Merging .TXT files

[4x2y]

You code for reading files is buggy, because FileData1.Split() is split by space while you must split by vbNewLine

try this one

        Try

            Dim File1Data As String() = IO.File.ReadAllLines(txtBoxXas1.Text) ' READ FILE #1
            Dim File2Data As String() = IO.File.ReadAllLines(txtBoxXas2.Text) ' READ FILE #2

            ' get the smallest upper bound to prevent 'out of range' exception inside the for...next 
            Dim intUpperBound As Integer
            If File1Data.Length < File2Data.Length Then
                intUpperBound = File1Data.Length
            Else
                intUpperBound = File2Data.Length
            End If

            For x = 0 To intUpperBound - 1
                If File1Data(x) <> File2Data(x) Then

                Else

                End If

            Next

        Catch ex As Exception

            MessageBox.Show(ex.ToString)

        End Try