27.2 API函数的类型

Posted 2022-07-13 VB.Net

API函数最开始是为了方便C语言开发者使用，所以现在MSDN也只有C语言的原型和对应说明。

以下转换是我的经验，大家需要根据实际情况设置对应的类型。

C类型	Vb6类型	Vb.Net类型
BOOL	Boolean	Boolean
Char	String	Char
BYTE	Byte	Byte
WORD	Integer	Short，Int16
DWORD	Long	Integer，Int32
LONG	Long	Integer，Int32
UINT	Long	Integer，Int32
HANDLE	Integer	IntPtr（注1）
HDC	Integer	IntPtr
P开始的类型（注2）	Integer	IntPtr
LP开始的类型（注2）	Long	IntPtr
LPTSTR	String	String
结构	类型（Type）	结构，IntPtr，Byte()

注1：根据操作系统位数，32位操作系统下IntPtr可视为Int32，64位操作系统下IntPtr可视为Int64。

注2：C语言中P和LP开头的类型通常都是指针。

上面的列表只是类型变化的一般情况，特殊情况特殊分析。

以GetSystemDirectory这个Api函数为例：

GetSystemDirectory：获得系统目录的路径，系统目录包含了系统文件（比如动态链接库文件和驱动等），通常是C:\\Windows\\System或者C:\\Windows\\System32。

C原型（参看MSDN）：

UINT WINAPI GetSystemDirectory( _Out_ LPTSTR lpBuffer, _In_  UINT   uSize );

Vb6中的声明：

Declare Function GetSystemDirectory Lib "kernel32" Alias "GetSystemDirectoryA" (ByVal lpBuffer As String, ByVal nSize As Long) As Long

VB.Net中通常的声明：

Declare Function GetSystemDirectory Lib "kernel32" Alias "GetSystemDirectoryA" (ByVal lpBuffer As String, ByVal nSize As Integer) As Integer

其实可以有更多的声明方式，在【例 27A.2】中将用4中不同的声明来使用GetSystemDirectory这个Api函数。

【例 27A.2】【项目：code27A-002】获得系统目录路径。

    Const Max_Path As Integer = 260

    Public Declare Function GetSystemDirectory Lib "kernel32" Alias "GetSystemDirectoryA" (ByVal lpBuffer As String, ByVal nSize As Integer) As Integer

    Public Declare Function GetSystemDirectory Lib "kernel32" Alias "GetSystemDirectoryA" (ByVal lpBuffer As IntPtr, ByVal nSize As Integer) As Integer

    Public Declare Function GetSystemDirectory Lib "kernel32" Alias "GetSystemDirectoryA" (ByVal lpBuffer() As Byte, ByVal nSize As Integer) As Integer

    Public Declare Function GetSystemDirectory Lib "kernel32" Alias "GetSystemDirectoryA" (ByVal lpBuffer As System.Text.StringBuilder, ByVal nSize As Integer) As Integer

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim systemPath As String = New String(" "c, 260)
        Dim returnValue As Integer
        returnValue = GetSystemDirectory(systemPath, Max_Path)
        TextBox1.Text &= vbCrLf & "String:" & vbCrLf & systemPath.Substring(0, returnValue)
    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        Dim pBuffer As IntPtr = System.Runtime.InteropServices.Marshal.AllocHGlobal(Max_Path)
        Dim returnValue As Integer
        returnValue = GetSystemDirectory(pBuffer, Max_Path)
        Dim systemPath As String
        systemPath = System.Runtime.InteropServices.Marshal.PtrToStringAnsi(pBuffer)
        TextBox1.Text &= vbCrLf & "IntPtr:" & vbCrLf & systemPath
    End Sub

    Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
        Dim btePath(Max_Path - 1) As Byte
        Dim returnValue As Integer
        returnValue = GetSystemDirectory(btePath, Max_Path)
        Dim systemPath As String = System.Text.Encoding.ASCII.GetString(btePath)
        TextBox1.Text &= vbCrLf & "Byte():" & vbCrLf & systemPath.Substring(0, returnValue)
    End Sub

    Private Sub Button4_Click(sender As Object, e As EventArgs) Handles Button4.Click
        Dim systemPath As New System.Text.StringBuilder(Max_Path)
        Dim returnValue As Integer
        returnValue = GetSystemDirectory(systemPath, Max_Path)
        TextBox1.Text &= vbCrLf & "StringBuilder:" & vbCrLf & systemPath.ToString
    End Sub

运行结果如下图所示：

 图27A-2 多种Api声明获取系统目录路径

 

由于.net平台下C#和vb.NET很相似，本文也可以为C#爱好者提供参考。

学习更多vb.net知识，请参看vb.net 教程 目录