source: Common/Dialogs.cs

using System.IO;
using System.Windows.Forms;

namespace Common
{
    class Dialogs
    {
        public const string AnyFile = "Any file";

        public static string OpenFileDialog(string fileName, string fileType, string fileExtension)
        {
            OpenFileDialog dlg = new OpenFileDialog { Filter = fileType + @"|*." + fileExtension + @"|" + AnyFile + @"|*.*", DefaultExt = fileExtension };
            if (!string.IsNullOrEmpty(fileName))
            {
                dlg.FileName = Path.GetFileName(fileName);
                dlg.InitialDirectory = Path.GetDirectoryName(fileName);
            }

            if (dlg.ShowDialog() == DialogResult.OK)
            {
                fileName = dlg.FileName;
            }
            return fileName;
        }

        public static string SaveFileDialog(string fileName, string fileType, string fileExtension)
        {
            SaveFileDialog dlg = new SaveFileDialog { Filter = fileType + @"|*." + fileExtension + @"|" + AnyFile + @"|*.*", DefaultExt = fileExtension };
            if (!string.IsNullOrEmpty(fileName))
            {
                dlg.FileName = Path.GetFileName(fileName);
                dlg.InitialDirectory = Path.GetDirectoryName(fileName);
            }

            if (dlg.ShowDialog() == DialogResult.OK)
            {
                fileName = dlg.FileName;
            }
            return fileName;
        }
    }
}