控件属性和InitializeComponent()关系：

namespace Test22
{
    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.numericUpDown1 = new System.Windows.Forms.NumericUpDown();
            ((System.ComponentModel.ISupportInitialize)(this.numericUpDown1)).BeginInit();
            this.SuspendLayout();
            // 
            // numericUpDown1
            // 
            this.numericUpDown1.DecimalPlaces = 4;//属性里对应！！！！！
            this.numericUpDown1.Location = new System.Drawing.Point(12, 12);
            this.numericUpDown1.Name = "numericUpDown1";
            this.numericUpDown1.Size = new System.Drawing.Size(120, 21);
            this.numericUpDown1.TabIndex = 0;
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 12F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(153, 53);
            this.Controls.Add(this.numericUpDown1);
            this.Name = "Form1";
            this.Text = "Form1";
            this.Load += new System.EventHandler(this.Form1_Load);
            ((System.ComponentModel.ISupportInitialize)(this.numericUpDown1)).EndInit();
            this.ResumeLayout(false);

        }

        #endregion

        private System.Windows.Forms.NumericUpDown numericUpDown1;
    }
}

控件中小数点位数和InitializeComponent()里面的代码相呼应，而下面的代码又设置了2，所以覆盖掉了，代码和运行结果如下：

using System;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
namespace Test22
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            numericUpDown1.Maximum = 20;
            numericUpDown1.Minimum = 1;
            numericUpDown1.DecimalPlaces = 2;
        }
    }
}