Note: An answer in either VB.NET or C# will be fine. I have no preference for this Q&A.

I’m trying to test my networking code and I’m running into a very strange problem: the GatewayAddresses collection on my mocked NetworkInterface always returns a Count of 0, even though its Enumerator‘s internal list clearly contains an item:

I inserted this debugging code just before the assertions, and it works as expected:

Dim oEnumerator2 As IEnumerator(Of GatewayIPAddressInformation) = oGatewayAddresses.GetEnumerator

While oEnumerator2.MoveNext()
  Dim oCurrentGatewayInfo As GatewayIPAddressInformation = oEnumerator2.Current
  Me.Output.WriteLine($"Gateway IP Address: {oCurrentGatewayInfo.Address}")
End While

The string Gateway IP Address: 192.168.1.1 is written to the test results.

This assertion also succeeds:

Dim lMoved = oGatewayAddresses.GetEnumerator.MoveNext
oGatewayAddresses.GetEnumerator.Current.Address.Should.Be(oIPAddress)

But no matter what I try, the collection count always returns zero.

What could be the problem? Here’s my code:

Service

Public Interface INicService
  Function GetActiveNetworkInterfaces() As IEnumerable(Of NetworkInterface)
End Interface

Test

<Fact>
Public Sub GetActiveNetworkInterfaces_Should_Return_Correct_Gateway_Address()
  ' Arrange
  Dim oMockGatewayAddresses As GatewayIPAddressInformationCollection
  Dim oGatewayAddresses As GatewayIPAddressInformationCollection
  Dim oMockGatewayInfo As GatewayIPAddressInformation
  Dim oGatewayList As List(Of GatewayIPAddressInformation)
  Dim oEnumerator As IEnumerator(Of GatewayIPAddressInformation)
  Dim oActiveNics As IEnumerable(Of NetworkInterface)
  Dim oNicService As INicService
  Dim oIPAddress As IPAddress
  Dim oMockNic1 As NetworkInterface

  oMockGatewayAddresses = Substitute.For(Of GatewayIPAddressInformationCollection)
  oMockGatewayInfo = Substitute.For(Of GatewayIPAddressInformation)
  oIPAddress = IPAddress.Parse("192.168.1.1")
  oMockNic1 = Substitute.For(Of NetworkInterface)

  ' Mock GatewayIPAddressInformation
  oMockGatewayInfo.Address.Returns(oIPAddress)
  oMockNic1.OperationalStatus.Returns(OperationalStatus.Up)

  ' Mock GatewayIPAddressInformationCollection
  oGatewayList = New List(Of GatewayIPAddressInformation) From {oMockGatewayInfo}

  ' Create an enumerator that implements IEnumerator(Of GatewayIPAddressInformation)
  oEnumerator = oGatewayList.GetEnumerator

  ' Configure the mock to return the enumerator
  oMockGatewayAddresses.GetEnumerator.Returns(oEnumerator)

  ' Configure GetIPProperties.GatewayAddresses to return the mocked collection
  oMockNic1.GetIPProperties.GatewayAddresses.Returns(oMockGatewayAddresses)

  oNicService = Substitute.For(Of INicService)
  oNicService.GetActiveNetworkInterfaces.Returns({oMockNic1})

  ' Act
  oActiveNics = oNicService.GetActiveNetworkInterfaces
  oGatewayAddresses = oActiveNics.First.GetIPProperties.GatewayAddresses

  ' Assert
  oGatewayAddresses.Should.NotBeNull()
  oGatewayAddresses.Should.HaveCount(1) ' <-- Fails here
  oGatewayAddresses.First.Address.Should.Be(oIPAddress)
End Sub