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 窗体设计器生成的代码
 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 窗体设计器生成的代码
 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 = "测试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

<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

F:>csc.exe TestForm1.cs