Mocking DbSet<T>

DbSetInitializer.cs

public static class DbSetInitializer
  {
    public static void InitDb<T>(IQueryable<T> set, IQueryable<T> data) where T : class
    {
      set.Provider.Returns(data.Provider);
      set.Expression.Returns(data.Expression);
      set.ElementType.Returns(data.ElementType);
      set.GetEnumerator().Returns(data.GetEnumerator());
      set.AsNoTracking().Returns(data);
    }

    public static void InitDb<T>(DbSet<T> set, List<T> data) where T : class
    {
      InitDb(set, data.AsQueryable());
      set.When(x => x.Add(Arg.Any<T>())).Do(x => data.Add((T)x.Args()[0]));
      set.When(x => x.AddAsync(Arg.Any<T>())).Do(x => data.Add((T)x.Args()[0]));
      set.When(x => x.AddRangeAsync(Arg.Any<IEnumerable<T>>())).Do(x => data.AddRange((IEnumerable<T>)x.Args()[0]));
      set.When(x => x.AddRange(Arg.Any<IEnumerable<T>>())).Do(x => data.AddRange((IEnumerable<T>)x.Args()[0]));
    }
  }
  
  // Usage:
var list = new List<MyClass>(){
  new MyClass(),new MyClass(),new MyClass()
};
var db = Substitute.For<DbSet<MyClass>, IQueryable<MyClass>>();
DbSetInitializer.InitDb(db, list);
_context.MyClassDb.Returns(db); // MyClassDb is an instance of DbSet<MyClass>