opened
alexhelms
username_0
**Operating System**: Windows

**DocFX Version Used**: 2.56.5

**Template used**: default

**Steps to Reproduce**:

1. Create a property like `public string MyString { get; init; }`
2. Run docfx

**Expected Behavior**: Documentation is generated.

**Actual Behavior**: An exception is thrown, see below for stack trace.

To find this I had to run docfx from source and in debug mode and trace back from the exception. The hint I had was the name of the property in a function further up the call stack.

I understand #6805 is for C# 9 support but I don't see any implementation or notes about `init` properties. At work we recently upgraded to .NET 5 and this is the first time I tried to use an `init` property and our CI failed during doc generation and took a considerable amount of time to find the root cause.

Thanks!

```
Microsoft.DocAsCode.Exceptions.DocfxException: Unable to generate spec reference for !: ---> System.IO.InvalidDataException: Fail to parse id for symbol  in namespace .
   at Microsoft.DocAsCode.Metadata.ManagedReference.YamlModelGenerator.AddSpecReference(ISymbol symbol, IReadOnlyList`1 typeGenericParameters, IReadOnlyList`1 methodGenericParameters, Dictionary`2 references, SymbolVisitorAdapter adapter)
   at Microsoft.DocAsCode.Metadata.ManagedReference.SymbolVisitorAdapter.AddSpecReference(ISymbol symbol, IReadOnlyList`1 typeGenericParameters, IReadOnlyList`1 methodGenericParameters)
   --- End of inner exception stack trace ---
   at Microsoft.DocAsCode.Metadata.ManagedReference.SymbolVisitorAdapter.AddSpecReference(ISymbol symbol, IReadOnlyList`1 typeGenericParameters, IReadOnlyList`1 methodGenericParameters)
   at Microsoft.DocAsCode.Metadata.ManagedReference.SymbolVisitorAdapter.AddMethodSyntax(IMethodSymbol symbol, MetadataItem result, IReadOnlyList`1 typeGenericParameters, IReadOnlyList`1 methodGenericParameters)
   at Microsoft.DocAsCode.Metadata.ManagedReference.SymbolVisitorAdapter.VisitMethod(IMethodSymbol symbol)
   at Microsoft.DocAsCode.Metadata.ManagedReference.SymbolVisitorAdapter.VisitNamedType(INamedTypeSymbol symbol)
   at Microsoft.DocAsCode.Metadata.ManagedReference.SymbolVisitorAdapter.VisitDescendants[T](IEnumerable`1 children, Func`2 getChildren, Func`2 filter)
   at Microsoft.DocAsCode.Metadata.ManagedReference.SymbolVisitorAdapter.VisitNamespace(INamespaceSymbol symbol)
   at Microsoft.DocAsCode.Metadata.ManagedReference.SymbolVisitorAdapter.VisitAssembly(IAssemblySymbol symbol)
   at Microsoft.DocAsCode.Metadata.ManagedReference.RoslynMetadataExtractor.Extract(ExtractMetadataOptions options)
   at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.GetMetadataFromProjectLevelCache(IBuildController controller, IInputParameters key)
   at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.<SaveAllMembersFromCacheAsync>d__13.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.<ExtractMetadataAsync>d__11.MoveNext()
        0 Warning(s)
        1 Error(s)
```
Support C# 9 init properties
issue