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7.16 Assignment Operators

The assignment operators assign a new value to a variable, a property, an event, or an indexer element.

assignment:
     unary-expression assignment-operator expression

assignment-operator:
    =
    +=
    -=
    *=
    /=
    %=
    &=
    |=
    ^=
    <<=
    right-shift-assignment

The left operand of an assignment must be an expression classified as a variable, a property access, an indexer access, or an event access.

The = operator is called the simple assignment operator. It assigns the value of the right operand to the variable, property, or indexer element given by the left operand. The left operand of the simple assignment operator may not be an event access (except as described in §10.8.1). The simple assignment operator is described in §7.16.1.

The assignment operators other than the = operator are called compound assignment operators. These operators perform the indicated operation on the two operands, and then assign the resulting value to the variable, property, or indexer element given by the left operand. The compound assignment operators are described in §7.16.2.

The += and -= operators with an event access expression as the left operand are called event assignment operators. No other assignment operator is valid with an event access as the left operand. The event assignment operators are described in §7.16.3.

The assignment operators are right-associative, meaning that operations are grouped from right to left. For example, an expression of the form a = b = c is evaluated as a = (b = c).

7.16.1 Simple Assignment

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.

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 runtime 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 (§6.1).
    • If the variable given by x is an array element of a reference-type, a runtime 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 (§6.1.6) 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 (§6.1).
    • The set accessor of x is invoked with the value computed for y as its value argument.

The array covariance rules (§12.5) 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 runtime 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;                     // Okay
oa[1] = "Hello";                  // Okay
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[].

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. Because of the points raised in §7.5.4, the same rule also applies to fields.

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; }
    }

    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, because r.A and r.B are not variables.

7.16.2 Compound Assignment

An operation of the form x op= y is processed by applying binary operator overload resolution (§7.2.4) as if the operation was written x op y. Then,

  • If the return type of the selected operator is implicitly convertible to the type of x, the operation is evaluated as x = x op y, except that x is evaluated only once.
  • Otherwise, if the selected operator is a predefined operator, if the return type of the selected operator is explicitly convertible to the type of x, and if y is implicitly convertible to the type of x or the operator is a shift operator, then the operation is evaluated as x = (T)(x op y), where T is the type of x, except that x is evaluated only once.
  • Otherwise, the compound assignment is invalid, and a compile-time error occurs.

The term “evaluated only once” means that in the evaluation of x op y, the results of any constituent expressions of x are temporarily saved and then reused when performing the assignment to x. For example, in the assignment A()[B()] += C(), where A is a method returning int[], and B and C are methods returning int, the methods are invoked only once, in the order A, B, C.

When the left operand of a compound assignment is a property access or indexer access, the property or indexer must have both a get accessor and a set accessor. If this is not the case, a compile-time error occurs.

The second rule permits x op= y to be evaluated as x = (T)(x op y) in certain contexts. The rule exists such that the predefined operators can be used as compound operators when the left operand is of type sbyte, byte, short, ushort, or char. Even when both arguments are of one of those types, the predefined operators produce a result of type int, as described in §7.2.6.2. Thus, without a cast, it would not be possible to assign the result to the left operand.

The intuitive effect of the rule for predefined operators is simply that x op= y is permitted if both of x op y and x = y are permitted. In the example

byte b = 0;
char ch = '\0';
int i = 0;

b += 1;               // Okay
b += 1000;            // Error, b = 1000 not permitted
b += i;               // Error, b = i not permitted
b += (byte)i;         // Okay

ch += 1;             // Error, ch = 1 not permitted
ch += (char)1;       // Okay

the intuitive reason for each error is that a corresponding simple assignment would also have been an error.

This also means that compound assignment operations support lifted operations. In the example

int? i = 0;
i += 1;     // Okay

the lifted operator +(int?,int?) is used.

7.16.3 Event Assignment

If the left operand of a += or -= operator is classified as an event access, then the expression is evaluated as follows:

  • The instance expression, if any, of the event access is evaluated.
  • The right operand of the += or -= operator is evaluated and, if required, converted to the type of the left operand through an implicit conversion (§6.1).
  • An event accessor of the event is invoked, with an argument list consisting of the right operand, after evaluation and, if necessary, conversion. If the operator was +=, the add accessor is invoked; if the operator was -=, the remove accessor is invoked.

An event assignment expression does not yield a value. Thus an event assignment expression is valid only in the context of a statement-expression (§8.6).

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