using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
public class Form1 : System.Windows.Forms.Form {
 private System.Windows.Forms.Button button1;
 private System.Windows.Forms.TextBox textBox1;
 private System.ComponentModel.Container components = null;
 [STAThread]
 static void Main() {
 Application.Run(new Form1());
 }
 public Form1()
 {
 InitializeComponent();
 }
 protected override void Dispose( bool disposing )
 {
 if( disposing )
 {
 if(components != null)
 {
 components.Dispose();
 }
 }
 base.Dispose( disposing );
 }
 #region Windows 窗體設(shè)計(jì)器生成的代碼
 private void InitializeComponent()
 {
 this.button1 = new System.Windows.Forms.Button();
 this.textBox1 = new System.Windows.Forms.TextBox();
 this.SuspendLayout();
 //
 // button1
 //
 this.button1.Location = new System.Drawing.Point(32, 24);
 this.button1.Name = "button1";
 this.button1.TabIndex = 0;
 this.button1.Text = "button1";
 this.button1.Click += new System.EventHandler(this.button1_Click);
 //
 // textBox1
 //
 this.textBox1.Location = new System.Drawing.Point(32, 64);
 this.textBox1.Name = "textBox1";
 this.textBox1.TabIndex = 1;
 this.textBox1.Text = "textBox1";
 //
 // Form1
 //
 this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
 this.ClientSize = new System.Drawing.Size(292, 266);
 this.Controls.Add(this.textBox1);
 this.Controls.Add(this.button1);
 this.Name = "Form1";
 this.Text = "Form1";
 this.ResumeLayout(false);
 }
 #endregion
 private void button1_Click(object sender, System.EventArgs e) {
 MessageBox.Show("This is button1 click!");
 }
}

using System;
using System.Text;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Runtime.InteropServices;
public class TestForm1 : System.Windows.Forms.Form {
 private System.Windows.Forms.Button button1;
 private System.Windows.Forms.Button button2;
 private System.ComponentModel.Container components = null;
 [STAThread]
 static void Main() {
 Application.Run(new TestForm1());
 }
 public TestForm1()
 {
 InitializeComponent();
 }
 protected override void Dispose( bool disposing )
 {
 if( disposing )
 {
 if(components != null)
 {
 components.Dispose();
 }
 }
 base.Dispose( disposing );
 }
 #region Windows 窗體設(shè)計(jì)器生成的代碼
 private void InitializeComponent()
 {
 this.button1 = new System.Windows.Forms.Button();
 this.button2 = new System.Windows.Forms.Button();
 this.SuspendLayout();
 //
 // button1
 //
 this.button1.Location = new System.Drawing.Point(32, 24);
 this.button1.Name = "button1";
 this.button1.TabIndex = 0;
 this.button1.Text = "button1";
 this.button1.Click += new System.EventHandler(this.button1_Click);
 //
 // button2
 //
 this.button2.Location = new System.Drawing.Point(32, 64);
 this.button2.Name = "button2";
 this.button2.TabIndex = 0;
 this.button2.Text = "button2";
 this.button2.Click += new System.EventHandler(this.button2_Click);
 //
 // TestForm1
 //
 this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
 this.ClientSize = new System.Drawing.Size(292, 266);
 this.Controls.Add(this.button1);
 this.Controls.Add(this.button2);
 this.Name = "TestForm1";
 this.Text = "TestForm1";
 this.ResumeLayout(false);
 }
 #endregion
 private void button1_Click(object sender, System.EventArgs e) {
 IntPtr hwnd_win ;
 IntPtr hwnd_button ;
 hwnd_win = FindWindow("WindowsForms10.Window.8.app3","Form1");
 hwnd_button = FindWindowEx(hwnd_win ,new IntPtr(0) ,"WindowsForms10.BUTTON.app3","button1");
 const int BM_CLICK = 0x00F5;
 Message msg = Message.Create(hwnd_button ,BM_CLICK ,new IntPtr(0),new IntPtr(0));
 PostMessage(msg.HWnd ,msg.Msg ,msg.WParam ,msg.LParam);
 }
 private void button2_Click(object sender, System.EventArgs e) {
 const int WM_CHAR = 0x0102;
 IntPtr hwnd_win ;
 IntPtr hwnd_textbox ;
 hwnd_win = FindWindow("WindowsForms10.Window.8.app3","Form1");
 hwnd_textbox = FindWindowEx(hwnd_win ,new IntPtr(0) ,"WindowsForms10.EDIT.app3","textBox1");
 string strtext = "測試aaa";
 UnicodeEncoding encode = new UnicodeEncoding();
 char[] chars = encode.GetChars(encode.GetBytes(strtext));
 Message msg ;
 foreach (char c in chars ) {
 msg = Message.Create(hwnd_textbox ,WM_CHAR ,new IntPtr(c),new IntPtr(0));
 PostMessage(msg.HWnd ,msg.Msg ,msg.WParam ,msg.LParam);
 }
 }
 [DllImport("user32.dll")]
 public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
 [DllImport("user32.dll")]
 public static extern IntPtr FindWindowEx(IntPtr hwndParent,IntPtr hwndChildAfter,string lpszClass,string lpszWindow);
 [DllImport("user32.dll",CharSet=CharSet.Unicode)]
 public static extern IntPtr PostMessage(IntPtr hwnd,int wMsg,IntPtr wParam,IntPtr lParam);
}

F:>csc.exe Form1.cs
F:>csc.exe TestForm1.cs