You can find a Table of Contents implementation in the Toolkit. That's the link for the source code.
The Toolkit components are currently in beta - you can get a NuGet package for the latest master build by adding the URL `https://ci.appveyor.com/nuget/arcgis-toolkit-dotnet` as a NuGet feed in the Visual Studio Package Manager. For more info see Setting up Local NuGet Feeds | Microsoft Docs.
If You are using version 100.4, then you need to add two nuget packages ArcGISRuntime Toolkit 100.4 and ArcGIS Runtime Toolkit Preview 100.4 . TableOfContents class is available in ArcGISRuntime Toolkit Preview Nuget Package.
Hope I could help