/// <summary>
/// 主窗体接口
/// </summary>
public interface IMdiParent
{
   void ParentFoo();
}

/// <summary>
/// 子窗体接口
/// </summary>
public interface IMyChildForm
{
   void Foo();
}

/// <summary>
/// 主窗体，实现IMdiParent接口
/// </summary>
public partial class frmParent : Form, IMdiParent
{
   public frmParent()
   {
      InitializeComponent();
   }
   
   private void form1ToolStripMenuItem_Click(object sender, EventArgs e)
   {
      //打开子窗体
      frmChildA child = new frmChildA();
      child.MdiParent = this;
      child.Show();
   }
   
   private void menuCallFoo_Click(object sender, EventArgs e)
   {
      //调用子窗体的Foo()方法
      Form activedChild = this.ActiveMdiChild;
      if ((activedChild != null) && (activedChild is IMyChildForm))
         (activedChild as IMyChildForm).Foo();
   }
   
   #region IMdiParent 成员
   
   public void ParentFoo()
   {
      MessageBox.Show("调用" this.GetType().FullName ".ParentFoo()方法！");
   }
   
   #endregion
}

//来源:C/S框架网(www.csframework.com) QQ:1980854898

/// <summary>
/// 子窗体，实现IMyChildForm接口
/// </summary>
public partial class frmChildA : Form, IMyChildForm
{
   public frmChildA()
   {
      InitializeComponent();
   }
   
   #region IMyChildForm 成员
   
   public void Foo()
   {
      MessageBox.Show("调用" this.GetType().FullName ".Foo()方法！");
   }
   
   #endregion
   
   private void btnParentFoo_Click(object sender, EventArgs e)
   {
      //调用父窗体的ParentFoo()方法
      if ((this.MdiParent != null) && (this.MdiParent is IMdiParent))
      (this.MdiParent as IMdiParent).ParentFoo();
   }
   
   private void btnErrCall_Click(object sender, EventArgs e)
   {
      //错误的调用
      (this.MdiParent as frmParent).ParentFoo();
   }
   
   //来源:C/S框架网(www.csframework.com) QQ:1980854898