14.13.1 Simple assignment

王朝other·作者佚名  2006-05-04
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The = operator is called the simple assignment operator. In a simple

assignment, the right operand must be

an expression of a type that is implicitly convertible to the type of the

left operand. The operation assigns the

value of the right operand to the variable, property, or indexer element

given by the left operand.

The result of a simple assignment expression is the value assigned to the

left operand. The result has the

same type as the left operand and is always classified as a value.

Chapter 14 Expressions

173

If the left operand is a property or indexer access, the property or

indexer must have a set accessor. If this is

not the case, a compile-time error occurs.

The run-time processing of a simple assignment of the form x = y consists

of the following steps:

?If x is classified as a variable:

x is evaluated to produce the variable.

y is evaluated and, if required, converted to the type of x through an

implicit conversion (?3.1).

If the variable given by x is an array element of a reference-type, a

run-time check is performed to ensure

that the value computed for y is compatible with the array instance of

which x is an element. The check

succeeds if y is null, or if an implicit reference conversion (?3.1.4)

exists from the actual type of the

instance referenced by y to the actual element type of the array instance

containing x. Otherwise, a

System.ArrayTypeMismatchException is thrown.

The value resulting from the evaluation and conversion of y is stored into

the location given by the

evaluation of x.

?If x is classified as a property or indexer access:

The instance expression (if x is not static) and the argument list (if x is

an indexer access) associated with

x are evaluated, and the results are used in the subsequent set accessor

invocation.

y is evaluated and, if required, converted to the type of x through an

implicit conversion (?3.1).

The set accessor of x is invoked with the value computed for y as its value

argument.

[Note: The array covariance rules (?9.5) permit a value of an array type

A[] to be a reference to an

instance of an array type B[], provided an implicit reference conversion

exists from B to A. Because of

these rules, assignment to an array element of a reference-type requires a

run-time check to ensure that the

value being assigned is compatible with the array instance. In the example

string[] sa = new string[10];

object[] oa = sa;

oa[0] = null; // Ok

oa[1] = "Hello"; // Ok

oa[2] = new ArrayList(); // ArrayTypeMismatchException

the last assignment causes a System.ArrayTypeMismatchException to be thrown

because an instance

of ArrayList cannot be stored in an element of a string[]. end note]

When a property or indexer declared in a struct-type is the target of an

assignment, the instance expression

associated with the property or indexer access must be classified as a

variable. If the instance expression is

classified as a value, a compile-time error occurs. [Note: Because of ?4.5.4

, the same rule also applies to

fields. end note]

[Example: Given the declarations:

struct Point

{

int x, y;

public Point(int x, int y) {

this.x = x;

this.y = y;

}

public int X {

get { return x; }

set { x = value; }

}

C# LANGUAGE SPECIFICATION

174

public int Y {

get { return y; }

set { y = value; }

}

}

struct Rectangle

{

Point a, b;

public Rectangle(Point a, Point b) {

this.a = a;

this.b = b;

}

public Point A {

get { return a; }

set { a = value; }

}

public Point B {

get { return b; }

set { b = value; }

}

}

in the example

Point p = new Point();

p.X = 100;

p.Y = 100;

Rectangle r = new Rectangle();

r.A = new Point(10, 10);

r.B = p;

the assignments to p.X, p.Y, r.A, and r.B are permitted because p and r are

variables. However, in the

example

Rectangle r = new Rectangle();

r.A.X = 10;

r.A.Y = 10;

r.B.X = 100;

r.B.Y = 100;

the assignments are all invalid, since r.A and r.B are not variables. end

example]

 
 
 
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