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Quickstart guide for deconstructions (C# 7.0)

  1. Install Visual Studio 2017
  2. Start a C# project
  3. Add a reference to the System.ValueTuple package from NuGet
    Install the ValueTuple package
  4. Use deconstructions:
public class C
{
        public static void Main()
        {
                
              int code;
              string message;

              var pair = (42, "hello");
              (code, message) = pair; // deconstruct a tuple into existing variables
              Console.Write(message); // hello

              (code, message) = new Deconstructable(); // deconstruct any object with a proper Deconstruct method into existing variables
              Console.Write(message); // world
              
              (int code2, string message2) = pair; // deconstruct into new variables
              var (code3, message3) = new Deconstructable(); // deconstruct into new 'var' variables
        }
}

public class Deconstructable
{
        public void Deconstruct(out int x, out string y)
        {
                x = 43;
                y = "world";
        }
}

Design

This design doc will cover two kinds of deconstruction: deconstruction into existing variables (deconstruction-assignment) and deconstruction into new variables (deconstruction-declaration).

Here is an example of deconstruction-assignment:

class C
{
    static void Main()
    {
        long x;
        string y;

        (x, y) = new C();
        System.Console.WriteLine(x + "" "" + y);
    }

    public void Deconstruct(out int a, out string b)
    {
        a = 1;
        b = ""hello"";
    }
}

Deconstruction-assignment (deconstruction into existing variables):

This doesn't introduce any changes to the language grammar. We have an assignment-expression (also simply called assignment in the C# grammar) where the unary-expression (the left-hand-side) is a tuple-expression containing values that can be assigned to. In short, what this does in the general case is find a Deconstruct method on the expression on the right-hand-side of the assignment, invoke it with the appropriate number of out var parameters, converts those output values (if needed) and assign them to the variables on the left-hand-side. But in the special case where the expression on the right-hand-side is a tuple (tuple expression or tuple type), then the elements of the tuple are assigned to the variables on the left-hand-side without calling Deconstruct.

If the left-hand-side is nested the process will be repeated. For instance, in (x, (y, z)) = deconstructable;, deconstructable will be deconstructed into two parts and its second part will be further deconstructed.

In the case where the expression on the right is a tuple expression, it is first given a type. So in long x; string y; (x, y) = (1, null); the literals on the right-hand-side are typed as long and string before the deconstruction even starts, which means that no conversions will be needed during the deconstruction steps.

We noted already that tuples (which are syntactic sugar for the System.ValueTuple underlying type) don't need to invoke Deconstruct. The .Net framework also includes a set of System.Tuple types. Those are not recognized as C# tuples, and so will rely on the Deconstruct pattern. Those Deconstruct methods will be provided as extension methods for System.Tuple for up to 3 nestings deep (that is 21 elements).

A deconstruction-assignment returns a tuple value (with elements using default names) which is shaped and typed like the left-hand-side and holds the (converted) parts resulting from deconstruction.

Evaluation order

The evaluation order can be summarized as: (1) all the side-effects on the left-hand-side, (2) all the Deconstruct invocations (if not tuple), (3) conversions (if needed), and (4) assignments.

In the general case, the lowering for deconstruction-assignment would translate: (expressionX, expressionY, expressionZ) = expressionRight into:

// do LHS side-effects
tempX = &evaluate expressionX
tempY = &evaluate expressionY
tempZ = &evaluate expressionZ

// do Deconstruct
evaluate right and evaluate Deconstruct in three parts (tempA, tempB and tempC)

// do conversions
tempConvA = convert tempA
tempConvB = convert tempB
tempConvC = convert tempC

// do assignments
tempX = tempConvA
tempY = tempConvB
tempZ = tempConvC

The evaluation order for nesting (x, (y, z)) is:

// do LHS side-effects
tempX = &evaluate expressionX
tempY = &evaluate expressionY
tempZ = &evaluate expressionZ

// do Deconstruct
evaluate right and evaluate Deconstruct into two parts (tempA and tempNested)
evaluate Deconstruct on tempNested intwo two parts (tempB and tempC)

// do conversions
tempConvA = convert tempA
tempConvB = convert tempB
tempConvC = convert tempC

// do assignments
tempX = tempConvA
tempY = tempConvB
tempZ = tempConvC

The evaluation order for the simplest cases (locals, fields, array indexers, or anything returning ref) without needing conversion:

evaluate side-effect on the left-hand-side variables
evaluate Deconstruct passing the references directly in

Resolution of the Deconstruct method

The resolution is equivalent to typing rhs.Deconstruct(out var x1, out var x2, ...); with the appropriate number of parameters to deconstruct into. It is based on normal overload resolution. This implies that rhs cannot be dynamic and that none of the parameters of the Deconstruct method can be type arguments. A Deconstruct<T>(out T x1, out T x2) method will not be found. Also, the Deconstruct method must be an instance method or an extension (but not a static method). It also must return void.

Deconstruction-declaration (deconstruction into new variables):

The deconstruction-declaration is also represented with an assignment, but the left-hand-side is either a declaration-expression or a tuple expression containing declaration-expressions. Deconstruction-declarations can be thought of as two steps: (1) declaring new locals, and (2) applying a deconstruction-assignment into those locals. The declaration of new locals by the left-hand-side of the assignment can take multiple forms. The simplest case is (int x, string y), that is a tuple expression containing declaration expressions. Variants include nested declarations like (int x, (string y, long z)) (which declares 3 locals) and implicitly typed declarations like (var x, var y). The latter can also be written using the shorthand var (x, y), which is a declaration expression with parenthesized designations. var is the only case where such shorthand is allowed (so int (x, y) is not legal).

As in the case of deconstruction-declarations, tuple expressions on the right-hand-side have their type inferred from the left-hand-side. With deconstruction-declaration, this is also the case, except that any type is var in the left-hand-side, then the natural type of the element in the right-hand-side is used. For example, in (string x, byte y, var z) = (null, 1, 2);, null has type string, the literal 1 has type byte (inferred from y) and the literal 2 has type int (its natural type).

In C#7.0, deconstruction-declarations are only allowed as top-level statements. They do not allow mixing of declaration expressions and assignable expressions (such as (existing, var declared) = (1, 2)).

Grammar changes

expression
	: ... // existing
	| declaration_expression // new (only allowed in C#7.0 in certain contexts, such as out var, deconstruction and pattern declarations)
	;

declaration_expression // new
	: type variable_designation
	;

variable_designation // new
	: single_variable_designation
	| parenthesized_variable_designation
	| discard_designation
	;

single_variable_designation // new
	: identifier
	;

parenthesized_variable_designation // new
	: '(' variable_designation (',' variable_designation)+ ')'
	;

discard_designation // new
	: '_'
	;

foreach_variable_statement // new
    : 'foreach' '(' declaration_expression 'in' expression ')' embedded_statement
    ;

References

C# Design Notes for Oct 25-26, 2016

C# Design Notes for Sep 6, 2016

C# Design Notes for July 13, 2016

C# Design Notes for May 3-4, 2016

C# Design Notes for Apr 12-22, 2016

Design for declaration expressions

The What's new in C# 7.0 post has a section on deconstructions.

Possible future expansions

See C# Lang repo for more up-to-date proposals and discussions.