/// <summary>
/// 根據圖像獲取圖像的擴展名
/// </summary>
/// <param name="image"></param>
/// <returns></returns>
public static String GetExtension(Image image)
{
    foreach (var pair in ImageFormats)
    {
        if (pair.Value.Guid == image.RawFormat.Guid)
        {
            return pair.Key;
        }
    }
    throw new BadImageFormatException();
}

using (var img = Image.FromFile(@"C:soar"))
{
    var ext = GetExtension(img);
}

public static bool CheckImgType(string strImg)
{
        if(strImg!=null&&strImg.ToString().Length>0)
        {
            int i = strImg.LastIndexOf(".");
            string StrType = strImg.Substring(i);
            if (StrType == ".jpg" || StrType == ".gif" || StrType == ".jpeg" || StrType == ".png")
            {
                return true;
            }
            else
            {
                return false;
            }
        }
        else
        {
                return false;
        }
}

string aFirstName = aFile.Substring(aFile.LastIndexOf("/") + 1, (aFile.LastIndexOf(".") - aFile.LastIndexOf("/") - 1));  //文件名
string aLastName = aFile.Substring(aFile.LastIndexOf(".") + 1, (aFile.Length - aFile.LastIndexOf(".") - 1));   //擴展名
string strFilePaht="文件路徑";
Path.GetFileNameWithoutExtension(strFilePath);這個就是獲取文件名的

strFilePaht.Substring(path.LastIndexOf("/") + 1, path.Length - 1 - path.LastIndexOf("/"));
strFilePaht.Substring(path.LastIndexOf("."), path.Length - path.LastIndexOf("."));

string path1 = System.IO.Path.GetDirectoryName(openFileDialog1.FileName) + @"";
string path = Path.GetFileName("C:My Documentpathimage.jpg");    //只獲取文件名image.jpg