Do not Restore my Back up file Please Help

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C#

Cursor.Current = Cursors.WaitCursor;

           try
           {
               if (txtFileName.Text == "")
               {
                   MessageBox.Show("Please Select Restore File.", Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Information);
                   txtFileName.Focus();
                   return;
               }
               FileInfo file = new FileInfo(txtFileName.Text);
               if (file.Exists)
               {
                   if (MessageBox.Show("Are you sure you restore?", "Back", MessageBoxButtons.YesNo, MessageBoxIcon.Question) == DialogResult.Yes)
                   {
                       if (file.Extension == ".mbk")
                       {

}
else
{

FileInfo filebak = new FileInfo(txtFileName.Text);

String Conn = File.ReadAllText("ConnectionString.txt");
SqlConnection _SqlConnection = new SqlConnection(Conn);
_SqlConnection.Open();
//_SqlCommand = new SqlCommand("use master", connect);
//_SqlCommand.ExecuteNonQuery();
_SqlCommand = new SqlCommand("USE master ALTER DATABASE " + "PragatiDB" + " SET SINGLE_USER WITH ROLLBACK IMMEDIATE; ALTER DATABASE " + "PragatiDB" + " SET MULTI_USER;", _SqlConnection);
_SqlCommand.ExecuteNonQuery();
_SqlCommand = new SqlCommand(@"restore database " + "PragatiDB" + " from disk = '" + BackupPath_bak + "\\" + filebak.Name + "' WITH REPLACE", _SqlConnection);
_SqlCommand.ExecuteNonQuery();
_SqlConnection.Close();
//Directory.Delete(BackupPath_bak, true);


}
MessageBox.Show("Database " + "PragatiDB" + " Restored successfully.", "Restoration", MessageBoxButtons.OK, MessageBoxIcon.Information);
}
}
else
MessageBox.Show(@"Do not make any endorsement above (or is not in the correct path)", "Restoration", MessageBoxButtons.OK, MessageBoxIcon.Information);

}
catch (Exception exp)
{
MessageBox.Show(exp.Message, Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Error);
}

}