效果如下：

1.创建一个winform的窗体项目（框架.NET Framework 4.7.2）
2.Nuget引入opencv的c#程序包（版本最好和我一致）

3.后台代码

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Drawing.Imaging;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
using OpenCvSharp;
using OpenCvSharp.Extensions;

namespace opencv读取摄像头
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            Control.CheckForIllegalCrossThreadCalls = false;//取消跨线程检查
            this.timer1.Start();
        }

        private VideoCapture capture;
        private bool isopen = false;
        private bool saveImgFlag = false;
        private string filePath = "";

        private void button1_Click(object sender, EventArgs e)
        {
            capture = new VideoCapture(0);
            if (!capture.IsOpened())
            {
                MessageBox.Show("无法打开摄像头");
                return;
            }
            isopen = true;
            Thread video_th = new Thread(StartCapturing);
            video_th.IsBackground = true;
            video_th.Start();
        }

        public void StartCapturing()
        {
            Mat frame = new Mat();
            while (true)
            {
                capture.Read(frame);//读取图像帧
                if (frame.Empty())
                {
                    break;
                }
                Bitmap bitmap = BitmapConverter.ToBitmap(frame);
                if (saveImgFlag == true)
                {
                    try
                    {
                        bitmap.Save(filePath, ImageFormat.Jpeg);
                        saveImgFlag = false;
                        MessageBox.Show("保存成功！");
                    }
                    catch (Exception ex)
                    {
                    }
                }
                pictureBox1.Image = bitmap;
                pictureBox1.Refresh();
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            SaveFileDialog saveFileDialog = new SaveFileDialog();
            // 设置对话框的标题
            saveFileDialog.Title = "Save Image";

            // 设置默认的文件名和文件类型过滤器
            saveFileDialog.FileName = DateTime.Now.ToString("yyyy_MM_dd_HH_mm_ss");
            saveFileDialog.Filter = "Image files (*.Png)|*.Png|Image files (*.Jpg)|*.Jpg";

            // 显示对话框并获取用户的操作结果
            DialogResult result = saveFileDialog.ShowDialog();

            if (result == DialogResult.OK)
            {
                // 用户点击了保存按钮
                filePath = saveFileDialog.FileName;
                //string filter=saveFileDialog.Filter;

                // 在这里进行保存文件的操作，例如：
                // File.WriteAllText(filePath, "Hello, world!");

                Console.WriteLine("File saved to: " + filePath);
            }
            else if (result == DialogResult.Cancel)
            {
                // 用户点击了取消按钮
                Console.WriteLine("Save cancelled");
            }
            saveImgFlag = true;
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            this.label1.Text = DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss");
        }
    }
}