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Ejemplo MsgBox

  • Artículo

En este ejemplo se muestra cómo pasar tipos de cadena por valor como parámetros In y cuándo utilizar los campos EntryPoint, CharSet y ExactSpelling.

En el ejemplo MsgBox se utiliza la siguiente función no administrada, que se muestra con su declaración de función original:

  • MessageBox exportada desde User32.dll.

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

En este ejemplo, la clase NativeMethods contiene un prototipo administrado para cada función no administrada a la que se llama desde la clase MsgBoxSample. Los métodos de prototipo administrados MsgBox, MsgBox2 y MsgBox3 tienen declaraciones diferentes para la misma función no administrada.

La declaración para MsgBox2 produce una salida incorrecta en el cuadro de mensaje porque el tipo de caracteres, especificado como ANSI, no corresponde con el punto de entrada MessageBoxW, que es el nombre de la función Unicode. La declaración de MsgBox3 genera una falta de coincidencia entre los campos EntryPoint, CharSet y ExactSpelling. Al llamar a MsgBox3, se produce una excepción. Para obtener información detallada sobre los nombres de cadena y la serialización de nombres, consulte Especificación de un juego de caracteres.

Declaración de prototipos

private ref class NativeMethods
{
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);
};

internal static class NativeMethods
{
    // Declares managed prototypes for unmanaged functions.
    [DllImport("User32.dll", EntryPoint = "MessageBox",
        CharSet = CharSet.Auto)]
    internal static extern int MsgBox(
        IntPtr hWnd, string lpText, string lpCaption, uint uType);

    // Causes incorrect output in the message window.
    [DllImport("User32.dll", EntryPoint = "MessageBoxW",
        CharSet = CharSet.Ansi)]
    internal static extern int MsgBox2(
        IntPtr hWnd, string lpText, string lpCaption, uint uType);

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

Friend Class NativeMethods
    ' Declares managed prototypes for unmanaged functions.
    Friend Declare Auto Function MsgBox Lib "User32.dll" Alias "MessageBox" (
        ByVal hWnd As IntPtr, ByVal lpText As String, ByVal lpCaption As String,
        ByVal uType As UInteger) As Integer

    ' Causes incorrect output in the message window.
    Friend Declare Ansi Function MsgBox2 Lib "User32.dll" Alias "MessageBoxW" (
        ByVal hWnd As IntPtr, ByVal lpText As String, ByVal lpCaption As String,
        ByVal uType As UInteger) As Integer

    ' Causes an exception to be thrown.
    ' ExactSpelling is True by default when Ansi or Unicode is used.
    Friend Declare Ansi Function MsgBox3 Lib "User32.dll" Alias "MessageBox" (
        ByVal hWnd As IntPtr, ByVal lpText As String, ByVal lpCaption As String,
        ByVal uType As UInteger) As Integer
End Class

Llamadas a funciones

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

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

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

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

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

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

Vea también