C# と VB.NET の質問掲示板

横より失礼します。
ちょっとコードを書いて見ました。

新規プロジェクトのフォームにラベル3個とボタン1個で動くと思います。

Public Class Form1

    Private Comp As Compare = New Compare()

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Me.Button1.Text = "Next"

        Comp.ReadTextA("C:\AAA.txt")
        Comp.ReadTextB("C:\BBB.txt")

        Comp.MoveFirst()

        AddHandler Comp.NextResult, AddressOf OnNextResult
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Comp.OnNextResult()
    End Sub

    Private Sub OnNextResult(value1 As String, value2 As String, value3 As String)
        Me.Label1.Text = value1
        Me.Label2.Text = value2
        Me.Label3.Text = value3
    End Sub

End Class


Public Class Compare
    Private TextA As List(Of String) = New List(Of String)
    Private TextB As List(Of String) = New List(Of String)

    Public Sub ReadTextA(path As String)
        TextA.Clear()
        TextA.AddRange(Read(path))
    End Sub
    Public Sub ReadTextB(path As String)
        TextB.Clear()
        TextB.AddRange(Read(path))
    End Sub

    Private Iterator Function Read(path As String) As IEnumerable(Of String)
        Using sr As New IO.StreamReader(path)
            While (Not sr.EndOfStream)
                Yield sr.ReadLine()
            End While
        End Using
    End Function

    Private _Current As Integer = 0

    Public Sub MoveFirst()
        _Current = 0
    End Sub

    Public Property EndOfList()
        Get
            Return (_Current >= TextA.Count And _Current >= TextB.Count)
        End Get
        Private Set(value)

        End Set
    End Property

    Public Event NextResult As Action(Of String, String, String)

    Public Sub OnNextResult()
        If EndOfList Then Exit Sub

        Dim retA As String = TextA.Skip(_Current).Take(1).FirstOrDefault()
        Dim retB As String = TextB.Skip(_Current).Take(1).FirstOrDefault()

        _Current += 1

        If retA Is Nothing OrElse retB Is Nothing Then
            If retA Is Nothing AndAlso retB Is Nothing Then
                RaiseEvent NextResult(retA, retB, Nothing) ' 両方ともNothing
            Else
                RaiseEvent NextResult(retA, retB, "") ' 片方だけNothing
            End If

            Exit Sub
        End If

        RaiseEvent NextResult(retA, retB, IIf(retA = retB, retA, ""))
    End Sub
End Class

# ファイル読み込みのエラー処理とかしてません。