I have found this code while I was busy searching for an answer!
private void btnOpenFile_Click(object sender, EventArgs e)
{
    OpenFileDialog saveFileDialogBrowse = new OpenFileDialog();
    saveFileDialogBrowse.Filter = "Pcap file|*.pcap";
    saveFileDialogBrowse.Title = "Save an pcap File";
    saveFileDialogBrowse.ShowDialog();
    var pcapFile = saveFileDialogBrowse.FileName; //do whatever you like with the selected filename
    if (pcapFile != "")
    {
        FileInfo fileInfo = new FileInfo(pcapFile);
        txtFilePath.Text = fileInfo.FullName;
    }
}
 
     
     
     
    