public int this[string key] => 0;

System.InvalidCastException: Cannot cast an object of type "Microsoft.CodeAnalysis.CSharp.Symbols.PublicModel.PropertySymbol" to type "Microsoft.CodeAnalysis.IMethodSymbol".
      at Microsoft.CodeAnalysis.ValueTracking.ValueTracker.<TrackParameterSymbolAsync>d__4.MoveNext()
      at Microsoft.CodeAnalysis.ValueTracking.ValueTracker.<TrackValueSourceAsync>d__1.MoveNext()
      at Microsoft.CodeAnalysis.Remote.RemoteValueTrackingService.<>c__DisplayClass3_0.<<TrackValueSourceAsync>b__0>d.MoveNext()
      at Microsoft.CodeAnalysis.Remote.BrokeredServiceBase.<RunServiceImplAsync>d__12`1.MoveNext()