form1 中 new form2的时候new form2(this)，form2的构造函数中接受form1的引用 public Form2(Form1 frm1),然后定义一个From1类型变量接受参数，就可以使用form1了，要使用其中的控件需要在form1中把他们定义成public就行。
form1：
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;

namespace f2parent
{
 /// <summary>
 /// Summary description for Form1.
 /// </summary>
 public class Form1 : System.Windows.Forms.Form
 {
  public System.Windows.Forms.TextBox textBox1;
  private System.Windows.Forms.Button button1;
  /// <summary>
  /// Required designer variable.
  /// </summary>
  private System.ComponentModel.Container components = null;

  public Form1()
  {
   //
   // Required for Windows Form Designer support
   //
   InitializeComponent();

   //
   // TODO: Add any constructor code after InitializeComponent call
   //
  }

  /// <summary>
  /// Clean up any resources being used.
  /// </summary>
  protected override void Dispose( bool disposing )
  {
   if( disposing )
   {
    if (components != null)
    {
     components.Dispose();
    }
   }
   base.Dispose( disposing );
  }

  #region Windows Form Designer generated code
  /// <summary>
  /// Required method for Designer support - do not modify
  /// the contents of this method with the code editor.
  /// </summary>
  private void InitializeComponent()
  {
   this.textBox1 = new System.Windows.Forms.TextBox();
   this.button1 = new System.Windows.Forms.Button();
   this.SuspendLayout();
   //
   // textBox1
   //
   this.textBox1.Location = new System.Drawing.Point(16, 32);
   this.textBox1.Name = "textBox1";
   this.textBox1.TabIndex = 0;
   this.textBox1.Text = "textBox1";
   //
   // button1
   //
   this.button1.Location = new System.Drawing.Point(88, 128);
   this.button1.Name = "button1";
   this.button1.TabIndex = 1;
   this.button1.Text = "button1";
   this.button1.Click += new System.EventHandler(this.button1_Click);
   //
   // Form1
   //
   this.AutoScaleBaseSize = new System.Drawing.Size(5, 12);
   this.ClientSize = new System.Drawing.Size(292, 266);
   this.Controls.Add(this.button1);
   this.Controls.Add(this.textBox1);
   this.IsMdiContainer = true;
   this.Name = "Form1";
   this.Text = "Form1";
   this.ResumeLayout(false);

  }
  #endregion

  /// <summary>
  /// The main entry point for the application.
  /// </summary>
  [STAThread]
  static void Main()
  {
   Application.Run(new Form1());
  }

  private void button1_Click(object sender, System.EventArgs e)
  {
   Form2 frm2=new Form2(this);
   frm2.ShowDialog();
  }
 }
}

form2
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;

namespace f2parent
{
 /// <summary>
 /// Summary description for Form2.
 /// </summary>
 public class Form2 : System.Windows.Forms.Form
 {
  private System.Windows.Forms.Button button1;
  /// <summary>
  /// Required designer variable.
  /// </summary>
  private System.ComponentModel.Container components = null;
  private Form1 form1;

  public Form2(Form1 frm1)
  {
   //
   // Required for Windows Form Designer support
   //
   InitializeComponent();
   form1=frm1;

   //
   // TODO: Add any constructor code after InitializeComponent call
   //
  }

  /// <summary>
  /// Clean up any resources being used.
  /// </summary>
  protected override void Dispose( bool disposing )
  {
   if( disposing )
   {
    if(components != null)
    {
     components.Dispose();
    }
   }
   base.Dispose( disposing );
  }

  #region Windows Form Designer generated code
  /// <summary>
  /// Required method for Designer support - do not modify
  /// the contents of this method with the code editor.
  /// </summary>
  private void InitializeComponent()
  {
   this.button1 = new System.Windows.Forms.Button();
   this.SuspendLayout();
   //
   // button1
   //
   this.button1.Location = new System.Drawing.Point(184, 192);
   this.button1.Name = "button1";
   this.button1.TabIndex = 0;
   this.button1.Text = "button1";
   this.button1.Click += new System.EventHandler(this.button1_Click);
   //
   // Form2
   //
   this.AutoScaleBaseSize = new System.Drawing.Size(5, 12);
   this.ClientSize = new System.Drawing.Size(292, 266);
   this.Controls.Add(this.button1);
   this.Name = "Form2";
   this.Text = "Form2";
   this.ResumeLayout(false);

  }
  #endregion

  private void button1_Click(object sender, System.EventArgs e)
  {
   form1.textBox1.Text="435134";
  }
 }
}