Redigeeri

Jagamisviis:

Array Conversions (Visual Basic)

  • Artikkel

You can convert an array type to a different array type provided you meet the following conditions:

  • Equal Rank. The ranks of the two arrays must be the same, that is, they must have the same number of dimensions. However, the lengths of the respective dimensions do not need to be the same.

  • Element Data Type. The data types of the elements of both arrays must be reference types. You cannot convert an Integer array to a Long array, or even to an Object array, because at least one value type is involved. For more information, see Value Types and Reference Types.

  • Convertibility. A conversion, either widening or narrowing, must be possible between the element types of the two arrays. An example that fails this requirement is an attempted conversion between a String array and an array of a class derived from System.Attribute. These two types have nothing in common, and no conversion of any kind exists between them.

A conversion of one array type to another is widening or narrowing depending on whether the conversion of the respective elements is widening or narrowing. For more information, see Widening and Narrowing Conversions.

Conversion to an Object Array

When you declare an Object array without initializing it, its element type is Object as long as it remains uninitialized. When you set it to an array of a specific class, it takes on the type of that class. However, its underlying type is still Object, and you can subsequently set it to another array of an unrelated class. Since all classes derive from Object, you can change the array's element type from any class to any other class.

In the following example, no conversion exists between types student and String, but both derive from Object, so all assignments are valid.

' Assume student has already been defined as a class.  
Dim testArray() As Object  
' testArray is still an Object array at this point.  
Dim names() As String = New String(3) {"Name0", "Name1", "Name2", "Name3"}  
testArray = New student(3) {}  
' testArray is now of type student().  
testArray = names  
' testArray is now a String array.

Underlying Type of an Array

If you originally declare an array with a specific class, its underlying element type is that class. If you subsequently set it to an array of another class, there must be a conversion between the two classes.

In the following example, students is a student array. Since no conversion exists between String and student, the last statement fails.

Dim students() As student  
Dim names() As String = New String(3) {"Name0", "Name1", "Name2", "Name3"}  
students = New Student(3) {}  
' The following statement fails at compile time.  
students = names

See also