About loading navigation properties

Are you connecting to a typed DbContext in your own assembly? If so, you must enable lazy-loading in your typed DbContext class as follows:

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    optionsBuilder.UseLazyLoadingProxies();
    base.OnConfiguring(optionsBuilder);
}

You will also need to add a NuGet package reference to Microsoft.EntityFrameworkCore.Proxies:

<PackageReference Include="Microsoft.EntityFrameworkCore.Proxies" Version="6.0" />

If you want to enable lazy loading only when querying from LINQPad, see https://linqpad.azureedge.net/public/SampleDbContext.cs

Hi,
maybe your navigation properties are not virtual. In that case, the proxy cannot replace them.

public class TestProject
{
    ...
    public virtual ICollection<TestItems> TestItems{get;set;}
}