IAMCameraControl NullReferenceException Error

[kevinlin]

Hello,erveryone, I want to write a program to get the exposure time of a
webcam and I was using IAMCameraControl. But there was an error in my program
and I couldn't find out what the problem is.

Private CamControl As IAMCameraControl

Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Dim data As Integer
        Dim iFlags As CameraControlFlags
        Dim result As Integer = CamControl.Get(CameraControlProperty.Exposure, data, iFlags)
        TextBox1.Text = String.Concat(data.ToString)

    End Sub

When the program excuted the get function, it showed an NullReferenceException
Error. Can anyone help me point out what the error is?

[kevinlin]

BTW,the error is System.NullReferenceException – Object reference not set to
an instance of an object.

[snarfle]

Where do you assign a value to CamControl?

[kevinlin]

Assign a value to CamControl?Why?I did't do that.

[kevinlin]

What value should I assigned to?

[kevinlin]

Hi,snarfle, I get another question, If I have more than one webcam used in my
program, how can I specify which camera is using?The belowing is my whole
program. Now my program have no error but can not get the expected value. All
the three values of exposure,focus,zoom are 0. I guess it's because of the
selected video device. Is it?Can you give me some guidline on how to solve it?
Thank you so much for your kind help.

Private capDevices As DsDevice()
    Private CamDevice As DsDevice
    Private CamFilter As IBaseFilter = Nothing
    Private CamControl As IAMCameraControl

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'Enumerate video Input filters and add them to comboBox1
        capDevices = DsDevice.GetDevicesOfCat(FilterCategory.VideoInputDevice)
        For Each ds As DsDevice In capDevices
            ComboBox1.Items.Add(ds.Name)
        Next

    End Sub

 Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click

        ' Setup()
        Dim expo, focus, zoom As Integer

        GetControlProperties(CameraControlProperty.Exposure, expo)
        TextBox1.Text = String.Concat(expo.ToString)

        GetControlProperties(CameraControlProperty.Focus, focus)
        TextBox2.Text = String.Concat(focus.ToString)

        GetControlProperties(CameraControlProperty.Zoom, zoom)
        TextBox3.Text = String.Concat(zoom.ToString)

    End Sub

    Private Sub Setup()
        CamControl = Nothing
        'Get the IAMCameraControl
        Dim devs As DsDevice() = DsDevice.GetDevicesOfCat(FilterCategory.VideoInputDevice)
        'Get the graphbuilder object
        Dim graphBuilder As IFilterGraph2 = TryCast(New FilterGraph(), IFilterGraph2)
        'add the video input device
        Dim capFilter As IBaseFilter = Nothing
        Dim hr As Integer = graphBuilder.AddSourceFilterForMoniker(devs(0).Mon, Nothing, devs(0).Name, capFilter)
        DsError.ThrowExceptionForHR(hr)
        CamControl = TryCast(capFilter, IAMCameraControl)
        Debug.Assert(CamControl IsNot Nothing)

    End Sub

    Private Sub GetControlProperties(ByVal CtrlProp As DirectShowLib.CameraControlProperty, ByRef value As Integer)
        Dim hr As Integer
        'Dim iValue As Integer
        Dim iFlags As CameraControlFlags
        hr = CamControl.Get(CtrlProp, value, iFlags)
        'Dim Result As Integer = CamControl.Get(CtrlProp, value, DirectShowLib.CameraControlFlags.Manual)
    End Sub


    Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        If ComboBox1.SelectedIndex <> -1 Then
            CamDevice = capDevices(ComboBox1.SelectedIndex)
            Dim graphBuilder As IFilterGraph2 = CType(New FilterGraph, IFilterGraph2)
            Dim hr As Integer = graphBuilder.AddSourceFilterForMoniker(CamDevice.Mon, Nothing, CamDevice.Name, CamFilter)
            CamControl = CType(CamFilter, IAMCameraControl)
        End If
    End Sub

    Private Function CreateFilter(ByVal category As Guid, ByVal friendlyname As String) As IBaseFilter
        Dim source As Object = Nothing
        Dim iid As Guid = GetType(IBaseFilter).GUID
        For Each device As DsDevice In DsDevice.GetDevicesOfCat(category)
            If device.Name.CompareTo(friendlyname) = 0 Then
                device.Mon.BindToObject(Nothing, Nothing, iid, source)
                Exit For
            End If
        Next

        Return DirectCast(source, IBaseFilter)
    End Function