System.Windows.Forms.SaveFileDialog saveFileDialog1;
saveFileDialog1 = new System.Windows.Forms.SaveFileDialog();
DialogResult dr=  saveFileDialog1.ShowDialog();
if (dr==DialogResult.OK)
{
    string filename = saveFileDialog1.FileName;
    //save file using stream.
}

SaveFileDialog oSaveFileDialog = new SaveFileDialog();
oSaveFileDialog.Filter = "All files (*.*) | *.*";
if (oSaveFileDialog.ShowDialog() == DialogResult.OK)
{
    string fileName = oSaveFileDialog.FileName;
    string extesion = Path.GetExtension(fileName);
    switch (extesion)
    {
        case ".txt"://do something here 
            break;
        case ".xls"://do something here 
            break;
        default://do something here
            break;
    }
}