Debug Launch Failures for ASP.NET Core MVC Projects since new Workspace-based development hybrid experience

[sherrsw]

"Right-Click > Debug > Start New Instance" fails intermittently with port binding errors on ASP.NET Core 8.0 MVC projects

Severity

High (blocks debugging workflow)

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Affected Project Details

• Framework: ASP.NET Core 8.0
• Launch Configuration:
  • IIS Express (port 44308)
  • Kestrel (ports 5000/5001)
• Project Complexity:
  • Large monorepo with 100+ dependency injections
  • 7 SQL Server database contexts
  • Azure AD authentication integration

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Description

After updating to the latest C# and C# Dev Kit extensions, the debugging experience for ASP.NET Core projects is severely degraded. Attempting to start a new debug instance (Right-Click Project > Debug > Start New Instance) fails with "localhost refused to connect" errors.

Regression Information

Works 100% of the time when reverting to the prior version of C# and C# Dev Kit.

Working Versions

• C# Extension: v2.130.5
• C# Dev Kit: v3.10.14

Broken Versions

• C# Extension: v2.140.8+
• C# Dev Kit: v3.10.15+

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Actual Behavior

• ❌ Browser displays: "ERR_CONNECTION_REFUSED" or "This site can't be reached - localhost refused to connect"
• ❌ Dev tools indicate no service is listening on the configured ports
• ❌ The dotnet process fails to bind to ports 5000/5001
• ❌ Stopping the debugger sometimes leaves zombie dotnet processes
• ❌ Frequency: 95% of the time; only randomly does it work

Steps To Reproduce

Steps to Reproduce

1. Open an ASP.NET Core 8.0 MVC project in VS Code
2. Right-click the project folder
3. Select "Debug > Start New Instance"
4. Attempt to navigate to the configured URL (e.g., https://localhost:5001 or http://localhost:5000)

Expected Behavior

Expected Behavior

• ✅ App compiles and starts listening on the configured ports
• ✅ Browser connects successfully to the application
• ✅ Stopping the debugger cleanly terminates the process

Actual Behavior

• ❌ Browser displays: "ERR_CONNECTION_REFUSED" or "This site can't be reached - localhost refused to connect"
• ❌ Dev tools indicate no service is listening on the configured ports
• ❌ The dotnet process fails to bind to ports 5000/5001
• ❌ Stopping the debugger sometimes leaves zombie dotnet processes
• ❌ Frequency: 95% of the time; only randomly does it work

Environment Information

Component	Version/Details
VS Code	Latest
C# Extension	v2.140.8+ (issue occurs; works on v2.130.5)
C# Dev Kit	v3.20.199+ (issue occurs; works on v3.10.14)
.NET SDK	.NET 8.0
OS	Windows
Project Type	ASP.NET Core Web Application (MVC)

[sherrsw]

I now know that this failure only happens using Debug > Start new instance, not while running without debugging.

I can see it first hit my breakpoint in the home controller, then after returning the view, this happens:

Additional Evidence - Heap Corruption in vsdbg-ui.exe

Found the root cause in Windows Event Viewer. The debugger itself is crashing with heap corruption at the exact moment the exception occurs during view rendering.

20:44:48.6750108Z - Action completes: "Executed action method ... returned result ViewResult"
20:44:48.67Z - vsdbg-ui.exe crashes (simultaneous)
Result: HTTP response lost, ERR_CONNECTION_REFUSED in browser

Windows Event Viewer crash dump:

AppCrash.txt
WER.8519b651-b491-455b-a9fc-d100593a3b4a.tmp.WERInternalMetadata.xml

Faulting application: vsdbg-ui.exe v18.7.10427.2
Exception code: 0xc0000374 (HEAP CORRUPTION)
Faulting module: ntdll.dll v10.0.26100.8328
Fault offset: 0x0000000000117175
Report ID: 08e23cbd-dd2d-4d98-beb3-30d4e6a12ab5

Regression: Occurs only in C# Dev Kit 2.140.8 (updated 6/2/2026). Did not occur in previous version.