VB.Net convert formatted string to double

February 7, 2024

I got a string by formatting a double value, like

Dim strFormat = "#,##0.00 weeks"
Dim strDisplayText As String = dblIn.ToString(strFormat) ' dblIn is type Double

Then the strDisplayText is displayed in a textbox (of a wpf.datagrid), and the user may edit it. Afterwards, the text should be converted back to an double.

Dim dblOut = ???(strDisplayText, strFormat)
 ' dblOut should be the same as dblIn, if strDisplayText wasnt changed.
if dblOut.ToString(strFormat) <> strDisplayText Then
    print("failed conversion.")
end if

Double.Parse (and TryParse) fail because of the " weeks" at the end and Val cant process the "," as group seperator.

strFormat is read from a file, so it might be any valid custom or standart numeric format (https://learn.microsoft.com/en-us/dotnet/standard/base-types/standard-numeric-format-strings or https://learn.microsoft.com/en-us/dotnet/standard/base-types/custom-numeric-format-strings)

>Solution :

You can use simple string methods to remove the " weeks" at the end:

Dim dblValue = strDisplayText.Remove(strDisplayText.Length - " weeks".Length)
Dim dbl As Double = double.Parse(dblValue)

strFormat is read from a file, so it might be any valid custom or
standart numeric format

You cannot revert a text that was formatted with an arbitrary format string to the double value. There is no Double.TryParseExact(like for Date). Especially if you even have different decimal or group separators.

I’d let the user edit the double value, not the formatted text.