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26 строки
2.3 KiB
XML
26 строки
2.3 KiB
XML
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<?xml version="1.0"?>
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<clause number="19.5" title="Array covariance">
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<paragraph>For any two <non_terminal where="11.2">reference-type</non_terminal>s A and B, if an implicit reference conversion (<hyperlink>13.1.4</hyperlink>) or explicit reference conversion (<hyperlink>13.2.3</hyperlink>) exists from A to B, then the same reference conversion also exists from the array type A[R] to the array type B[R], where R is any given <non_terminal where="19.1">rank-specifier</non_terminal> (but the same for both array types). This relationship is known as array covariance. Array covariance, in particular, means that a value of an array type A[R] may actually be a reference to an instance of an array type B[R], provided an implicit reference conversion exists from B to A. </paragraph>
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<paragraph>Because of array covariance, assignments to elements of reference type arrays include a run-time check which ensures that the value being assigned to the array element is actually of a permitted type (<hyperlink>14.13.1</hyperlink>). </paragraph>
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<paragraph>
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<example>[Example: For example: <code_example><![CDATA[
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class Test
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{
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static void Fill(object[] array, int index, int count, object value) {
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for (int i = index; i < index + count; i++) array[i] = value;
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}
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static void Main() {
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string[] strings = new string[100];
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Fill(strings, 0, 100, "Undefined");
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Fill(strings, 0, 10, null);
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Fill(strings, 90, 10, 0);
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}
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}
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]]></code_example></example>
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</paragraph>
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<paragraph>
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<example>The assignment to array[i] in the Fill method implicitly includes a run-time check, which ensures that the object referenced by value is either null or an instance of a type that is compatible with the actual element type of array. In Main, the first two invocations of Fill succeed, but the third invocation causes a System.ArrayTypeMismatchException to be thrown upon executing the first assignment to array[i]. The exception occurs because a boxed <keyword>int</keyword> cannot be stored in a string array. end example]</example>
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</paragraph>
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<paragraph>Array covariance specifically does not extend to arrays of <non_terminal where="11.1">value-type</non_terminal>s. For example, no conversion exists that permits an int[] to be treated as an object[]. </paragraph>
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</clause>
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