MsgBox 示例 - .NET Framework
此示例演示如何按值传递字符串类型作为 In 参数以及何时使用EntryPoint和CharSetExactSpelling字段。
MsgBox 示例使用以下非托管函数，并显示其原始函数声明：
MessageBox 从 User32.dll导出 。
int MessageBox(HWND hWnd, LPCTSTR lpText, LPCTSTR lpCaption,
UINT uType);
在此示例中，NativeMethods 类为由 MsgBoxSample 类调用的每个非托管函数提供托管原型。 托管原型方法 MsgBox、MsgBox2 和 MsgBox3 对同一个非托管函数有不同的声明。
由于指定为 ANSI 的字符类型与 Unicode 函数名称的入口点 MsgBox2 不匹配，MessageBoxW 的声明会在消息框中生成不正确的输出。 声明MsgBox3导致字段EntryPoint、CharSet和ExactSpelling之间的不匹配。 调用时， MsgBox3 引发异常。 有关字符串命名和名称封送处理的详细信息，请参阅指定字符集。
声明原型
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
调用函数
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
另请参阅
封送字符串
字符串的默认封送处理
在托管代码中创建原型
指定字符集