The solution to the problem is very simple. The followings are few approaches to solution.

• Use inline code where required. Use left and right function to separate “hh” and “mm” respectively and then concatenate the strings with “:” character in between.

• Write a common function “FormatHhMmString” that accepts string as input parameter to be formatted. Include the logic as above.

• Write a class that has a shared method “FormatHhMmString” and include the logic from 1^st bullet.

Imagine that you are storing the date also in simple string format as “mmddyyyy” with no slashes or hyphens in between or phone number as 10 digits with no slashes or hyphens in between. So now you will have to write the 2 functions FormatMmDdYyyyString and FormatTelPhoneString or inline code wherever required.

If every developer writes the inline code then there will not be unique approach to come up with the solution. If you write a class then each time there is new user defined format there will be a new method added to the class.

To wrap up all we need to do is define a class which implements IFormattable interface and its version of ToString function to custom format the strings. This approach allows developers to use standard ToString function transparently leaving implementation details behind the scene.

The ToString function accepts two parameters: the first parameter is a format string (custom format specification) and the second parameter is format provider object. The format provider object usually carries additional information specific to culture, date & time, numbers of targeted users. To keep things simple we will pass Nothing to format provider and apply general (default) formatting to strings when Nothing is passed.

The code that implements above scenarios is as follows:

Public Class FormatClass

Implements IFormattable

' Assign a value for the class.

Private _myValue As String

' Add a constructor.

Public Sub New(ByVal value As String)

End Sub

' Write a custom Format method for the type.

Public Overloads Function ToString(ByVal format As String, ByVal fp As IFormatProvider) As String Implements IFormattable.ToString

If format.Equals("hhmm") Then

Return Left(_myValue, 2) & "-" & Right(_myValue, 2)

ElseIf format.Equals("mmddyyyy") Then

Return Left(_myValue, 2) & "-" & Mid(_myValue, 3, 2) & "-" & Right(_myValue, 4)

ElseIf format.Equals("9999999999") Then

Return Left(_myValue, 3) & "-" & Mid(_myValue, 4, 3) & "-" & Right(_myValue, 4)

Else

Return _myValue.ToString(fp)

End If

End Function

End Class

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

Dim myTime As New FormatClass(TextBox1.Text)

Dim myDate As New FormatClass(TextBox2.Text)

Dim myTelNo As New FormatClass(TextBox3.Text)

Try

lblTimeToString.Text = myTime.ToString("hhmm", Nothing)

lblDateToString.Text = myDate.ToString("mmddyyyy", Nothing)

lbltelToString.Text = myTelNo.ToString("9999999999", Nothing)

Catch ex As Exception

End Try

End Sub

How to work with this example:

Input: