GridView Sorting Issue without using a datasource

protected void CompaniesGridView_PageIndexChanging(object sender, GridViewPageEventArgs e)
{   
    CompaniesGridView.PageIndex = e.NewPageIndex;
    CompaniesGridView.DataSource = GetDataSource();
}

private DataSet GetDataSource()
{
    string _connString = ConfigurationManager.ConnectionStrings["DBConnection"].ConnectionString;
    ICompanyCollection coll = (ICompanyCollection)new CompanyCollection(_connString);
    // TODO: Pass a proper usrId to this method
    CompanySearchDataSet ds = coll.Find(0, CompanyNameTextBox.Text);
    CompaniesGridView.DataSource = ds;
    CompaniesGridView.DataBind();
    return ds;
}

protected void CompaniesGridView_Sorting(object sender, GridViewSortEventArgs e)
{
    GridViewSortExpression = e.SortExpression;
    int pageIndex = CompaniesGridView.PageIndex;
    CompaniesGridView.DataSource = GetDataSource();        
    CompaniesGridView.PageIndex = pageIndex;
}