Error: Object reference not set to an instance of an object

[HansW]

Hi there,
I have a class with the following method in it:

 
Public Class RedControlTracker
    Dim frmainInst As New frmMain

    Sub New()
        ' Constructor
        GetRedControl()
    End Sub

    Public Function GetRedControl() As Control
        Dim c As Control
        For Each c In frmainInst.Panel3.Controls
            If c.BackColor.Equals(Color.Red) Then
                Return c
            End If
        Next
    End Function
End Class

I have a button in my frmMain form with the following code:

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Dim c As New RedControlTracker
        MessageBox.Show("Name of the Red Control is: " & c.GetRedControl.Name)
    End Sub

When I press the button I get the following error:
Object reference not set to an instance of an object

What am I doing wrong?

There is a control with red backround in my Panel3 control. I have not had this error before, probably because my knowledge of classes is very limited. I can get it to work, doing all the coding in my frmMain (form) class, but it is getting kind of crouded.
Thank you for any help,
Hans

[Gizmo001]

You need to make the following changes:

1. Dim frmainInst As frmMain = New frmMain
2. Dim c As RedControlTracker = New RedControlTracker

3. You don't need to call getRedControl from Sub New.
4. Because of 'return c' statement, the sub will only show the first control with red backcolor and get out of the loop. If that is what is intended, fine.

Good luck.

[HansW]

Changed to the following:

Public Class RedControlTracker
    Dim frmainInst As frmMain = New frmMain

    Sub New()
        ' Constructor
     End Sub

    Public Function GetRedControl() As Control
        Dim c As Control
        For Each c In frmainInst.Panel3.Controls
            If c.BackColor.Equals(Color.Red) Then
                Return c
            End If
        Next
    End Function
End Class

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Dim c As RedControlTracker = New RedControlTracker
        MessageBox.Show("Name of the Red Control is: " & c.GetRedControl.Name)
    End Sub

It still throws the same error at the same line of code

MessageBox.Show("Name of the Red Control is: " & c.GetRedControl.Name)

Hans

[sotoasty]

Here is my solution.

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Dim c As New RedControlTracker(Me)
        MessageBox.Show("Name of the Red Control is: " & c.GetRedControl.Name)
    End Sub

Public Class RedControlTracker
    Dim frmainInst As frmMain

    Sub New(ByVal frm As frmMain)
        ' Constructor
        frmainInst = frm
    End Sub

    Public Function GetRedControl() As Control
        Dim c As Control
        For Each c In frmainInst.Panel3.Controls
            If c.BackColor.Equals(Color.Red) Then
                Return c
            End If
        Next
    End Function
End Class

[sotoasty]

Here is more flexible Class you can use to find the first red control.

Public Class Form1
    Inherits System.Windows.Forms.Form

#Region " Windows Form Designer generated code "

    Public Sub New()
        MyBase.New()

        'This call is required by the Windows Form Designer.
        InitializeComponent()

        'Add any initialization after the InitializeComponent() call

    End Sub

    'Form overrides dispose to clean up the component list.
    Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
        If disposing Then
            If Not (components Is Nothing) Then
                components.Dispose()
            End If
        End If
        MyBase.Dispose(disposing)
    End Sub

    'Required by the Windows Form Designer
    Private components As System.ComponentModel.IContainer

    'NOTE: The following procedure is required by the Windows Form Designer
    'It can be modified using the Windows Form Designer.  
    'Do not modify it using the code editor.
    Friend WithEvents PictureBox1 As System.Windows.Forms.PictureBox
    Friend WithEvents Button1 As System.Windows.Forms.Button
    Friend WithEvents Panel1 As System.Windows.Forms.Panel
    Friend WithEvents Panel2 As System.Windows.Forms.Panel
    Friend WithEvents Panel3 As System.Windows.Forms.Panel
    Friend WithEvents TextBox1 As System.Windows.Forms.TextBox
    Friend WithEvents Label1 As System.Windows.Forms.Label
    Friend WithEvents TextBox2 As System.Windows.Forms.TextBox
    <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()
        Me.PictureBox1 = New System.Windows.Forms.PictureBox
        Me.Button1 = New System.Windows.Forms.Button
        Me.Panel1 = New System.Windows.Forms.Panel
        Me.Panel2 = New System.Windows.Forms.Panel
        Me.Panel3 = New System.Windows.Forms.Panel
        Me.TextBox1 = New System.Windows.Forms.TextBox
        Me.Label1 = New System.Windows.Forms.Label
        Me.TextBox2 = New System.Windows.Forms.TextBox
        Me.Panel1.SuspendLayout()
        Me.Panel3.SuspendLayout()
        Me.SuspendLayout()
        '
        'PictureBox1
        '
        Me.PictureBox1.Location = New System.Drawing.Point(46, 327)
        Me.PictureBox1.Name = "PictureBox1"
        Me.PictureBox1.Size = New System.Drawing.Size(760, 149)
        Me.PictureBox1.TabIndex = 0
        Me.PictureBox1.TabStop = False
        '
        'Button1
        '
        Me.Button1.Location = New System.Drawing.Point(73, 16)
        Me.Button1.Name = "Button1"
        Me.Button1.TabIndex = 1
        Me.Button1.Text = "Button1"
        '
        'Panel1
        '
        Me.Panel1.Controls.Add(Me.TextBox1)
        Me.Panel1.Location = New System.Drawing.Point(246, 84)
        Me.Panel1.Name = "Panel1"
        Me.Panel1.Size = New System.Drawing.Size(162, 60)
        Me.Panel1.TabIndex = 2
        '
        'Panel2
        '
        Me.Panel2.Location = New System.Drawing.Point(431, 215)
        Me.Panel2.Name = "Panel2"
        Me.Panel2.Size = New System.Drawing.Size(162, 60)
        Me.Panel2.TabIndex = 3
        '
        'Panel3
        '
        Me.Panel3.Controls.Add(Me.Label1)
        Me.Panel3.Controls.Add(Me.TextBox2)
        Me.Panel3.Location = New System.Drawing.Point(532, 54)
        Me.Panel3.Name = "Panel3"
        Me.Panel3.Size = New System.Drawing.Size(162, 60)
        Me.Panel3.TabIndex = 3
        '
        'TextBox1
        '
        Me.TextBox1.BackColor = System.Drawing.Color.Red
        Me.TextBox1.Location = New System.Drawing.Point(29, 25)
        Me.TextBox1.Name = "TextBox1"
        Me.TextBox1.Size = New System.Drawing.Size(33, 20)
        Me.TextBox1.TabIndex = 0
        Me.TextBox1.Text = "TextBox1"
        '
        'Label1
        '
        Me.Label1.Location = New System.Drawing.Point(25, 17)
        Me.Label1.Name = "Label1"
        Me.Label1.Size = New System.Drawing.Size(51, 22)
        Me.Label1.TabIndex = 1
        Me.Label1.Text = "Label1"
        '
        'TextBox2
        '
        Me.TextBox2.BackColor = System.Drawing.Color.Red
        Me.TextBox2.Location = New System.Drawing.Point(36, 32)
        Me.TextBox2.Name = "TextBox2"
        Me.TextBox2.Size = New System.Drawing.Size(33, 20)
        Me.TextBox2.TabIndex = 4
        Me.TextBox2.Text = "TextBox2"
        '
        'Form1
        '
        Me.AutoScaleBaseSize = New System.Drawing.Size(5, 13)
        Me.ClientSize = New System.Drawing.Size(855, 511)
        Me.Controls.Add(Me.Panel1)
        Me.Controls.Add(Me.Button1)
        Me.Controls.Add(Me.PictureBox1)
        Me.Controls.Add(Me.Panel3)
        Me.Controls.Add(Me.Panel2)
        Me.Name = "Form1"
        Me.Text = "Form1"
        Me.Panel1.ResumeLayout(False)
        Me.Panel3.ResumeLayout(False)
        Me.ResumeLayout(False)

    End Sub

#End Region

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim c As New RedControlTracker
        c.RetRedControl(Me)
        MessageBox.Show("Name of the first Red Control on the form is: " & c.Name & ".  It is located on " & c.Parent)
        MessageBox.Show("Name of the first Red Control on Panel3 is: " & c.GetRedControl(Me.Panel3).Name)
    End Sub
End Class

For the Class

Public Class RedControlTracker
    Private mvarName As String
    Private mvarControl As Control
    Private mvarParent As String
    Friend ReadOnly Property Parent() As String
        Get
            Return mvarControl.Parent.Name
        End Get
    End Property
    Friend ReadOnly Property Name() As String
        Get
            Return mvarControl.Name
        End Get
    End Property
    Friend ReadOnly Property ControlObject() As Control
        Get
            Return mvarControl
        End Get
    End Property

    Public Sub RetRedControl(ByRef ParentObject As Object)
        Dim c As Control
        For Each c In ParentObject.Controls
            If c.BackColor.Equals(Color.Red) Then
                mvarControl = c
                Exit Sub
            End If
        Next

        For Each c In ParentObject.Controls
            Select Case c.GetType.ToString.ToUpper
                Case "System.Windows.Forms.TabControl".ToUpper
                    RetRedControl(c)
                    Exit Sub
                Case "System.Windows.Forms.TabPage".ToUpper
                    RetRedControl(c)
                    Exit Sub
                Case "System.Windows.Forms.GroupBox".ToUpper
                    RetRedControl(c)
                    Exit Sub
                Case "System.Windows.Forms.Panel".ToUpper
                    RetRedControl(c)
                    Exit Sub
            End Select
        Next
        mvarControl = Nothing
    End Sub

    Public Function GetRedControl(ByRef ParentObject As Object) As Control
        Dim c As Control
        For Each c In ParentObject.Controls
            If c.BackColor.Equals(Color.Red) Then
                Return c
            End If
        Next

        For Each c In ParentObject.Controls
            Select Case c.GetType.ToString.ToUpper
                Case "System.Windows.Forms.TabControl".ToUpper
                    Return GetRedControl(c)
                Case "System.Windows.Forms.TabPage".ToUpper
                    Return GetRedControl(c)
                Case "System.Windows.Forms.GroupBox".ToUpper
                    Return GetRedControl(c)
                Case "System.Windows.Forms.Panel".ToUpper
                    Return GetRedControl(c)
            End Select
        Next
        Return Nothing
    End Function
End Class

Note, This code will error out if there is no red control on the form. To run, create a new project with Form1. Add a new class. Replace ALL the code in the form with the top Code Section. Replace ALL the code in the class with the bottom code section.

[Gizmo001]

Hans:

I tried the code you have posted after making the changes I suggested. It works as long as you have at least one control with red backcolor. If there is no control with red backcolor, an exception is thrown because c.GetRedControl.Name is not defined. To avoid the exception from being displayed, put the messagebox.show in a try catch end try as follows:

Try
Messagebox.Show(("Name of the Red Control is: " & c.GetRedControl.Name)
catch
end try

Good luck.

[HansW]

sotoasty,
Thank you for your examples. Works great!

Gizmo001,
Thank you for helping as well. I could not get it to work your way, maybe because the "red" control was created dynamically.

In any case thatnk you both for your time and effort,
Hans