How to update Format function from VB to VB.NET - vb.net

Question

I am trying to port a VB function to VB.NET, but I cannot get the function to work correctly and update properly.
rFormat = Format(Format(Value, fmt), String$(Len(fmt), "#"))
It seems like the problem lies with the String$() function parameter which is used to align decimal points of values. How would I be able to properly fix this, or is there another way to achieve this?
EDIT
The following is an example console application that shows the issues that I am having.
Imports Microsoft.VisualBasic
Module Module1
Sub Main()
Dim rFormat As String
Dim fmt As String
Dim value As Object
fmt = "########.000"
value = 12345.2451212
'value = 12345
'~~~~~~~~~~~~~~~~~~~~~
'rFormat = Microsoft.VisualBasic.Format(Microsoft.VisualBasic.Format(value, fmt), "".PadLeft(fmt.Length, "#"c))
'Console.WriteLine(rFormat) ' <<Not working prints all "#" for any value!>>>
'rFormat = Microsoft.VisualBasic.Format(Microsoft.VisualBasic.Format(value, fmt), "".PadLeft(fmt.Length))
'Console.WriteLine(rFormat) '<<Not working prints nothing>>
'rFormat = (String.Format(value, fmt)).PadLeft(Len(fmt))
'Console.WriteLine(rFormat) ' <<Not working prints the value 12345.2451212>>> should print>>>>> 12345.245
'for integer values< works good>
rFormat = String.Format("{0," + fmt.Length.ToString + "}", String.Format(value, fmt))
Console.WriteLine(rFormat) ' <<Not working prints the value 12345.2451212>>> should print>>>>> 12345.245
'for integer values< works good>
End Sub
End Module

Answer 1

All String$ does is repeat the character specified in the second parameter the number of times specified in the first parameter.
So if fmt is, for example "9999", then the String$ command will produce "####".
You can replace this with the String.PadLeft method and continue to use the VB Format function from the Microsoft.VisualBasic namespace:
rFormat = Microsoft.VisualBasic.Format(Microsoft.VisualBasic.Format(value, fmt), "".PadLeft(fmt.Length, "#"c))
EDIT:
Based on the edit in the question, the correct format logic should be:
rFormat = String.Format("{0:" & fmt & "}", value)
It is very helpful to review the String.Format documentation since it has a lot of examples and explanation.

Answer 2

It sounds like you're wanting to pad out your results so they are a fixed length. How about using the String.PadLeft Method or the String.PadLeft(int32,char) Method to Pad out rFormat.
Something like this for spaces:
rFormat = (String.Format(value, fmt)).PadLeft(Len(fmt))
Edit
Boy is it hard to find VB6 documentation online. It appears that the # in a VB6 Custom Format has to do with String justification per this Forum posting and this SO answer they suggest something something like this.
rFormat = String.Format("{0," + fmt.Length.ToString + "}", String.Format(value, fmt))
This is using the Composite Formatting Alignment Component
Alignment Component
The optional alignment component is a signed integer indicating the preferred formatted field width. If the value of alignment is less than the length of the formatted string, alignment is ignored and the length of the formatted string is used as the field width. The formatted data in the field is right-aligned if alignment is positive and left-aligned if alignment is negative. If padding is necessary, white space is used. The comma is required if alignment is specified.
The main issue that I see in your updated example is that you are using an object to store your Double. By changing values declaration to a Decimal and changing the format function I was able to get it to work.
Sub Main()
Dim rFormat As String
Dim fmt As String
Dim value As Double
fmt = "#######0.000"
value = 12345.2451212
rFormat = String.Format("{0," + fmt.Length.ToString + "}", value.ToString(fmt))
Console.WriteLine(rFormat)
Console.ReadLine()
End Sub

Answer 3

In VBNet, you can also do this:
Dim rFormat As String = String.Empty
Dim fmt As String = "########.000"
Dim value As Object = 12345.2451212
rFormat = (CDbl(value)).ToString(fmt)