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Expertise: Intermediate
Language: .NET

July 3, 2002

Return Strings From an API

In .NET, strings are immutable: When you pass them out to an API, you can’t 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

VB.NET, however, doesn’t allow you to specify that marshaling attribute on parameters, so you must use the first declaration. Use a StringBuilder object as an alternative to using the VBByRefStr attribute.