SL5 String Bug in MAC SL5 Runtime

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macOS

Silverlight

I've come across a problem that happens on some and not all MACs - approx 5% of our MAC users. It does not appear to have anything to do with 32/64bit or OS version or Hardware type.

It is to do with a custom search and replace routine to fix CR/LF in a string that is up to 2,000 characters long.

Here's the routine:

Public Function FixCRLF(ByVal InputString As String) As String
        Return FixCR(FixLF(InputString))
    End Function

    Public Function FixCR(ByVal InputString As String) As String

        If InputString Is Nothing Then Return Nothing

        Dim result As New StringBuilder()
        Dim oldValue As String = vbCr
        Dim lenOldValue As Integer = oldValue.Length
        Dim curPosition As Integer = 0

        Dim idxNext As Integer = InputString.IndexOf(oldValue)
        While idxNext >= 0
            result.Append(InputString, curPosition, idxNext - curPosition)
            If Not InputString.IndexOf(vbLf, idxNext) = idxNext + 1 Then
                result.Append(vbCrLf)
            Else
                result.Append(vbCr)
            End If
            curPosition = idxNext + 1
            idxNext = InputString.IndexOf(oldValue, curPosition)
        End While

        result.Append(InputString, curPosition, InputString.Length - curPosition)
        Return result.ToString()

    End Function

    Public Function FixLF(ByVal InputString As String) As String

        If InputString Is Nothing Then Return Nothing

        Dim result As New StringBuilder()
        Dim oldValue As String = vbLf
        Dim lenOldValue As Integer = oldValue.Length
        Dim curPosition As Integer = 0

        Dim idxNext As Integer = InputString.IndexOf(oldValue)
        While idxNext >= 0
            result.Append(InputString, curPosition, idxNext - curPosition)
            Dim idxBack As Integer = idxNext - 1
            If Not InputString.IndexOf(vbCr, idxBack) = idxBack Then
                result.Append(vbCrLf)
            Else
                result.Append(vbLf)
            End If
            curPosition = idxNext + 1
            idxNext = InputString.IndexOf(oldValue, curPosition)
        End While

        result.Append(InputString, curPosition, InputString.Length - curPosition)
        Return result.ToString()

    End Function

We see the CPU spike to 100% for approx 5 to 25 minutes showing the pinwheel (busy) cursor then eventually return.

Windows SL5 runtimes do not experience this problem.

Is this a runtime bug or an error in my coding?

Graeme