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How to: Call Windows APIs (Visual Basic)

This example defines and calls the MessageBox function in user32.dll and then passes a string to it.

' Defines the MessageBox function. 
Public Class Win32
    Declare Auto Function MessageBox Lib "user32.dll" (
        ByVal hWnd As Integer, ByVal txt As String,
        ByVal caption As String, ByVal Type As Integer
        ) As Integer 
End Class 

' Calls the MessageBox function. 
Public Class DemoMessageBox
    Public Shared Sub Main()
        Win32.MessageBox(0, "Here's a MessageBox", "Platform Invoke Sample", 0)
    End Sub 
End Class

This example requires:

  • A reference to the System namespace.

The following conditions may cause an exception:

  • The method is not static, is abstract, or has been previously defined. The parent type is an interface, or the length of name or dllName is zero. (ArgumentException)

  • The name or dllName is Nothing. (ArgumentNullException)

  • The containing type has been previously created using CreateType. (InvalidOperationException)

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