Parse floats to a custom numeric format string

Question

I'm working to implement a custom string format. These are the float values to convert into a string:

1,02458
0,4145805
0,06231292
0,04362812
1,16458
1,172608
0,06011338
0,05324263
0,0653288
0,0285034
0,0992517
0,0861678

This is the output I'm getting:

1.02458f
.4145805f
6.231292E-02f
4.362812E-02f
1.16458f
1.172608f
6.011338E-02f
5.324263E-02f
.0653288f
.0285034f
.0992517f
.0861678f

The idea is, if the decimal places are greater than 8, use the scientific notation. If not, remove the left zeros. Now I'm using this code, but I would like to know if there's another more efficient way of doing this but always getting the same output:

Dim numericProvider As New NumberFormatInfo With {
    .NumberDecimalSeparator = "."
}

'Get the number of decimal places
Dim stringNum As String = waveDuration.ToString(numericProvider)
Dim decimalPlaces As Integer = 0
If InStr(1, stringNum, ".", CompareMethod.Binary) Then
    decimalPlaces = stringNum.Substring(stringNum.IndexOf(".")).Length
End If

'Format String
Dim waveDurationFormat As String
If decimalPlaces > 8 Then
    waveDurationFormat = waveDuration.ToString("0.######E+00", numericProvider)
Else
    waveDurationFormat = waveDuration.ToString(".#######", numericProvider)
End If

ListBox3.Items.Add(waveDurationFormat)

Answer 1

Welcome to Code Review! Nice first question.

Instead of using Instr() to check if stringNum contains a . you can just use String.IndexOf() and check if the result is > -1 which means the string representation of waveDuration contains a ..

If IndexOf() returns a value greater than -1 we can just check if stringNum.Length - stringNum.IndexOf(".") > 8. This has the advantage that the code won't create a new string each time it is called because we don't need to call Substring() anymore.

I would place this whole stuff inside an Extension method having an optional parameter stating the numbers of decimalplaces like so

Imports System.Globalization
Imports System.Runtime.CompilerServices
Module StringExtensions
    Dim numericProvider As New NumberFormatInfo With {.NumberDecimalSeparator = "."}

    <Extension()>
    Public Function ToCustomString(ByVal value As Single, Optional scientificNotationDecimalPlaces As Integer = 8) As String
        If scientificNotationDecimalPlaces < 0 Then
            Throw New ArgumentOutOfRangeException(NameOf(scientificNotationDecimalPlaces))
        End If

        Dim stringValue As String = value.ToString(numericProvider)

        Dim decimalPointIndex As Integer = stringValue.IndexOf(".")

        If decimalPointIndex > -1 AndAlso stringValue.Length - decimalPointIndex > scientificNotationDecimalPlaces Then
            Return value.ToString("0.######E+00", numericProvider)
        End If

        Return value.ToString(".#######", numericProvider)

    End Function
End Module  

You could then call it like so

ListBox3.Items.Add(waveDuration.ToCustomString())

or e.g if you want to have the sientific notation only if the decimal places are greater than 5 like so

ListBox3.Items.Add(waveDuration.ToCustomString(5))