/// <summary>
/// 原文链接：https://blog.csdn.net/makenothing/article/details/39493779
/// </summary>
/// <param name="fileName"></param>
/// <returns></returns>
/// <exception cref="Exception"></exception>
private static string GetMD5HashFromFile(string fileName)
{
    try
    {
        FileStream file = new FileStream(fileName, FileMode.Open);
        System.Security.Cryptography.MD5 md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
        byte[] retVal = md5.ComputeHash(file);
        file.Close();

        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < retVal.Length; i++)
        {
            sb.Append(retVal[i].ToString("x2"));
        }
        return sb.ToString();
    }
    catch (Exception ex)
    {
        throw new Exception("GetMD5HashFromFile() fail,error:" + ex.Message);
    }
}

private void btnSelectFile1_Click(object sender, EventArgs e)
{
    using (OpenFileDialog openFileDialog = new OpenFileDialog())
    {
        openFileDialog.InitialDirectory = "C:\\Users\\jay.star\\Desktop"; // 初始目录
        openFileDialog.Title = "选择文件"; // 对话框标题
        openFileDialog.Filter = "所有文件(*.*)|*.*"; // 文件过滤器

        if (openFileDialog.ShowDialog() == DialogResult.OK)
        {
            // 获取文件路径
            string filePath = openFileDialog.FileName;
            var md5Str = GetMD5HashFromFile(filePath);
            txtFile1.Text = md5Str;
        }
    }
}