VB.NET在可移植类库中使用dllimport

Question

I wanted to change my usercontrol to a portable class library.

However, in this line of my code...

<DllImport("User32.dll", CharSet:=CharSet.Auto, CallingConvention:=CallingConvention.StdCall)> _
Public Function UnhookWindowsHookEx(ByVal idHook As Integer) As Boolean
End Function

... I get the error "Type ''DllImport'' is not defined''.

What am I doing wrong here and how could I fix that? Thank you!

Answer 1

In order for a library to be classified as portable it needs to adhere to a set of restrictions that include not supporting DllImport . Across the various operating systems that can use a portable class library, directly using native operating system calls isn't supported.

In this case, you would need to break the component into two or more pieces. The control that needs the call to UnhookWindowsHookEx would need to use a second platform specific component which implements the functionality, abstracting it from the UserControl. This could be done for example by using a platform specific class that implements an interface: INativeAccess . At run time, it would use the platform specific class if available. There is more on MSDN about this.

Answer 2

Just like any type you want to use in code, you must have referenced the assembly that the DllImportAttribute class is defined in in order to use it. You must also have imported the namespace that it's a member of to use it unqualified. Have you done both of those things?

Even if you have, the whole point of a portable class library is that it's portable. Is that API going to be supported on all the platforms you might port it to?