C# 7 Additions – Pattern Matching

C# 7 has started to introduce Pattern Matching. This is a concept found in functional programming, and although it isn’t fully implemented compared to F#, it is a step in that direction. Microsoft has announced they intend on expanding it in future releases.

Constant Patterns

The is keyword has been expanded to allow all constants on the right side of the operator instead of just a type. Previously, C#’s only valid syntax was similar to:

Now it is possible to compare a variable to anything which is a constant: null, a value, etc.

Behind the scenes, the is statement is converted to calling the Equals function in IL code. The following two functions produce roughly the same code (they call different overloads of the Equals function).

CheckIsNull

CheckEqualsNull

This can also be combined with other features allowing variable assignment through the is operator.

In Visual Studio Preview 4, the scoping rules surrounding variables assigned in this manner are more restrictive than in the final version. Right now, they can only be used within the scope of the conditional statement.

Switch Statements

The new pattern matching extensions have also extended and changed the use of case statements. Patterns can now be used in switch statements.

Like in previous versions, the default statement will always be evaluated last, but the location of the other case statements now matter.

In this example, case int n will never evaluate, because the statement above it will always be true. Fortunately, the C# compiler will evaluate this, determine that it can’t be reached and raise a compiler error.

The variables declared in patterns behave differently than others. Each variable in a pattern can have the same name without running into a collision with other statements. Just as before, in order to declare a variable of the same name inside the case statement, you must still explicitly enforce scope by adding braces ({}).

Pattern matching has a ways to go when compared to its functional language equivalent, but it is still a nice addition and will become more complete as the language evolves.

C# 7 Additions – Literals

A small, but nice chance in C# 7 is increased flexibility in literals. Previously, large numeric constants had no separator, and it was difficult to easily read a large number. For example, if you needed a constant for the number of stars in the observable universe (1,000,000,000,000,000,000,000), you’d have to do the following:

If you hadn’t caught the error, the constant is too short, and it’s difficult to tell looking at the numbers without a separator. In C# 7, it’s now possible to use the underscore (_) in between the numbers. So the previous example now becomes much easier to read, and it is easily recognizable the number is off.

The new version adds binary constants too. Instead of writing a constant in hex, or decimal, a constant can now be written like so:

C# 7 Additions – Throw Expressions

In previous versions, throwing exceptions had certain limitations where they could be used. Although not hampering, at times it caused additional work to validate and throw an exception, and C# 7 has removed much of the developer overhead for validation and execution.

Expressions

Previously to throw an exception in the middle of an expression there were really two options:

or

It is now possible to also throw an exception in the middle of an expression. Instead of checking for null, it is possible to throw as the second condition in the Null Coalescing Operator.

It is also possible in the Conditional Operator as well.

Expression Bodied Members

C# 6 added the ability to write a method with a single statement with a “fat arrow” (=>) and the statement. What used to be

can now be condensed to:

If you need a method stub, because you don’t know how to complete the method, and it was appropriate to use an Expression Bodied Member, you were left with two possibilities as throwing an expression wasn’t allowed by the compiler.

or

The first is error prone, because if program calls the method, there is no indication that it isn’t functioning properly. (Is null an expected return or an indicator of an error?) The second is better, but it is a little cumbersome that you must convert it to a standard function just to throw the exception. C# 7 solves this inconvenience and is now possible to throw exceptions in the Expression Bodied Member.

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 property, 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 property 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 property 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 property 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 properties 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 – 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 – 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#.