Object reference not set to an instance of an object.

harry harish

why I am getting following error for below code :-

at InterviewTest.Form1.Button1_Click(Object sender, EventArgs e) in h:\visual studio 2010\Projects\InterviewTest\InterviewTest\Form1.vb:line 8

1. Public Class Form1

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

4. Dim classOfService As New ClassOfService

5. Dim travelInformation As New TravelInformation

7. classOfService.code = "GT"

8. travelInformation.classes.Add(classOfService)

9. classOfService.code = "HIK"

10. travelInformation.classes.Add(classOfService)

11. TextBox1.Text = travelInformation.classes(1).code

12. End Sub

13. End Class

14.

15. Public Class TravelInformation

16. Public Property classes As List(Of ClassOfService)

17. End Class

It basically means we are trying to pull something out of nothing.am I correct , as we don't have following stuff in the code triggered at nterviewTest.Form1.Button1_Click

5 Replies

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Tapan Patel10y ago
Accepted answer
No. It's a property of travelinformation class
harry harish10y ago
Thank you Tapan, I will go thru the link provided by you now.
So it means we only have two classes here Class Form1 andClass TravelInformation.
Public Property classes As List(Of ClassOfService)- is not a separate class?
Tapan Patel10y ago
Public Class TravelInformation
Public Property classes As List(Of ClassOfService) <-- List here states that it is a collection
End Class
so classes variable is a list which can hold the objects of the class named ClassOfService.
Please go through the below link for further information.
https://msdn.microsoft.com/en-us/library/system.collections%28v=vs.110%29.aspx?f=255&MSPPError=-2147217396
harry harish10y ago
Thank you Tapan, it answered my query but I have one more basic doubt here, you mentioned that TravelInformation class has a collection called Classes, how do we define classes here(how come it's a cl=ollection of classes not just a class) and how many classes are we using in this code , does line number 16 is a class as well ?
16. Public Property classes As List(Of ClassOfService)
Tapan Patel10y ago
TextBox1.Text = travelInformation.classes(1).code

TravelInformation class has a collection called Classes which is not initialized in your case and that is why you are getting the error.
for any collection or object to be used, one has to initialize first (one you initialize CLR allocates the memory for that object and then only you can write some data in it).
you need to change it from above code to...

travelInformation.classes = new List();
travelInformation.classes.Add(classofService)

For adding the second object you need to create seperate object, else you are just modifying the same object and adding it again that's not going to help.
so, it should be

classofservice = new classofService()
classofservice.code ="HIK"
travelInfomration.classes.Add(classofService)
and then your code will work.