在Windows平台，可以利用很多系统API，而在.net平台下用C#调用系统API是比较方便而且也是比较轻松的事情。
我们可以利用C#调用API实现底层驱动程序驱动某一个部件（如串口通讯）和或者外设进行工作，也可以利用系统API实现图形化GUI功能
系统API一般分为:核心级(kernel32.dll),用户级(User32.dll), 应用级（gdi32.dll)和其他一些外设驱动等.
下面我们来看一个简单例子，用系统API获取窗口句柄。
代码
public class User32API
{
private static Hashtable processWnd = null;
public delegate bool WNDENUMPROC(IntPtr hwnd, uint lParam);
static User32API()
{
if (processWnd == null)
{
processWnd = new Hashtable();
}
}
[DllImport("user32.dll", EntryPoint = "EnumWindows", SetLastError = true)]
public static extern bool EnumWindows(WNDENUMPROC lpEnumFunc, uint lParam);
[DllImport("user32.dll", EntryPoint = "GetParent", SetLastError = true)]
public static extern IntPtr GetParent(IntPtr hWnd);
[DllImport("user32.dll", EntryPoint = "GetWindowThreadProcessId")]
public static extern uint GetWindowThreadProcessId(IntPtr hWnd, ref uint lpdwProcessId);
[DllImport("user32.dll", EntryPoint = "IsWindow")]
public static extern bool IsWindow(IntPtr hWnd);
[DllImport("kernel32.dll", EntryPoint = "SetLastError")]
public static extern void SetLastError(uint dwErrCode);
[DllImport("User32.dll", EntryPoint = "SendMessage")]
private static extern int SendMessage(IntPtr hWnd, int Msg, int wParam, int lParam);
Process[] processes;
const int WM_CLOSE = 0x0010;
public void GetCurrentWindowHandle()
{
Microsoft.Win32.RegistryKey oRegistryKey = Microsoft.Win32.Registry.CurrentUser.OpenSubKey("Software").OpenSubKey("Amada Software");
string[] subKeys = oRegistryKey.GetSubKeyNames();
IntPtr ptrWnd = IntPtr.Zero;
foreach (string subkey in subKeys)
{
processes = Process.GetProcessesByName(subkey);
foreach (Process proc in processes)
{
uint uiPid = (uint)proc.Id;
object objWnd = processWnd[uiPid];
if (objWnd != null)
{
ptrWnd = (IntPtr)objWnd;
if (ptrWnd != IntPtr.Zero && IsWindow(ptrWnd))
{
SendMessage(ptrWnd, WM_CLOSE, 0, 0);
}
else
{
ptrWnd = IntPtr.Zero;
}
}
bool bResult = EnumWindows(new WNDENUMPROC(EnumWindowsProc), uiPid);
if (!bResult && Marshal.GetLastWin32Error() == 0)
{
objWnd = processWnd[uiPid];
if (objWnd != null)
{
ptrWnd = (IntPtr)objWnd;
SendMessage(ptrWnd, WM_CLOSE, 0, 0);
}
}
}
}
}
private static bool EnumWindowsProc(IntPtr hwnd, uint lParam)
{
uint uiPid = 0;
if (GetParent(hwnd) == IntPtr.Zero)
{
GetWindowThreadProcessId(hwnd, ref uiPid);
if (uiPid == lParam)
{
processWnd[uiPid] = hwnd;
SetLastError(0);
return false;
}
}
return true;
}
}