Improve Dev Proxy API port conflict handling with helpful error messages

[Copilot]

When Dev Proxy tries to start and the API port (default 8897) is already in use, it previously showed an unhelpful error message with a long stack trace. This PR improves the user experience by providing clear, actionable guidance.

Changes Made

• Added specific IOException handling for API port binding failures in DevProxyCommand.cs
• When the API port is already in use, Dev Proxy now displays:
  • A clear error message indicating which port is blocked
  • Step-by-step resolution instructions
  • Example configuration showing how to set a custom API port
  • Reminder about updating Dev Proxy Toolkit extension settings if applicable

Before

fail    Hosting failed to start
      System.IO.IOException: Failed to bind to address http://127.0.0.1:8897: address already in use.
       ---> Microsoft.AspNetCore.Connections.AddressInUseException: Only one usage of each socket address...
       [Long stack trace continues...]

After

fail    Hosting failed to start
      [Stack trace for debugging...]
fail    Dev Proxy API port 8897 is already in use.
fail    To resolve this issue:
fail    1. Add the 'apiPort' property to your configuration file and set it to a different port number
fail    2. If you're using Dev Proxy Toolkit, update the API port extension setting to match your custom port
fail    Example configuration: { "apiPort": 8898 }

Testing

• ✅ Verified normal startup when API port is available
• ✅ Confirmed improved error message for default port (8897) conflicts
• ✅ Tested with custom API port configurations
• ✅ Ensured existing error handling for other exceptions remains unchanged
• ✅ Build passes all analyzer checks

The changes are minimal and surgical - only 8 lines of new error handling code were added while preserving all existing functionality.

Fixes #1273.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• aka.ms
  • Triggering command: curl -I -sSL --retry 5 --retry-delay 2 --connect-timeout 15 REDACTED (dns block)
  • Triggering command: /home/REDACTED/work/dev-proxy/dev-proxy/DevProxy/bin/Debug/net9.0/devproxy --help (dns block)
  • Triggering command: /home/REDACTED/work/dev-proxy/dev-proxy/DevProxy/bin/Debug/net9.0/devproxy (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.