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Specifying an Entry Point

An entry point identifies the location of a function in a DLL. Within a managed project, the original name or ordinal entry point of a target function identifies that function across the interoperation boundary. Further, you can map the entry point to a different name, effectively renaming the function.

Following is a list of possible reasons to rename a DLL function:

  • To avoid using case-sensitive API function names

  • To comply with existing naming standards

  • To accommodate functions that take different data types (by declaring multiple versions of the same DLL function)

  • To simplify using APIs that contain ANSI and Unicode versions

This topic demonstrates how to rename a DLL function in managed code.

Visual Basic uses the Function keyword in the Declare statement to set the DllImportAttribute.EntryPoint field. The following example shows a basic declaration.

Imports System.Runtime.InteropServices

Public Class Win32
    Declare Auto Function MessageBox Lib "user32.dll" _
       (ByVal hWnd As Integer, ByVal txt As String,_
       ByVal caption As String, ByVal Typ As Integer) As Integer
End Class

You can replace the MessageBox entry point with MsgBox by including the Alias keyword in your definition, as shown in the following example. In both examples the Auto keyword eliminates the need to specify the character-set version of the entry point. For more information about selecting a character set, see Specifying a Character Set.

Imports System.Runtime.InteropServices

Public Class Win32
    Declare Auto Function MsgBox Lib "user32.dll" _
       Alias "MessageBox" (ByVal hWnd As Integer, ByVal txt As String,_
       ByVal caption As String, ByVal Typ As Integer) As Integer
End Class

You can use the DllImportAttribute.EntryPoint field to specify a DLL function by name or ordinal. If the name of the function in your method definition is the same as the entry point in the DLL, you do not have to explicitly identify the function with the EntryPoint field. Otherwise, use one of the following attribute forms to indicate a name or ordinal:

[DllImport("dllname", EntryPoint="Functionname")]
[DllImport("dllname", EntryPoint="#123")]

Notice that you must prefix an ordinal with the pound sign (#).

The following example demonstrates how to replace MessageBoxA with MsgBox in your code by using the EntryPoint field.

using System.Runtime.InteropServices;

public class Win32 {
    [DllImport("user32.dll", EntryPoint="MessageBoxA")]
    public static extern int MsgBox(int hWnd, String text, String caption,
                                    uint type);
}
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