我无法使用FolderBrowserDialog设置位图

我正在尝试制作一个绘图应用程序。我创建了一个调用FolderBrowserDialog的函数，它可以工作。现在我的问题是，当试图使用它的地方设置。bmp文件，我得到"路径不是一个合法的形式"这里的代码

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;

namespace Paint_AppLication
{
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }
    private bool mouse_down = false;
    static string folderPath = "";
    private Color col = Color.Black;
    private void panel1_MouseDown(object sender, MouseEventArgs e)
    {
        mouse_down = true;
    }
    private void panel1_MouseUp(object sender, MouseEventArgs e)
    {
        mouse_down = false;
    }
    private void panel1_MouseMove(object sender, MouseEventArgs e)
    {
        toolStripStatusLabel1.Text = e.X + ", " + e.Y;
        if(mouse_down == true)
        {
            bit = new Bitmap(bit, panel1.Size);
            panel1.BackgroundImage = bit;
            bit.SetPixel(e.X, e.Y, col);
        }
    }
    private void button1_Click(object sender, EventArgs e)
    {
        colorDialog1.ShowDialog();
        col = colorDialog1.Color;
    }
    private void Form1_Load(object sender, EventArgs e)
    {
        FolderBrowserDialog directchoosedlg = new FolderBrowserDialog();
        if (directchoosedlg.ShowDialog() == DialogResult.OK)
        {
            folderPath = directchoosedlg.SelectedPath;
        }
    }
   //My Error Is here
     private Bitmap bit = new Bitmap(folderPath);
}
}