Diagnostic console for C# & .NET exceptions
Paste an exception message, stack trace, or the offending code. The more context you give, the sharper the diagnosis.
.NET Doctor is a free, in-browser tool for developers debugging C# and .NET applications. Paste an exception message, a stack trace, or a suspicious snippet of code, and it will identify the most likely root cause, explain it in plain English, show a tested fix with before-and-after code, and verify that the fix actually resolves the matched pattern. If nothing in its library matches, it says so directly and points you to Microsoft Learn, Stack Overflow, and the .NET runtime issue tracker instead of guessing.
The most common runtime error in C# — calling a member on a null reference.
Blocking on async code with .Result or .Wait().
InvalidOperationException from mutating a list mid-foreach.
Entity Framework Core objects used after disposal or across scopes.
Captive dependencies between singleton and scoped services.
Cross-origin requests blocked by missing or misordered middleware.
It matches your pasted text against a library of known C# and .NET failure signatures and returns the closest diagnosis, an explanation of the root cause, and a fix.
No — everything runs client-side as pattern matching. It does not execute your code. For live execution, pair it with a sandbox such as dotnetfiddle.net.
It tells you plainly, rather than inventing an answer, and links to Microsoft Learn, Stack Overflow, and the dotnet/runtime GitHub issues so you can keep digging.
Yes, no sign-up required.