C# MDI子窗体互相操作

 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Data;
 using System.Drawing;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 using System.Windows.Forms;

 namespace WindowsFormsApplication1
 {
     public partial class Form1 : Form
     {
         private string name;
         public Form1()
         {
             InitializeComponent();
         }

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

         private void button2_Click(object sender, EventArgs e)
         {
             Form3 frm1 = new Form3();
             frm1.MdiParent = this;
             frm1.Show();
         }
         public event Action<string> OnText;
         public void ToChangeText(string obj)
         {
             if (OnText != null)
             {
                 OnText(obj);}
         }
         private void Form1_Load(object sender, EventArgs e)
         {

         }
     }
 }

 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Data;
 using System.Drawing;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 using System.Windows.Forms;

 namespace WindowsFormsApplication1
 {
     public partial class Form2 : Form
     {
         public Form2()
         {
             InitializeComponent();
         }

         private void button1_Click(object sender, EventArgs e)
         {
             (this.ParentForm as Form1).ToChangeText("form2来的消息啊？");
         }
     }
 }

 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Data;
 using System.Drawing;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 using System.Windows.Forms;

 namespace WindowsFormsApplication1
 {
     public partial class Form3 : Form
     {
         public Form3()
         {
             InitializeComponent();
         }

         private void Form3_Load(object sender, EventArgs e)
         {
             MessageBox.Show(this.ParentForm.Name);
             (this.ParentForm as Form1).OnText += new Action<string>(Form3_OnText);
         }
         private void Form3_FormClosed(object sender, FormClosedEventArgs e)
         {
             (this.ParentForm as Form1).OnText -= new Action<string>(Form3_OnText);
         }
         void Form3_OnText(string obj)
         {
             textBox1.Text = textBox1.Text+obj;
         }
     }
 }