if richtextbox2.lines.doesnt.contains(richtextbox1.lines

then add richtextbox2.lines to richtextbox3
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my code:

For Each RTB1 As String In RichTextBox1.Lines
For Each RTB2 As String In RichTextBox1.Lines
If RTB1.Contains(RTB2) Then
Else
RichTextBox4.AppendText(Environment.NewLine & RTB2)
End If
Next
Next
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error:

Cannot access a disposed object.
Object name: 'RichTextBox'.