Prywatna klasa?

ControllerExtensions.cs

public static class ControllerExtensions
{
    public static string GetUserClaim(this ApiController controller, string claimName)
    {
        var principal = controller.User as ClaimsPrincipal;

        if (principal == null)
        {
            return null;
        }

        var claim = principal.Claims.FirstOrDefault(c => c.Type == claimName);

        if (claim == null)
        {
            return null;
        }

        return claim.Value;
    }

    public static T GetUserClaim<T>(this ApiController controller, string claimName)
    {
        var val = controller.GetUserClaim(claimName);

        return (T)Convert.ChangeType(
            val,
            typeof(T)
        );
    }
}

ControllerExtensionsTest.cs

public class ControllerExtensionsTest
{
    private string _claimName = "userId";
    private int _claimValue = 123;

    private TestController Create()
    {
        var controller = new TestController();

        controller.User = new ClaimsPrincipal(new ClaimsIdentity(new [] {new Claim(_claimName, _claimValue.ToString())}));

        return controller;
    }
    [Fact]
    public void GetUserClaim_Return_Null_When_User_Is_Null()
    {
        var controller = Create();
        controller.User = null;

        var claim = controller.GetUserClaim("claimName");

        claim.Should().BeNull();
    }

    [Fact]
    public void GetUserClaim_Return_Null_When_Claim_Does_Not_Exist()
    {
        var controller = Create();

        var claim = controller.GetUserClaim("claimName");

        claim.Should().BeNull();
    }

    [Fact]
    public void GetUserClaim_Return_Claim()
    {
        var controller = Create();

        var claim = controller.GetUserClaim(_claimName);

        claim.Should().NotBeNull();
        claim.Should().Be(_claimValue.ToString());
    }

    [Fact]
    public void Generic_GetUserClaim_Return_Claim()
    {
        var controller = Create();

        var claim = controller.GetUserClaim<int>(_claimName);

        claim.Should().Be(_claimValue);
    }

    private class TestController : ApiController
    {

    }
}

OuterClass.cs

public class OuterClass
{
    private class InnerClass
    {

    }
}

PrivateClass.cs

private class PrivateClass
{
}