Use next available port when the default port is already used

src/Azure.Functions.Cli/Actions/HostActions/StartHostAction.cs

@@ -91,7 +91,7 @@ public override ICommandLineParserResult ParseArgs(string[] args)
             Parser
                 .Setup<int>('p', "port")
                 .WithDescription($"Local port to listen on. Default: {DefaultPort}")
-                .SetDefault(hostSettings.LocalHttpPort == default(int) ? DefaultPort : hostSettings.LocalHttpPort)
+                .SetDefault(hostSettings.LocalHttpPort == default(int) ? NetworkHelpers.GetNextAvailablePort(DefaultPort) : hostSettings.LocalHttpPort)
                 .Callback(p => Port = p);
 
             Parser

src/Azure.Functions.Cli/Helpers/NetworkHelpers.cs

@@ -1,4 +1,8 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
 using System.Net;
+using System.Net.NetworkInformation;
 using System.Net.Sockets;
 
 namespace Azure.Functions.Cli.Helpers
@@ -33,5 +37,40 @@ public static int GetAvailablePort()
                 listener.Stop();
             }
         }
+
+        public static int GetNextAvailablePort(int startPort)
+        {
+            // Check if the port is in the valid range
+            // Port numbers are in the range of 0 to 65535, but ports below 1024 are reserved for system use
+            if (startPort < 1024 || startPort > 65535)
+            {
+                throw new ArgumentOutOfRangeException(nameof(startPort), "Port number must be between 1024 and 65535.");
+            }
+
+            var usedPorts = new HashSet<int>();
+            var ipGlobalProperties = IPGlobalProperties.GetIPGlobalProperties();
+
+            usedPorts.UnionWith(ipGlobalProperties
+                                .GetActiveTcpConnections()
+                                .Where(c => c.LocalEndPoint.Port >= startPort)
+                                .Select(c => c.LocalEndPoint.Port));
+
+            usedPorts.UnionWith(ipGlobalProperties
+                                .GetActiveTcpListeners()
+                                .Where(l => l.Port >= startPort)
+                                .Select(l => l.Port));
+
+            // Find the next available port starting from the specified port
+            for (int port = startPort; port <= 65535; port++)
+            {
+                if (!usedPorts.Contains(port))
+                {
+                    return port;
+                }
+            }
+
+            // If no port is available after the specified start port, return the first available port
+            return GetAvailablePort();
+        }
     }
-}
+}

test/Azure.Functions.Cli.Tests/NetworkHelpersTests.cs

@@ -0,0 +1,56 @@
+using System;
+using System.Net.Sockets;
+using Xunit;
+using Azure.Functions.Cli.Helpers;
+using Xunit.Abstractions;
+
+namespace Azure.Functions.Cli.Tests.E2E
+{
+  public class NetworkHelpersTests : BaseE2ETest
+  {
+    public NetworkHelpersTests(ITestOutputHelper output) : base(output) { }
+
+    [Fact]
+    public void IsPortAvailable_PortIsAvailable_ReturnsTrue()
+    {
+      int port = NetworkHelpers.GetAvailablePort();
+      bool result = NetworkHelpers.IsPortAvailable(port);
+      Assert.True(result);
+    }
+
+    [Fact]
+    public void IsPortAvailable_PortIsNotAvailable_ReturnsFalse()
+    {
+      int port = NetworkHelpers.GetAvailablePort();
+      var listener = new TcpListener(System.Net.IPAddress.Any, port);
+      listener.Start();
+
+      bool result = NetworkHelpers.IsPortAvailable(port);
+      listener.Stop();
+
+      Assert.False(result);
+    }
+
+    [Fact]
+    public void GetAvailablePort_ReturnsValidPort()
+    {
+      int port = NetworkHelpers.GetAvailablePort();
+      Assert.InRange(port, 1024, 65535);
+    }
+
+    [Fact]
+    public void GetNextAvailablePort_ValidStartPort_ReturnsNextAvailablePort()
+    {
+      int startPort = 5000;
+      int port = NetworkHelpers.GetNextAvailablePort(startPort);
+      Assert.InRange(port, startPort, 65535);
+    }
+
+    [Fact]
+    public void GetNextAvailablePort_InvalidStartPort_ThrowsArgumentOutOfRangeException()
+    {
+      Assert.Throws<ArgumentOutOfRangeException>(() => NetworkHelpers.GetNextAvailablePort(1023));
+      Assert.Throws<ArgumentOutOfRangeException>(() => NetworkHelpers.GetNextAvailablePort(65536));
+    }
+  }
+}