Add using System.ComponentModel.DataAnnotations 
Add using System.ComponentModel.DataAnnotations 

In the code for Speaker.cs - using statement is missing System.ComponentModel.DataAnnotations.
Added an entry to  System.ComponentModel.DataAnnotations in the using statements
csharpfritz/aspnetcore-app-workshop,csharpfritz/aspnetcore-app-workshop,dotnet-presentations/aspnetcore-app-workshop,csharpfritz/aspnetcore-app-workshop,dotnet-presentations/aspnetcore-app-workshop,jongalloway/aspnetcore-app-workshop,dotnet-presentations/aspnetcore-app-workshop,csharpfritz/aspnetcore-app-workshop,jongalloway/aspnetcore-app-workshop,dotnet-presentations/aspnetcore-app-workshop