Validating groups of entities

We've seen how we can validate single properties or entities, but what if we need to validate several entities at the same time? This may be because we cannot allow duplicate values, or for any other reason. Let's look at a possible solution.

We will be using NuGet Package Manager to install the Entity Framework Core 1 package, Microsoft.EntityFrameworkCore. We will also be using a SQL Server database for storing the data, so we will also need Microsoft.EntityFrameworkCore.SqlServer.

Finally, xunit is the package we will be using for the unit tests and dotnet-text-xunit adds tooling support for Visual Studio. Note that the UnitTests project is a .NET Core App 1.0 (netcoreapp1.0), where Microsoft.EntityFrameworkCore.Design ...

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