Issue with Custom Date Format in VB.NET

Question

Good day everyone!

I have a problem about custom date format in VB.NET and I already tried to use these codes:

• Display date in dd/mm/yyyy format in vb.net
• VB.NET DateTime Format Functions - Dot Net Perls

but none of them are working for me.

First, this the source code where I get the first and last day of the month

• Get the first/last day of the month with VB.NET

Code:

Public Function FirstDayOfMonth(ByVal sourceDate As DateTime) As DateTime    
    Return New DateTime(sourceDate.Year, sourceDate.Month, 1)
End Function

Public Function LastDayOfMonth(ByVal sourceDate As DateTime) As DateTime
    Dim lastDay As DateTime = New DateTime(sourceDate.Year, sourceDate.Month, 1)
    Return lastDay.AddMonths(1).AddDays(-1)
End Function

This is the output

3/1/2018 & 3/31/2018

Example:

My first attempt based on Source# 1

Dim startDayMonth As String
Dim endDayMonth As String

startDayMonth = startDayMonth.ToString("dd")
endDayMonth = endDayMonth.ToString("dd")

MsgBox(startDayMonth & " " & endDayMonth)

OUTPUT:

An unhandled exception of type 'System.InvalidCastException' occurred in TIMELOG.exe

Additional information: Unable to cast object of type 'System.String' to type 'System.IFormatProvider'.

My second attempt based on Source# 2

Dim startDayMonth As DateTime
Dim endDayMonth As DateTime
Dim dateFormat As String

dateFormat = "%d"

startDayMonth = FirstDayOfMonth(Now)
endDayMonth = LastDayOfMonth(Now)

startDayMonth = startDayMonth.ToString(dateFormat)
endDayMonth = endDayMonth.ToString(dateFormat)

OUTPUT:

An unhandled exception of type 'System.InvalidCastException' occurred in Microsoft.VisualBasic.dll

Additional information: Conversion from string "1" to type 'Date' is not valid.

Own Method #1

Dim startDayMonth As DateTime
Dim endDayMonth As DateTime
Dim dateFormat As String

dateFormat = "yyyy-MM-dd"

startDayMonth = Convert.ToString(FirstDayOfMonth(Now))
endDayMonth = Convert.ToString(LastDayOfMonth(Now))

startDayMonth = startDayMonth.ToString(dateFormat)
endDayMonth = endDayMonth.ToString(dateFormat)

OUTPUT

3/1/2018 3/31/2018

Own Method #2

Dim startDayMonth As DateTime
Dim endDayMonth As DateTime
Dim dateFormat As String

dateFormat = "%yyyy-MM-dd"

startDayMonth = Convert.ToString(FirstDayOfMonth(Now))
endDayMonth = Convert.ToString(LastDayOfMonth(Now))

startDayMonth = startDayMonth.ToString(dateFormat)
endDayMonth = endDayMonth.ToString(dateFormat)

OUTPUT

An unhandled exception of type 'System.InvalidCastException' occurred in Microsoft.VisualBasic.dll

Additional information: Conversion from string "182018-03-01" to type 'Date' is not valid.

Answer 1

Pay better attention to what data types you are using in what situations. Strings are not DateTime values, and DateTime values are not strings. You need variables declared with the correct type to hold each kind of value.

The FirstDayOfMonth() and LastDayOfMonth() functions use DateTime values. This is good. However, DateTime values always use a binary format for their data which is not human readable. You cannot change this, but that's okay; you can easily convert the values to strings formatted however you want. You just need to use different variables to store the string values that are declared to use the string type:

Dim format As String = "%d"
Dim startDayMonth As DateTime = FirstDayOfMonth(Now)
Dim endDayMonth As DateTime = LastDayOfMonth(Now)
Dim startDayMonthFormatted As String = startDayMonth.ToString(format)
Dim lastDayMonthFormatted As String = lastDayMonth.ToString(format)