窗口显示后设置按钮为默认按钮

//设置本窗体的活动控件为某个控件
this.ActiveControl = this.button2;

窗口显示后设置焦点在按钮上

//调用Focus方法设置某个控件获取焦点
this.button2.Focus();

代码放不同位置，是否生效

在窗体实例化——加载——绘制——显示完毕四个过程中使用两种方法设置效果有区别，具体如下：

构造方法中：
this.ActiveControl = this.button2;//生效

this.button2.Focus(); //不生效

Load（加载）事件中：
this.ActiveControl = this.button2;//生效

this.button2.Focus(); //不生效

Paint（绘制）事件中：
this.ActiveControl = this.button2;//生效

this.button2.Focus(); //生效

Shown（显示完毕）事件中：
this.ActiveControl = this.button2;//生效

this.button2.Focus(); //生效

测试代码

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            //this.ActiveControl = this.button2;//生效
            //this.button2.Focus();//不生效
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            //this.ActiveControl = this.button2;//生效
            //this.button2.Focus(); //不生效
        }

        private void Form1_Shown(object sender, EventArgs e)
        {
            //this.ActiveControl = this.button2;//生效
            //this.button2.Focus(); //生效
        }

        private void Form1_Paint(object sender, PaintEventArgs e)
        {
            //this.ActiveControl = this.button2;//生效
            this.button2.Focus(); //生效
        }
    }
}

https://blog.csdn.net/sinat_27305053/article/details/79161757