MsgBox Sample

This sample demonstrates how to pass string types by value as In parameters and when to use the EntryPoint, CharSet, and ExactSpelling fields.

The MsgBox sample uses the following unmanaged function, shown with its original function declaration:

• MessageBox exported from User32.dll.

  int MessageBox(HWND hWnd, LPCTSTR lpText, LPCTSTR lpCaption, 
     UINT uType);
  

In this sample, the LibWrap class contains a managed prototype for each unmanaged function called by the MsgBoxSample class. The managed prototype methods MsgBox, MsgBox2, and MsgBox3 have different declarations for the same unmanaged function.

The declaration for MsgBox2 produces incorrect output in the message box because the character type, specified as ANSI, is mismatched with the entry point MessageBoxW, which is the name of the Unicode function. The declaration for MsgBox3 creates a mismatch between the EntryPoint, CharSet, and ExactSpelling fields. When called, MsgBox3 throws an exception. For detailed information on string naming and name marshaling, see Specifying a Character Set.

Declaring Prototypes

Public Class LibWrap
   ' Declares managed prototypes for unmanaged functions.
   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

   ' Causes incorrect output in the message window.
   Declare Ansi Function MsgBox2 Lib "User32.dll" Alias "MessageBoxW" ( _
       ByVal hWnd As Integer, ByVal txt As String, ByVal caption As String, _
       ByVal type As Integer ) As Integer

   ' Causes an exception to be thrown.
   ' ExactSpelling is True by default in when
   ' Ansi or Unicode is used.
   Declare Ansi Function MsgBox3 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

public class LibWrap
{
    // Declares managed prototypes for unmanaged functions.
    [DllImport( "User32.dll", EntryPoint="MessageBox",
        CharSet=CharSet.Auto)]
    public static extern int MsgBox(int hWnd, string text, string caption,
      uint type);

    // Causes incorrect output in the message window.
    [DllImport( "User32.dll", EntryPoint="MessageBoxW",
        CharSet=CharSet.Ansi )]
    public static extern int MsgBox2(int hWnd, string text,
       string caption, uint type);

    // Causes an exception to be thrown. EntryPoint, CharSet, and
    // ExactSpelling fields are mismatched.
    [DllImport( "User32.dll", EntryPoint="MessageBox",
        CharSet=CharSet.Ansi, ExactSpelling=true )]
    public static extern int MsgBox3(int hWnd, string text,
        string caption, uint type);
}

public ref class LibWrap
{
public:
    // Declares managed prototypes for unmanaged functions.
    [DllImport( "User32.dll", EntryPoint="MessageBox",
        CharSet=CharSet::Auto)]
    static int MsgBox(int hWnd, String^ text, String^ caption,
      unsigned int type);

    // Causes incorrect output in the message window.
    [DllImport( "User32.dll", EntryPoint="MessageBoxW",
        CharSet=CharSet::Ansi )]
    static int MsgBox2(int hWnd, String^ text,
       String^ caption, unsigned int type);

    // Causes an exception to be thrown. EntryPoint, CharSet, and
    // ExactSpelling fields are mismatched.
    [DllImport( "User32.dll", EntryPoint="MessageBox",
        CharSet=CharSet::Ansi, ExactSpelling=true )]
    static int MsgBox3(int hWnd, String^ text,
        String^ caption, unsigned int type);
};

Calling Functions

Public Class MsgBoxSample
    Public Shared Sub Main()
        LibWrap.MsgBox(0, "Correct text", "MsgBox Sample", 0)
        LibWrap.MsgBox2(0, "Incorrect text", "MsgBox Sample", 0)

        Try
            LibWrap.MsgBox3(0, "No such function", "MsgBox Sample", 0)
        Catch e As EntryPointNotFoundException
            Console.WriteLine("EntryPointNotFoundException thrown as expected!")
        End Try
    End Sub
End Class

public class MsgBoxSample
{
    public static void Main()
    {
        LibWrap.MsgBox(0, "Correct text", "MsgBox Sample", 0);
        LibWrap.MsgBox2(0, "Incorrect text", "MsgBox Sample", 0);

        try
        {
            LibWrap.MsgBox3(0, "No such function", "MsgBox Sample", 0);
        }
        catch(EntryPointNotFoundException)
        {
           Console.WriteLine("EntryPointNotFoundException thrown as expected!");
        }
    }
}

public class MsgBoxSample
{
public:
    static void Main()
    {
        LibWrap::MsgBox(0, "Correct text", "MsgBox Sample", 0);
        LibWrap::MsgBox2(0, "Incorrect text", "MsgBox Sample", 0);

        try
        {
            LibWrap::MsgBox3(0, "No such function", "MsgBox Sample", 0);
        }
        catch (EntryPointNotFoundException^)
        {
           Console::WriteLine("EntryPointNotFoundException thrown as expected!");
        }
    }
};

See Also

Concepts

Marshaling Strings

Platform Invoke Data Types

Default Marshaling for Strings

Creating Prototypes in Managed Code

Other Resources

Specifying a Character Set