namespace text
{
    partial class Form1
    {
        /// <summary>
        /// 必需的设计器变量。
        /// </summary>
        
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// 清理所有正在使用的资源。
        /// </summary>
        /// <param name="disposing">如果应释放托管资源，为 true；否则为 false。</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows 窗体设计器生成的代码

        /// <summary>
        /// 设计器支持所需的方法 - 不要
        /// 使用代码编辑器修改此方法的内容。
        /// </summary>
        private void InitializeComponent()
        {
            this.toolStrip1 = new System.Windows.Forms.ToolStrip();
            this.panel1 = new System.Windows.Forms.Panel();
            this.SuspendLayout();
            // 
            // toolStrip1
            // 
            this.toolStrip1.Location = new System.Drawing.Point(0, 0);
            this.toolStrip1.Name = "toolStrip1";
            this.toolStrip1.Size = new System.Drawing.Size(553, 25);
            this.toolStrip1.TabIndex = 4;
            this.toolStrip1.Text = "toolStrip1";
            // 
            // panel1
            // 
            this.panel1.BackColor = System.Drawing.SystemColors.ControlDarkDark;
            this.panel1.Dock = System.Windows.Forms.DockStyle.Fill;
            this.panel1.Location = new System.Drawing.Point(0, 25);
            this.panel1.Name = "panel1";
            this.panel1.Size = new System.Drawing.Size(553, 382);
            this.panel1.TabIndex = 5;
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 12F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(553, 407);
            this.Controls.Add(this.panel1);
            this.Controls.Add(this.toolStrip1);
            this.IsMdiContainer = true;
            this.MinimumSize = new System.Drawing.Size(561, 439);
            this.Name = "Form1";
            this.StartPosition = System.Windows.Forms.FormStartPosition.CenterScreen;
            this.Text = "Form1";
            this.Load += new System.EventHandler(this.Form1_Load);
            this.ResumeLayout(false);
            this.PerformLayout();

        }
        #endregion

        private System.Windows.Forms.ToolStrip toolStrip1;
        public  System.Windows.Forms.Panel panel1;
    }
}

------解决方案--------------------

有一个变通的办法可以达到这种效果，
就是建立一个控件大小的不规则窗口，看起来是一个固定在父窗口的控件，其实下面隐含了一个只有这个控件大小的form
C# code
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 WindowsFormsApplication27
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            this.IsMdiContainer = true;

            Form HideF = new Form();
            HideF.MdiParent = this;

            TextBox TB = new TextBox();
            TB.Parent = HideF;
            HideF.Region = new Region(TB.Bounds);
            HideF.FormBorderStyle = FormBorderStyle.None;
            HideF.Show();


            Form F = new Form();
            F.MdiParent = this;
            F.Show();
        }
    }
}