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;
}
}