WinAPI.cs

public class WinAPI
	{
		[DllImport("user32.dll")]
		public static extern bool PostMessage(int hhwnd, uint msg, IntPtr wparam, IntPtr lparam);

		[DllImport("user32.dll")]
		public static extern IntPtr LoadKeyboardLayout(string pwszKLID, uint Flags);

		public static uint WM_INPUTLANGCHANGEREQUEST = 0x0050;
		public static int HWND_BROADCAST = 0xffff;
		public static string en_US = "00000409"; //英文
		public static string cn_ZH = "00000804";
		public static uint KLF_ACTIVATE = 1;
	}

引用

WinAPI.PostMessage(WinAPI.HWND_BROADCAST, WinAPI.WM_INPUTLANGCHANGEREQUEST, IntPtr.Zero, WinAPI.LoadKeyboardLayout(WinAPI.en_US, WinAPI.KLF_ACTIVATE));