Imports

 <  Day Day Up  >  

Namespaces are very useful for organizing declarations, but as a practical matter, typing fully qualified names can get very tiresome. Having to type System in front of Console.WriteLine may be useful the first few times to understand where the type Console lives, but by the fiftieth or hundredth time (or thousandth time), you've probably gotten the point. To help this situation, a namespace or a type can be imported into a particular source file. Importing a namespace or type makes the members of that declaration available in that source file without qualification. For example:

 Imports System Module Test   Sub Main()     Console.WriteLine("Hello, world!")   End Sub End Module 

The example imports the System namespace into the source file, so Console can be used without qualification.

NOTE

The Visual Basic .NET compiler also allows imports to be defined at the project level, which are equivalent to Imports statements placed at the top of each source file in the project. For example, by default the System namespace is imported in all projects. This is why the examples in this book do not explicitly include an Imports System statement at the top of each example.


Advanced

The names in Imports statements are handled differently from other names ”the names in an Imports statement must always be fully qualified names.


In general, only namespaces are really useful to import, but importing a class can be useful in the case of an enumeration.

 Imports Colors Enum Colors   Red   Green   Blue End Enum Module Test   Sub Main()     Dim c As Colors = Red     If c = Blue Then       ...     End If   End Sub End Module 

If a name is imported through two separate imports, attempting to use the name will result in an ambiguity error. In the following example, the name Test is ambiguous between namespace A and namespace B .

 Imports A Imports B Namespace A   Class Test   End Class End Namespace Namespace B   Class Test   End Class End Namespace Module TestModule   Sub Main()     Dim x As Test ' Error: Test is imported through A and B   End Sub End Module 

To help resolve such ambiguities , an import can be assigned an alias . An alias is an alternate name given to the imported namespace or type and allows access to the members of the imported namespace or type through that alternate name. Using the previous example, we can resolve the ambiguity by assigning A.Text and B.Text aliases.

 Imports ATest = A.Test Imports BTest = B.Test Namespace A   Class Test   End Class End Namespace Namespace B   Class Test   End Class End Namespace Module TestModule   Sub Main()     Dim x As Test  ' Error: Test is only imported through an alias     Dim y As ATest     Dim z As BTest   End Sub End Module 

An import alias doesn't actually declare anything in the file, so it is possible to declare something else in the file with the same name as the import alias. However, the name of the alias must be unique in the top-level scope of the source file.

 <  Day Day Up  >  


The Visual Basic .NET Programming Language
The Visual Basic .NET Programming Language
ISBN: 0321169514
EAN: 2147483647
Year: 2004
Pages: 173
Authors: Paul Vick

Similar book on Amazon

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net