how can i define "DisplayErrorMessage" in this case?

this is a code that i wrote for my open button...but i have error on "DisplayErrorMessage" part...what should i write instead? or how can i define it in order to don't error again

protected void btnOpen_Click(object sender, EventArgs e)
{
    txtFileName.Text = txtFileName.Text.Trim();
    if (txtFileName.Text == string.Empty)
    {
        string strErrorMessage = "you did Not specify file for opening!";
        DisplayErrorMessage(strErrorMessage);
    }

    string strFileName = txtFileName.Text;
    string strRootRelativePath = "~/app_data/pageContent";
    string strRootRelativePathName =
    string.Format("{0}/{1}", strRootRelativePath, strFileName);
    string strPathName = Server.MapPath(strRootRelativePathName);

    System.IO.StreamReader ostreamReader = null;

    try
    {
        ostreamReader = new System.IO.StreamReader(strPathName, System.Text.Encoding.UTF8);
        litPageMessages.Text = ostreamReader.ReadToEnd();
    }
    catch (Exception ex)
    {
        litPageMessages.Text = ex.Message;
    }
    finally
    {
        if (ostreamReader != null)
        {
            ostreamReader.Dispose();
            ostreamReader= null;
        }

    }
}