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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Hyperlinked ECMA C# Language Specification</title><meta name="author" content="Jon Jagger" /><link rel="stylesheet" href="ecma334.css"></link></head><body><div align="right"><em><a href="http://www.jaggersoft.com">Jon Jagger</a></em></div><div align="right"><a href="mailto:jon@jaggersoft.com">jon@jaggersoft.com</a></div><form method="get" action="http://search.atomz.com/search/"><input size="30" name="sp-q"></input><input type="submit" value="Search C# Spec"></input><input type="hidden" name="sp-a" value="sp10024177"></input><input type="hidden" name="sp-f" value="ISO-8859-1"></input></form><a href="toc.htm">Table of Contents</a> <a href="1.htm">1</a> <a href="2.htm">2</a> <a href="3.htm">3</a> <a href="4.htm">4</a> <a href="5.htm">5</a> <a href="6.htm">6</a> <a href="7.htm">7</a> <a href="8.htm">8</a> <a href="9.htm">9</a> <a href="10.htm">10</a> <a href="11.htm">11</a> <a href="12.htm">12</a> <a href="13.htm">13</a> <a href="14.htm">14</a> <a href="15.htm">15</a> <a href="16.htm">16</a> <a href="17.htm">17</a> <a href="18.htm">18</a> <a href="19.htm">19</a> <a href="20.htm">20</a> <a href="21.htm">21</a> <a href="22.htm">22</a> <a href="23.htm">23</a> <a href="24.htm">24</a> <a href="25.htm">25</a> <a href="notes.htm">Notes</a> <a href="HyperlinkedCSharpECMA.zip">Download</a><span class="ruler"></span><span class="heading">ECMA-334 C# Language Specification</span><span class="navigate"><a href="14.13.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="14.13.2.htm"><img src="next.gif" alt="next" border="0" /></a></span><span class="clause-depth"><a href="13.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="15.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="14.htm">14</a></span><span class="clause-title-previous"> Expressions</span></span><span class="clause-depth"><a href="14.12.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="14.14.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="14.13.htm">14.13</a></span><span class="clause-title-previous"> Assignment operators</span></span><span class="clause-depth"><a href="14.13.1.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="14.13.2.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">14.13.1</span><span class="clause-title"> Simple assignment</span></span><span class="locator">
Paragraph 1</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P1S1"></a>The = operator is called the simple assignment operator.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P1S2"></a>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.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P1S3"></a>The operation assigns the value of the right operand to the variable, property, or indexer element given by the left operand.</span> </span><span class="locator">
Paragraph 2</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P2S1"></a>The result of a simple assignment expression is the value assigned to the left operand.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P2S2"></a>The result has the same type as the left operand and is always classified as a value.</span> </span><span class="locator">
Paragraph 3</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P3S1"></a>If the left operand is a property or indexer access, the property or indexer must have a set accessor.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P3S2"></a>If this is not the case, a compile-time error occurs.</span> </span><span class="locator">
Paragraph 4</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P4S1"></a>The run-time processing of a simple assignment of the form x = y consists of the following steps: </span><ul><li><span class="sentence"><span class="sentence-number">2</span> <a name="P4S2"></a> If x is classified as a variable: </span></li><ul><li><span class="sentence"><span class="sentence-number">3</span> <a name="P4S3"></a> x is evaluated to produce the variable.</span> </li><li><span class="sentence"><span class="sentence-number">4</span> <a name="P4S4"></a> y is evaluated and, if required, converted to the type of x through an implicit conversion (<a href="13.1.htm">§13.1</a>).</span> </li><li><span class="sentence"><span class="sentence-number">5</span> <a name="P4S5"></a> If the variable given by x is an array element of a <span class="non-terminal"><a href="11.2.htm#reference-type">reference-type</a></span>, 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.</span> <span class="sentence"><span class="sentence-number">6</span> <a name="P4S6"></a>The check succeeds if y is null, or if an implicit reference conversion (<a href="13.1.4.htm">§13.1.4</a>) exists from the actual type of the instance referenced by y to the actual element type of the array instance containing x.</span> <span class="sentence"><span class="sentence-number">7</span> <a name="P4S7"></a>Otherwise, a System.ArrayTypeMismatchException is thrown.</span> </li><li><span class="sentence"><span class="sentence-number">8</span> <a name="P4S8"></a> The value resulting from the evaluation and conversion of y is stored into the location given by the evaluation of x.</span> </li></ul><li><span class="sentence"><span class="sentence-number">9</span> <a name="P4S9"></a> If x is classified as a property or indexer access: </span></li><ul><li><span class="sentence"><span class="sentence-number">10</span> <a name="P4S10"></a> 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.</span> </li><li><span class="sentence"><span class="sentence-number">11</span> <a name="P4S11"></a> y is evaluated and, if required, converted to the type of x through an implicit conversion (<a href="13.1.htm">§13.1</a>).</span> </li><li><span class="sentence"><span class="sentence-number">12</span> <a name="P4S12"></a> The set accessor of x is invoked with the value computed for y as its value argument.</span> </li></ul></ul></span><span class="paragraph"><span class="note">[Note: The array covariance rules (<a href="19.5.htm">§19.5</a>) 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 <span class="non-terminal"><a href="11.2.htm#reference-type">reference-type</a></span> requires a run-time check to ensure that the value being assigned is compatible with the array instance. In the example <pre class="code-example">
string[] sa = new string[10];
object[] oa = sa;
oa[0] = null; // Ok
oa[1] = "Hello"; // Ok
oa[2] = new ArrayList(); // ArrayTypeMismatchException
</pre>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]</span> </span><span class="locator">
Paragraph 5</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P5S1"></a>When a property or indexer declared in a <span class="non-terminal"><a href="11.1.htm#struct-type">struct-type</a></span> is the target of an assignment, the instance expression associated with the property or indexer access must be classified as a variable.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P5S2"></a>If the instance expression is classified as a value, a compile-time error occurs.</span> <span class="note">[Note: Because of <a href="14.5.4.htm">§14.5.4</a>, the same rule also applies to fields. end note]</span> </span><span class="paragraph"><span class="example">[Example: Given the declarations: <pre class="code-example">
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; }
}
}
</pre>in the example <pre class="code-example">
Point p = new Point();
p.X = 100;
p.Y = 100;
Rectangle r = new Rectangle();
r.A = new Point(10, 10);
r.B = p;
</pre>the assignments to p.X, p.Y, r.A, and r.B are permitted because p and r are variables. However, in the example <pre class="code-example">
Rectangle r = new Rectangle();
r.A.X = 10;
r.A.Y = 10;
r.B.X = 100;
r.B.Y = 100;
</pre>the assignments are all invalid, since r.A and r.B are not variables. end example]</span> </span><span class="ruler"></span><table><tr><td><table align="left" bgcolor="navy"><tr bgcolor="navy"><td><font face="Arial,sans-serif" size="6" color="yellow"><strong>{ JSL }</strong></font></td></tr></table></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>Jagger Software Ltd</strong></font></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>Company # 4070126</strong></font></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>VAT # 762 5213 42</strong></font></td></tr></table><img src="valid-html401.png" align="left" height="31" width="88" alt="Valid HTML 4.01" /><img src="vcss.gif" align="left" height="31" width="88" alt="Valid CSS" /></body></html>⌨️ 快捷键说明
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