C# 7 Additions – ref Variables

C# 7 expands the use of the ref keyword. Along with its previous use, it can now be used in return statements, and local variables can store a reference to the object as well. At first glance, the question is “What is the real difference between returning a ref variable, and setting it through an out parameter?” Previously you could set a variable passed into a function with ref (or out) to a different value. In C# 7, you can return the reference of a field, variable etc. and store that in a local variable for later use.

The following is an examples showing its expanded use.

As expected, the PersonInformation object is passed into the GetName function which returns a reference to the string field Name. This is then passed into the MakeCapitalized function which capitalizes the name “jenny” (making it “Jenny”) in the original PersonInformation object. Compare this to the example here showing how the previous version of C# would not allow the modification of the original field in the same scenario.

Classes vs Structs

If the PersonInformation is changed to be a struct (value type) instead of a class (reference type), the following code won’t work without a slight modification, but it is still completely possible.

Structs are passed by value meaning that passing a struct into a method creates a copy of it. Returning a reference to the struct’s field would return a reference to the copied struct and would go out of scope as soon as the method completes. There would be no point, and it would cause errors pointing to fields to objects which didn’t exist.

Caveats

With these new features there are some restrictions to it. Consider this. A string can be treated as an array of characters. With the new functionality, it should be possible to pass back a reference to a character location in that string and update it, because you have the reference to the character location in the string.

Fortunately, this isn’t allowed. The compiler prevents from it being a valid option, because if this were possible, it would break the string’s immutability and cause havoc with C#’s ability to intern strings.
ref string not allowed.

The compiler is also smart enough to not allow references to variables which fall out of scope. The following is also not allowed:

After the method exits someNumber no longer exists, and when another part of the application tries to access it, it won’t be available. (You could say this might not be the case if it were a reference type like a string, but it still wouldn’t matter, because all the reference has is a location to where the object is, not the actual object itself. This causes 2 problems: One, currently there is no way to get the value from the reference. Two, the object isn’t rooted, so it could still be garbage collected at any point in time.)

The compiler is also smart enough to trace the variable use through the calling methods. This is also not allowed:

C# 7 Additions – Out Variables

C# 7 removes the need for out variables to be predeclared before passing them into a function.

It also now allows the use of the var keyword to declare the variable type, because the compiler will infer the type based on the declared parameter type. This is not allowed when the compiler can’t infer the type because of method overloading. It would be nice if the compiler would attempt to infer it’s type based on the use later on in the method similar to F#’s inferred types, but this isn’t slated to be in the current release.

compiler confused because of method overloading.

In Visual Studio 15 Preview 4, the out variable isn’t working exactly as it will in the final release. Wild cards will hopefully be added so extraneous variables don’t need to be declared.

The following code won’t work until the scope restrictions on out variables is updated. (They have said they intend on doing this before the release.)

In this example, the scope is limited to the method call where the strings are set. To get it work currently, variable scope must be extended and can be like so:

The conditional statement wraps the variables and they can now be used in the Console.WriteLine. This will be corrected in the final release and won’t be necessary.

C# 7 Additions – Deconstructors

C# has a new type of method, the Deconstructor. When a type implements this method type with the name of Deconstruct, multiple variables maybe directly assigned as a return type would.

The method must be named Deconstruct and have a return type of void. The parameters to be assigned all must be out parameters, and because they are out parameters with a return type of void, C# allows the Deconstruct method to be overloaded solely based on these parameters. This is how the new System.ValueTuple allows it’s properties to be assigned to separate variables without assigning each one individually.

Deconstruct also does not need to be directly attached to the class. C# allows the method to be implemented as an extension method as well.

At the moment it is uncertain if wildcards will be added allowing unneeded variables to be omitted from being assigned. This addition would allow the insertion of the * to indicate a parameter is not needed (similar to _ in F#)

C# 7 Additions – Local Functions

In C# 7 it is now possible to create a function within a function termed a Local Function. This is for instances where a second function is helpful, but it’s not really needed in the rest of the class. It’s created just like regular functions except in the middle of another function.

Just like normal functions, you can create expression bodied members as well

Local variables in the outer functions are accessible, and it’s possible to embed local functions inside other local functions:

So how does it work? Looking at the IL code, the compiler has converted the internal function into a private static one inside the class.

IL Code showing private static function

The name is generated at compile time, so it is not accessible to other methods, but it is still possible to access it through reflection with the private and static binding flags.

reflection shows local function.

Someone I know asked what would be a good use case of Local Functions vs. Lambdas. Lambdas can’t contain enumerators, and by encasing an enumerations in a local function it allows others parts of the outer method to be eagerly evaluated. For example, if you have a method which takes a parameter and returns an enumeration, the evaluation of the parameter won’t occur until program starts to enumerate the collection. Encapsulating the enumeration in a local function allows the other parts of the outer function to be eagerly evaluated. You can find an example of the difference between using one and not using one here.

C# 7 Additions – Tuples

In C# 7 Microsoft has introduced an updated Tuple type. It has a streamlined syntax compared to it’s predecessor making it fall it look more like F#. Instead of declaring it like previous versions, the new Tuple type looks like:

Likewise to declare it as a return type, the syntax is similar to declaring it:

The first thing to note about the new type is that it is not included automatically in a new project. If you immediately use it, you’ll see the following error.

As of VS 15 preview 4 (not to be confused with VS 2015), you must include the System.ValueTuple Nuget package to take advantage of it.

This raises the question about how the new Tuple type and the previous one included since .NET 4 are related? They’re not. They are treated as two different types and are not compatible with each other.  System.Tuple is a reference type and System. ValueTuple is a value type.

So what are advantages over the previous version? The syntax simpler, and there are several other advantages.

Named Properties

In the System.Tuple version, properties of the return object were referenced as Item1, Item2 etc. This gets confusing when there are multiples of the same type in the Tuple as you have to know what position had which value type.

Now it’s possible to explicitly name the item types to reduce confusion.

The Item properties (Item1, Item2, etc.) have also been included allowing methods to be updated to the new type without breaking code based on it’s predecessor.

It’s also possible to explicitly name the values when creating the object:

Deconstruction

It is now possible to name and assign variable values upon creating (or returning) a tuple. Although not necessary, it reduces the amount of code necessary to pull values out of the type.

It’s not certain if C# will get wildcards like F# to automatically discard values which aren’t needed. If they are allowed then it’s possible to only create a variable for the name like so:

Updating Values

System.Tuple is immutable.  Once created it’s not possible to update any of the values.  This restriction has been removed in the new version.  From a purely functional perspective this could be considered a step backwards, but in C# many people find this approach more forgiving and beneficial.

Like all value types, when it is passed into a method, a copy of the tuple is created, so modifying it in the in the method does not affect the original.

However if you compare two different tuples and they have the same values, the Equals method compares the values in each and if they are all equal, it considers them equal.

Integrations with Other Languages

Unfortunately, C#’s new tuple type doesn’t automatically allow it to translate tuples from F#.

F# can’t desconstruct the values like it can with it’s native tuples, and to return it, you have to explicitly instantiate the object type and add the values.

Either way, the translation to F# isn’t horrible as it acts like any other object passed to it by C#.