using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;
using System.Windows.Forms;
using System.Drawing;
using System.Diagnostics;

namespace AnimatedTransform
{
    static class FormTransform
    {
        public static void TransformSize(Form frm, int newWidth, int newHeight)
        {
            TransformSize(frm, new Size(newWidth, newHeight));
        }

        public static void TransformSize(Form frm, Size newSize)
        {
            ParameterizedThreadStart threadStart = new ParameterizedThreadStart(RunTransformation);
            Thread transformThread = new Thread(threadStart);
            transformThread.Start(new object[] { frm, newSize });
        }

        private delegate void RunTransformationDelegate(object paramaters);
        private static void RunTransformation(object parameters)
        {
            Form frm = (Form)((object[])parameters)[0];
            if (frm.InvokeRequired)
            {
                RunTransformationDelegate del = new RunTransformationDelegate(RunTransformation);
                frm.Invoke(del, parameters);
            }
            else
            {
                //动画的变量参数
                double FPS = 300.0;
                long interval = (long)(Stopwatch.Frequency / FPS);
                long ticks1 = 0;
                long ticks2 = 0;

                //传进来的新的窗体的大小
                Size size = (Size)((object[])parameters)[1];

                int xDiff = Math.Abs(frm.Width - size.Width);
                int yDiff = Math.Abs(frm.Height - size.Height);

                int step = 10;

                int xDirection = frm.Width < size.Width ? 1 : -1;
                int yDirection = frm.Height < size.Height ? 1 : -1;

                int xStep = step * xDirection;
                int yStep = step * yDirection;

                //要调整的窗体的宽度是否在步长之内
                bool widthOff = IsWidthOff(frm.Width, size.Width, xStep);
                //要调整的窗体的高度是否在步长之内
                bool heightOff = IsHeightOff(frm.Height, size.Height, yStep);


                while (widthOff || heightOff)
                {
                    //获取当前的时间戳
                    ticks2 = Stopwatch.GetTimestamp();
                    //允许调整大小仅在有足够的时间来刷新窗体的时候
                    if (ticks2 >= ticks1 + interval) 
                    {
                        //调整窗体的大小
                        if (widthOff)
                            frm.Width += xStep;

                        if (heightOff)
                            frm.Height += yStep;

                        widthOff = IsWidthOff(frm.Width, size.Width, xStep);
                        heightOff = IsHeightOff(frm.Height, size.Height, yStep);

                        //允许窗体刷新
                        Application.DoEvents();

                        //保存当前的时间戳
                        ticks1 = Stopwatch.GetTimestamp();
                    }

                    Thread.Sleep(1);
                }

            }
        }

        private static bool IsWidthOff(int currentWidth, int targetWidth, int step)
        {
            //目标宽度与当前宽度是否在步长之内，如果是，返回false
            if (Math.Abs(currentWidth - targetWidth) <= Math.Abs(step)) return false;

            return (step > 0 && currentWidth < targetWidth) ||
                   (step < 0 && currentWidth > targetWidth); 
        }

        private static bool IsHeightOff(int currentHeight, int targetHeight, int step)
        {
            //目标高度与当前高度是否在步长之内，如果是，返回false
            if (Math.Abs(currentHeight - targetHeight) <= Math.Abs(step)) return false;

            return (step > 0 && currentHeight < targetHeight) || 
                   (step < 0 && currentHeight > targetHeight); 
        }
    }
}

调用方法：Random random = new Random();            int width = random.Next(100, 500);//随机数产生窗体的宽            int height = random.Next(50, 800);//随机数产生窗体的高            FormTransform.TransformSize(this, width, height);