Debugger: Some extension method evaluated in watch window would lead an error

## Environment data

`dotnet --info` output:
VS Code version:
C# Extension version:
## Steps to reproduce

1 Set a breakpoint at line of " _appSettings = options.Value; " in HomeController.cs
2 Press F5 to start debugging
3 Add a expression " "Hello".AsParallel()" in watch window
4 Press Enter
## Expected behavior

The value should display with "This expression causes side effects and will not be evaluated"
## Actual behavior

The value displays with '
error CS1061: 'string' does not contain a definition for 'AsParallel' and no extension method 'AsParallel' accepting a first argument of type 'string' could be found (are you missing a using directive or an assembly reference?)'