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Specifying a Character Set

The DllImportAttribute.CharSet field controls string marshaling and determines how platform invoke finds function names in a DLL. This topic describes both behaviors.

Some APIs export two versions of functions that take string arguments: narrow (ANSI) and wide (Unicode). The Win32 API, for instance, includes the following entry-point names for the MessageBox function:

  • MessageBoxA

    Provides 1-byte character ANSI formatting, distinguished by an "A" appended to the entry-point name. Calls to MessageBoxA always marshal strings in ANSI format, as is common on Windows 95 and Windows 98 platforms.

  • MessageBoxW

    Provides 2-byte character Unicode formatting, distinguished by a "W" appended to the entry-point name. Calls to MessageBoxW always marshal strings in Unicode format, as is common on Windows NT, Windows 2000, and Windows XP platforms.

The CharSet field accepts the following values:

CharSet.Ansi (default value)

  • String marshaling

    Platform invoke marshals strings from their managed format (Unicode) to ANSI format.

  • Name matching

    When the DllImportAttribute.ExactSpelling field is true, as it is by default in Visual Basic 2005, platform invoke searches only for the name you specify. For example, if you specify MessageBox, platform invoke searches for MessageBox and fails when it cannot locate the exact spelling.

    When the ExactSpelling field is false, as it is by default in C++ and C#, platform invoke searches for the unmangled alias first (MessageBox), then the mangled name (MessageBoxA) if the unmangled alias is not found. Notice that ANSI name-matching behavior differs from Unicode name-matching behavior.

CharSet.Unicode

  • String marshaling

    Platform invoke copies strings from their managed format (Unicode) to Unicode format.

  • Name matching

    When the ExactSpelling field is true, as it is by default in Visual Basic 2005, platform invoke searches only for the name you specify. For example, if you specify MessageBox, platform invoke searches for MessageBox and fails if it cannot locate the exact spelling.

    When the ExactSpelling field is false, as it is by default in C++ and C#, platform invoke searches for the mangled name first (MessageBoxW), then the unmangled alias (MessageBox) if the mangled name is not found. Notice that Unicode name-matching behavior differs from ANSI name-matching behavior.

CharSet.Auto

  • Platform invoke chooses between ANSI and Unicode formats at run time, based on the target platform.

The following example declares the MessageBox function three times, each time with different character-set behavior. You can specify character-set behavior in Visual Basic by adding the Ansi, Unicode, or Auto keyword to the declaration statement.

If you omit the character-set keyword, as is done in the first declaration statement, the DllImportAttribute.CharSet field defaults to the ANSI character set. The second and third statements in the example explicitly specify a character set with a keyword.

Imports System.Runtime.InteropServices

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

   Declare Unicode Function MessageBoxW Lib "user32.dll" _
       (ByVal hWnd As Integer, ByVal txt As String, _
        ByVal caption As String, ByVal Typ As Integer) As Integer

   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

The DllImportAttribute.CharSet field identifies the underlying character set as ANSI or Unicode. The character set controls how string arguments to a method should be marshaled. Use one of the following forms to indicate the character set:

[DllImport("dllname", CharSet=CharSet.Ansi)]
[DllImport("dllname", CharSet=CharSet.Unicode)]
[DllImport("dllname", CharSet=CharSet.Auto)]

The following example shows three managed definitions of the MessageBox function attributed to specify a character set. In the first definition, by its omission, the CharSet field defaults to the ANSI character set.

[DllImport("user32.dll")]
    public static extern int MessageBoxA(int hWnd, String text, 
        String caption, uint type);
[DllImport("user32.dll", CharSet=CharSet.Unicode)]
    public static extern int MessageBoxW(int hWnd, String text, 
        String caption, uint type);
[DllImport("user32.dll", CharSet=CharSet.Auto)]
    public static extern int MessageBox(int hWnd, String text, 
        String caption, uint type);
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