When you migrate code from VB6 - regardless of whether you are using an automatic conversion tool - chances are that string-intensive code will actually run slower under VB.NET, if it uses a lot of string concatenation operations. For example, this code takes 2.8 seconds when it runs in VB6 and 27 seconds after its conversion to VB.NET on my 3GHz system:

Dim s As String = ""
For i As Integer = 1 To 100000
    s = s + "*"
Next

The solution is of course trivial: just replace the string variable with a StringBuilder object. Alas, this fix requires that you completely revise your source code, because you need to replace all + and & operators with the Append method, not to mention cases where the StringBuilder is used as an argument to string functions such as Trim or Len.

Is there a way to speed up the previous code with a minimal impact on the code itself? The answer is yes and the solution is actually very simple: you just need to create a VB.NET class that is based on the System.Text.StringBuilder object, that redifines the + and & operators, and that supports the implicit conversion to and from the System.String type. Authoring such a StringBuilder6 class is a matter of minutes:

Public Class StringBuilder6

    Private buffer As New System.Text.StringBuilder

    ' return the inner string

    Public Overrides Function ToString() As String
        Return buffer.ToString()
    End Function

    Public Shared Operator +(ByVal op1 As StringBuilder6, ByVal op2 As String) As StringBuilder6
        op1.buffer.Append(op2)
        Return op1
    End Operator

    Public Shared Operator &(ByVal op1 As StringBuilder6, ByVal op2 As String) As StringBuilder6
        op1.buffer.Append(op2)
        Return op1
    End Operator

    ' convert to string

    Public Shared Widening Operator CType(ByVal op As StringBuilder6) As String
        Return op.ToString()
    End Operator

    ' convert from string

    Public Shared Widening Operator CType(ByVal str As String) As StringBuilder6
        Dim op As New StringBuilder6()
        op.buffer.Append(str)
        Return op
    End Operator

End Class

Once the StringBuilder6 class is in place, you just need to replace the type of the String variable:

Dim s As StringBuilder6 = ""