class Program
{
    static void Main(string[] args)
    {
        Console.Write("文件地址：");
        string file = Console.ReadLine();
        Console.WriteLine("转换结果：{0}", FileToBase64String(file));
        Console.ReadLine();
    }

    static string FileToBase64String(string file)
    {
        string result = string.Empty;

        try
        {
            using (FileStream stream = new FileStream(file, FileMode.Open))
            {
                byte[] bt = new byte[stream.Length];
                stream.Read(bt, 0, bt.Length);
                result = Convert.ToBase64String(bt);
            }
        }
        catch { }

        return result;
    }
}

using System.Drawing;
using System.Drawing.Imaging;
using System.IO;

public class ImageHelper
{
    /// <summary>
    /// GetEncoderInfo
    /// </summary>
    /// <param name="mimeType"></param>
    /// <returns></returns>
    private static ImageCodecInfo GetEncoderInfo(string mimeType)
    {
        ImageCodecInfo[] encoders = ImageCodecInfo.GetImageEncoders();
        for (int i = 0; i < encoders.Length; ++i)
        {
            if (encoders[i].MimeType == mimeType)
                return encoders[i];
        }
        return null;
    }

    /// <summary>
    /// 图片压缩(降低质量以减小文件的大小)
    /// </summary>
    /// <param name="srcBitmap">传入的Bitmap对象</param>
    /// <param name="destStream">压缩后的Stream对象</param>
    /// <param name="level">压缩等级，0-100</param>
    private static void Compress(Bitmap srcBitmap, Stream destStream, long level)
    {
        ImageCodecInfo myImageCodecInfo;
        Encoder myEncoder;
        EncoderParameter myEncoderParameter;
        EncoderParameters myEncoderParameters;

        // Get an ImageCodecInfo object that represents the JPEG codec.
        myImageCodecInfo = GetEncoderInfo("image/jpeg");

        // Create an Encoder object based on the GUID

        // for the Quality parameter category.
        myEncoder = Encoder.Quality;

        // Create an EncoderParameters object.
        // An EncoderParameters object has an array of EncoderParameter
        // objects. In this case, there is only one

        // EncoderParameter object in the array.
        myEncoderParameters = new EncoderParameters(1);

        // Save the bitmap as a JPEG file with 给定的 quality level
        myEncoderParameter = new EncoderParameter(myEncoder, level);
        myEncoderParameters.Param[0] = myEncoderParameter;
        srcBitmap.Save(destStream, myImageCodecInfo, myEncoderParameters);
    }

    /// <summary>
    /// 图片压缩(降低质量以减小文件的大小)
    /// </summary>
    /// <param name="srcFile"></param>
    /// <param name="destFile"></param>
    /// <param name="level"></param>
    public static void Compress(string srcFile, string destFile, long level)
    {
        using (Image image = Image.FromFile(srcFile))
        {
            using (Bitmap bitmap = new Bitmap(image))
            {
                using (FileStream destStream = new FileStream(destFile, FileMode.Create))
                {
                    Compress(bitmap, destStream, level);
                }
            }
        }
    }
}