public string transsformValue
{
 get
   {
    return this.textBox1.Text;
   }
 set
   {
    this.textBox1.Text=value; 
  }
}

FORM2 f=new FORM2();
f.transsformValue="aaaa";
textBox1=f.transsformValue;
f.Show();

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
namespace WindowsFormsApplication17
{
  public partial class Form1 : Form
  {
    public Form1()
    {
      InitializeComponent();
    }
    private void Form1_Load(object sender, EventArgs e)
    {
    }
    private void button1_Click(object sender, EventArgs e)
    {
      InputBox f = new InputBox();
      f.Title = "请输入对话框";
      f.TipText = "请输入年龄";
      if (f.ShowDialog() == DialogResult.OK)
        this.label1.Text = f.Message;
    }
  }
}
//InputBox的FORMl里这么写
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
namespace WindowsFormsApplication17
{
  public partial class InputBox : Form
  {
    public InputBox()
    {
      InitializeComponent();
    }
    public string Title
    {
      set { this.Text = value; }
    }
    public string Message
    {
      get { return this.Input.Text; }
    }
    public string TipText
    {
      set { this.Tip.Text = value; }
    }
    private void InputBox_Load(object sender, EventArgs e)
    {
      this.AcceptButton = this.btnOK;
      this.CancelButton = this.btnCancel;
      this.btnOK.DialogResult = DialogResult.OK;
      this.btnCancel.DialogResult = DialogResult.Cancel;
    }
  }
}