我不确定原来的要求;例如，我不知道为什么UserControl本身应该设置窗体的AcceptButton，或者如果窗体上有多个此类控件的示例，预期的行为是什么。设置窗体的AcceptButton似乎不是UserControl的责任，可能有更好的解决方案，例如依赖事件和设置AcceptButton。
无论如何，下面的代码示例演示如何设置窗体的AcceptButton;也许它能帮助你找到一个解决方案。代码的亮点：

• 代码使用dispose将AcceptButton设置为null。
• 程式码实作ISupportInitialize，以在控件初始化完成后设定接受按钮。如果您在执行阶段使用程式码建立控件执行严修，请不要忘记呼叫EndInit，如下所示：((System.ComponentModel.ISupportInitialize)(userControl11)).EndInit();，但如果您使用设计器，则设计器会处理此问题。
• 代码调用NotifyDefault(true)只是为了在窗体上承载它时在设计时获得视觉效果。

下面是一个例子：

using System;
using System.ComponentModel;
using System.Windows.Forms;
namespace WindowsFormsApp1
{
    public class UserControl1 : UserControl, ISupportInitialize
    {
        /// <summary> 
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        #region Component 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.button2 = new System.Windows.Forms.Button();
            this.textBox1 = new System.Windows.Forms.TextBox();
            this.SuspendLayout();
            // 
            // button1
            // 
            this.button1.Location = new System.Drawing.Point(15, 57);
            this.button1.Name = "button1";
            this.button1.Size = new System.Drawing.Size(75, 23);
            this.button1.TabIndex = 0;
            this.button1.Text = "button1";
            this.button1.UseVisualStyleBackColor = true;
            this.button1.Click += new System.EventHandler(this.button1_Click);
            // 
            // button2
            // 
            this.button2.Location = new System.Drawing.Point(96, 57);
            this.button2.Name = "button2";
            this.button2.Size = new System.Drawing.Size(75, 23);
            this.button2.TabIndex = 1;
            this.button2.Text = "button2";
            this.button2.UseVisualStyleBackColor = true;
            this.button2.Click += new System.EventHandler(this.button2_Click);
            // 
            // textBox1
            // 
            this.textBox1.Location = new System.Drawing.Point(15, 17);
            this.textBox1.Name = "textBox1";
            this.textBox1.Size = new System.Drawing.Size(100, 20);
            this.textBox1.TabIndex = 2;
            // 
            // UserControl1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.Controls.Add(this.textBox1);
            this.Controls.Add(this.button2);
            this.Controls.Add(this.button1);
            this.Name = "UserControl1";
            this.Size = new System.Drawing.Size(236, 106);
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion
        private System.Windows.Forms.TextBox textBox1;
        public System.Windows.Forms.Button button1;
        public System.Windows.Forms.Button button2;
        public UserControl1()
        {
            InitializeComponent();
            //Just for visual effect in design time when it's hosted on a form
            button2.NotifyDefault(true); 
        }
        private void button1_Click(object sender, EventArgs e)
        {
            MessageBox.Show("1");
        }
        private void button2_Click(object sender, EventArgs e)
        {
            MessageBox.Show("2");
        }
        public void BeginInit()
        {
        }
        public void EndInit()
        {
            var f = this.FindForm();
            if (f != null)
                f.AcceptButton = button2;
        }
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            if (disposing)
            {
                var f = this.FindForm();
                if (f != null)
                    f.AcceptButton = null;
            }
            base.Dispose(disposing);
        }
    }
}