如何P /調用一個可能不存在的函數？

Question

有在本機代碼以下，需要在託管代碼編寫：

HINSTANCE hUser = LoadLibrary("user32.dll"); /* Can't fail -- it's already loaded */ 
BOOL (*dpi)() = (BOOL (*)())GetProcAddress(hUser, "SetProcessDPIAware"); 
if(dpi) dpi(); 

功能SetProcessDPIAware沒有最低端的平臺，我們支持上存在的，所以我們會碰到裝載機問題簡單地聲明函數並試圖稱它。

但是，我必須根據除操作系統版本以外的條件調用SetProcessDPIAware來做出運行時決定，所以我不能使用清單。

Answer 1

你可以P /調用LoadLibrary和GetProcAddress以類似的方式：

using System; 
using System.Runtime.InteropServices; 

static class Program 
{ 
    [DllImport("kernel32", SetLastError = true)] 
    static extern IntPtr LoadLibrary(string lpFileName); 

    [DllImport("kernel32", CharSet = CharSet.Ansi, ExactSpelling = true, SetLastError = true)] 
    static extern IntPtr GetProcAddress(IntPtr hModule, string procName); 

    delegate bool MyDelegate(); 

    static void Main() 
    { 
     var hUser = LoadLibrary("user32.dll"); 
     var res = GetProcAddress(hUser, "SetProcessDPIAware"); 
     if (res != IntPtr.Zero) 
     { 
      // The function we are looking for exists => 
      // we can now get the corresponding delegate and invoke it 
      var del = (MyDelegate)Marshal.GetDelegateForFunctionPointer(res, typeof(MyDelegate)); 

      bool x = del(); 
      Console.WriteLine(x); 
     } 
    } 
}