dotnet/src/webdriver/Internal/PortUtilities.cs

// <copyright file="PortUtilities.cs" company="Selenium Committers">
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//
//   http://www.apache.org/licenses/LICENSE-2.0
//
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using System.Net;
using System.Net.Sockets;

namespace OpenQA.Selenium.Internal;

/// <summary>
/// Encapsulates methods for working with ports.
/// </summary>
public static class PortUtilities
{
    /// <summary>
    /// Finds a random, free port to be listened on.
    /// </summary>
    /// <remarks>
    /// Prefers IPv4, but falls back to IPv6 if necessary.
    /// </remarks>
    /// <returns>A random, free port to be listened on.</returns>
    public static int FindFreePort()
    {
        try
        {
            using var ipV4socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            ipV4socket.Bind(new IPEndPoint(IPAddress.Loopback, 0));
            return ((IPEndPoint)ipV4socket.LocalEndPoint!).Port;
        }
        catch (SocketException)
        {
            using var ipV6socket = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
            ipV6socket.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));
            return ((IPEndPoint)ipV6socket.LocalEndPoint!).Port;
        }
    }
}