In .NET, strings are immutable: When you pass them out to an API, you cant modify them. However, VB.NET applies the VBByRefStr unmanaged type-marshaling attribute to the string. This allows VB.NET to create a temporary buffer, copy that back to a new string, then point the original string to the new string:
Public Declare Function GetWindowText _
Lib "User32.Dll" _
(ByVal hwnd As Int32, _
ByVal lpString As String, _
ByVal cch As Int32) As Int32
To use this declaration, simply initialize the string to the right size:
Dim s As String = Space(256)
Dim rtn as Int32 = GetWindowText(hwnd, s, 256)
The API declaration is equivalent to:
Public Declare Function GetWindowText _
Lib "User32.Dll" (ByVal hwnd As Int32, _
<System.Runtime.InteropServices.MarshalAs( _
Runtime.InteropServices.UnmanagedType.VBByRefStr)> _
ByRef lpString As String, _
ByVal cch As Int32) As Int32
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