Is it possible to define a local struct, within a method, in C#?

Question

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.

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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!

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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.

Answer 1

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 };
}

Answer 2

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.

Answer 3

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).

Answer 4

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);
}

Answer 5

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

Answer 6

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.

Answer 7

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
    };

Answer 8

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

Answer 9

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.