability to be reused creating in method types limits you ability to create reusable blocks of code that operates on common data.

What is the advantage of this type inference?

Using type inference we can write:
var myList = new List<int>();
but it's not really helping is it? Because the still requires us to only add ints, so wouldnt it have been easier to just declare the variable of 'int' instead of 'var' ?

The var doesn't change the output of the compiler - it only makes the code shorter.
The only time that var offers anything else is in regards to anonymous types:
var foo = new { Bar = 123 };
It is important to understand that var doesn't mean dynamic or variable (in the sense that it allows the type of the object to vary) - it just allows you to declare a variable and have the compiler infer the type as if you had declared it.

IEnumerable<MadeUpEmbarrassinglyLongClassName> stupidVariable = new IEnumerable<MadeUpEmbarrassinglyLongClassName>();
vs
var stupidVariable = new IEnumerable<MadeUpEmbarrassinglyLongClassName>();
Which is easier to read?

Its more concise than:
List<int> myList = new List<int>();
I guess this might be easier to read that using var though, although in this trivial case there's not much in it.
In addition, if the variable is named sensibly, then the what type it is should be fairly clear anyway.

The var is of unknown type until it is assigned a value. As soon as you assign a value, it becomes the typeof to which it is assigned. This var is used in situation when you can't infer at compile time as to what value will be passed to it. The other situation where var comes in handy is in LINQ queries. Rather than creating a special class to handle your query returns, you simple assign the result to var and it does the rest. You will surely compliment var once you start using LINQ

In your example, there is no advantage. In fact, I believe that code readability suffers a bit (opinion).
The answer to this quesiton boils down to personal preference. The compiler will infer List<int>() if var is used. Which would you prefer to look at? Which would your peers prefer to look at?

In C#, how do inferred variables using var behave with respect to memory and lifecycle?

Ok, I've been reading everything I can find on var, and I'm pretty sure I've got a handle on their basic behaviors and type inference. I've always been a big proponent of explicit types, so var makes me a little edgy when I'm reading code. So I have a couple of questions regarding its memory usage and the behavior of a variable declared with var over the course of that variable's lifecycle in memory.
Since var is an inference to an explicit or anonymous type, will its memory be allocated in the same place its corresponding type would be or is var universally created on the heap and accessed as if it were an object. As an example:
int i = 5; // puts this on the stack
var i = 5; // does this also go on the stack?
Does a declared var have a constant type once it is initialized or can it be adjusted as well? I ask this because I can't find in the documentation that specifies this, and I just read something in this SO question by #Eric Lippert:
a variable is a storage location whose
contents change
Having tested the following code, I see an implicit conversion exception even at the IDE level. I don't have the experience with LINQ at this point to run a similar test with respect to anonymous types. Do they follow this same behavior? Will the IDE recognize a type mismatch at design time or would such code receive a run-time exception?
var i = 5; // Initializes as int, value is 5
i = "Steve"; // Type casting error
Finally, is there ever a situation that you can think of where you may know a type at design time but it would be prudent to use var anyway? I ask because I've seen code samples where people do this, and from what I've read it strikes me as just lazy-coding.
EDIT: I realize there are a lot of articles talking about this topic, but I haven't found any that answer these questions specifically (though some hint one way or another). I would be happy to read any document you feel is relevant to these topics, just please post a link.

var is 100% syntactic sugar in the C# compiler. When the actual IL is generated, the type is explicitly defined. var-declared variables do not behave any differently from their explicitly-defined counterparts. The dearth of information you're seeing is coming from the fact that var is simpler than you're assuming. There is quite literally no difference between these two lines:
var i = 10;
int j = 10;
(Other than the fact that you see the words var and int, of course; they are functionally completely identical).

var variables are infered by the compiler, they are really just syntactical sugar, there is no difference between:
var i =0;
and
int i = 0;
in the compiled code.
They were added for the purpose of allowing anonymous types, e.g.
var MyVar = new { prop1="A string", prop2=5};
Yes var varables recognise a type mismatch at compile time, the compiler dynamically creates a class for them when it compiles. e.g. the below would not compile:
var MyVar = new { prop1="A string", prop2=5};
MyVar = "Fred";

As others have said, var has no effect on how the variables behave in terms of memory - it just means you don't specify the name. For anonymous types you couldn't specify the name, first because you don't know it at compile time, and second because the names are deliberately "unspeakable" - they're not valid in C#. (Generally they contain <> for example.)
The IL generated for code using var and code using an explicit name is exactly the same. Just as a normal variable can't change type, nor can a variable declared using var... so in your example where you try to assign the value "Steve" to a variable which has an implicit type of int, you'll get a compile-time error exactly as if you'd explicitly declared it to be of type int.
As for when to use var and when not to, I have a few rules of thumb:
Obviously if you want the variable to have a type other than the compile-time type of the assigned expression, you've got to do it explicitly
For constructor calls, it's always clear what the the type is even if you're using var
var is useful for emphasizing what the code is meant to do rather than how
Often I use var if the explicit name would be extremely long - particularly for generics with multiple type arguments.
Basically it's all about readability: if the code is easier to read using var, go for it. If not, don't. Note that this isn't the same as saving typing... code is generally read more than it's written, so think about your reader. Eric Lippert wrote a great note when tech reviewing the first edition of C# in Depth, which is worth a read.

Simple answer: exactly the same as normal variables.
var declarations are translated into specific (strong) types at compile-timee. var is simply a method of automatic type inference, and has nothing to do with dynamic languages. At the end of the day, it's just the compiler being clever and translating var into what actual type you want.

"Since var is an inference to an explicit or anonymous type, will its memory be allocated in the same place its corresponding type would be or is var universally created on the heap and accessed as if it were an object"
The important thing about var, is that the compiler changes the var statement to the actual type at compile time, so:
var number = 1;
Will get changed to:
System.Int32 number = 1;
...by the compiler. And as such the memory locations for storage of these types is no different. var is essentially syntactic sugar. So, method local declarations of value types will get stored on the stack, reference pointers will get stored on the stack with the referenced objects on the heap.
Once you have declared a variable as var, because this is translated into its full type by the compiler, you can't then dynamically change the type:
var i = 5;
i = "Steve";
...is invalid, because its already been declared as an Int32.
Anonymous types follow a similar behaviour, where a type is created by the compiler during compilation.
"Finally, is there ever a situation that you can think of where you may know a type at design time but it would be prudent to use var anyway?"
Personally, I follow a simple pattern:
For primitives, I always put the type:
int i = 5;
string name = "Matt";
For complex types, I mostly do this:
var instance = new MyComplexTypeInstance();
For LINQ results, I stick to var:
var result = from i in something select i;
For methods, if the method is descriptive of the type being returned, then I will use var.
var instance = GetInstance();
Whereas, more complex method names, I'd put the actual type:
Result result = DoSomethingWeirdAndWonderful();
Each developer will find something they are comfortable with, the choice is yours. As var is only really a design-time thing, its all syntactic sugar.

Advantage of var keyword in C# 3.0 [duplicate]

This question already has answers here:
Closed 13 years ago.
Duplicate:
What to use var or object name type
I couldn't understand the need of var keyword in C# 3.0 What is the advantage in using it.
i saw this question but did not understand the real purpose of using it

It's mostly present for LINQ, when you may use an anonymous type as the projection:
var query = from person in employees
where person.Salary > 10000m
select new { FullName=person.Name, person.Department };
Here the type of query can't be declared explicitly, because the anonymous type has no name. (In real world cases the anonymous type often includes values from multiple objects, so there's no one named class which contains all the properties.)
It's also practically useful when you're initializing a variable using a potentially long type name (usually due to generics) and just calling a constructor - it increases the information density (reduces redundancy). There's the same amount of information in these two lines:
List<Func<string, int>> functions = new List<Func<string, int>>();
var functions = new List<Function<string, int>>();
but the second one expresses it in a more compact way.
Of course this can be abused, e.g.
var nonObviousType = 999999999;
but when it's obvious what the type's variable is, I believe it can significantly increase readability.

The primary reason for its existence is the introduction of anonymous types in C#. You can construct types on the fly that don't have a name. How would you specify their name? The answer: You can't. You just tell the compiler to infer them for you:
var user = users.Where(u=> u.Name == "Mehrdad")
.Select(u => new { u.Name, u.Password });

It's a shorthand way of declaring a var. Although "int i = new int()" isn't too much to type, when you start getting to longer types, you end up with a lot of lines that look like:
SomeReallyLong.TypeName.WithNameSpaces.AndEverything myVar = new SomeReallyLong.TypeName.WithNameSpaces.AndEverything();
It eventually occurred to someone that the compiler already knew what type you were declaring thanks to the information you were using to initialize the var, so it wouldn't be too much to ask to just have the compiler do the right thing here.

Here are a couple of advantages
Less typing with no loss of functionality
Increases the type safety of your code. A foreach loop using an iteration variable which is typed to var will catch silently casts that are introduced with explicit types
Makes it so you don't have to write the same name twice in a variable declaration.
Some features, such as declaring a strongly typed anonymous type local variable, require the use of var
Shameless self promotion. I wrote a blog entry on this subject awhile back that dived into when I thought the use of var was appropriate and contains relative information to this topic.
http://beta.blogs.msdn.com/jaredpar/archive/2008/09/09/when-to-use-type-inference.aspx

Linq expressions don't return a predefined type, so you need a 'generic' variable declaration keyword to capture that and other places where anonymous types are used.
Used carefully, it can make refactoring easier by decoupling a method's return type from the variable that captures it.
Having to put the same name on the same line twice for the same statement is really kind of silly. It's a pain to type something like this:
.
ReallyLongTypeName<SomeOtherLongTypeName> MyVariable = new ReallyLongTypeName<SomeOtherLongTypeName>();

In short:
minimize the need for typing the variable type twice
essential in supporting anonymous types, e.g. as returned by LINQ queries

The real need for the var keyword was to support anonymous types in C# 3.0--which in turn were required to properly support LiNQ (Language Integrated Querying).
Without using var you could never do something like this:
var person = new { Name = "Peter", Age=4};
Which means that you couldn't execute execute the following linq query because you wouldn't know how to assign it to a variable:
[var/sometype] dogsFixedQuery = from myDog in kennel select new {dogName = myDog.FirstName + " " + myDog.OwnerLastName, myDog.IsNeutered, dogLocation = kennel.Address};
The utility of anonymous types will be more apparent if you start to create more complex linq queries with multiple levels of returns and joins.
The fact that you can use var in other ways to avoid typing out something like IEnumerable<Dictionary<List<string>,IOrderedCollection<DateTime>> myBadDesign = getBadDesignController().BadDesignResults("shoppingCart");
is just a side-effect/bonus in case you're a lazy typer =)
There are cons for readability if you start calling vars in disparate locations but if you're using var for a strong type and the compiler can determine the type of your variable than any reader should be able to determine it as well.

Using implicitly typed local variables [duplicate]

This question already has answers here:
Why would var be a bad thing?
(18 answers)
Closed 8 years ago.
I just installed a trial version of ReSharper and one of the first things I noticed is that it always suggests to replace explicitly typed local variables with implicitly typed ones, e.g:
public string SomeMethod(int aParam)
{
int aNumber = SomeOtherMethod(aParam);
// should be changed to:
var aNumber = SomeOtherMethod(aParam);
}
I think explicitly typed variables are more readable (more explicit).
What do you think about ReSharper's suggestion? Is there any advantage in using implicitly typed variables? When do you use implicit/explict vars?

I personally only use “var” when I can clearly distinguish the variable Type by just reading the declaration, for example:
var someVariable = new List<int>();
In the example above, its evident that “var” refers to “List<int>”.
I don’t like to use “var” when I have to go to some method definition to find out what variable type “var” represents or by having to rely on visual studio intelli-popup or whatever that is called, for example this in not ok to me:
var someVaraible = SomeMethod();
I mean, what is the “SomeMethod” function supposed to return? Can you tell just by looking at the line of code? No you can’t, so that is why I avoid using “var” on those situations.

There's a lot of discussion about this, but I think it all comes down to personal taste, just like using the 'this' keyword almost everywhere.
I personally prefer explictly typed variables, but when using nested generic collections things can become more readable using an implicitly typed variable. Look at:
Dictionary<string, Dictionary<string, string>> myDictionary = new Dictionary<string, Dictionary<string, string>>();
vs:
var myDictionary = new Dictionary<string, Dictionary<string, string>>();
EDIT: this SO topic covers the same topic, with some nice replies: What to use: var or object name type?
EDIT2: Working a lot with async nowadays, I find that using explicity typed variables can sometimes prevent nasty bugs. Consider this silly example where you would want to return the Id of a user. Also consider that GetUserAsync returns a Task<User>. If you use implicitly typed variables, you would end up using something like this:
public long GetUserId()
{
var user = GetUserAsync();
return user.Id;
}
This compiles, but it is wrong. 'user' is actually a Task<User>. And it compiles as Task also has an Id property. In this case, one would accidentally return the Id of a Task instead of the User.
public long GetUserId()
{
User user = GetUserAsync();
return user.Id;
}
The above does not compile, as the compiler will complain that you cannot cast a Task to a User. Adding the await keyword of course solves this.
I've actually had this happen to me once :-)

Just in case some haven’t noticed yet, you can easily change the “suggestions” in Reshaper (Reshaper -> Options -> Languages -> Context Actions -> “Replace explicit type specification with ‘var’”).
I personally prefer to have explicit type specifications everywhere, but I'm not too fussy about it.

It's just easier to type the var pseudo-keyword at times than a huge type name, especially if a generic could be involved. However, you should know they're functionally identical. There's no performance difference or anything either way. The compiler derives the type of the right-side of the assignment and replaces var with that type. It's not happening at run-time like a VB variant.

FWIW, the var keyword is plainly readable in many cases. Especially if...
The right-side of the assignment is a constructor expression.
var map = new Dictionary>();
Local variables have good names.
HTH