[AdamSpeight2008]

In vb.net the are different type of datatypes.

The first is a Value Type these are mostly involve the type to do with numbers. (Double, Integer etc)
Dim x As Double
At this point the variable x has the value of 0 the default value for a double.

The second is a Reference Type these are mostly involve classes but it includes Strings.
Dim s As String
Dim c As someClass
At this point the variables s & c don't have a value, they are set to Nothing, think of them as a pigeon hole for the value.

And this is the reason why you get the

Quote

Object reference not set to an instance of an object.

For a reference types you have at some point give the declaration an instance of the type.
For example
Dim s As String=""
Dim c As someClass = New someClass
or

Dim c As someClass
c = New someClass

[AdamSpeight2008]

Note: If you pass a Reference Type to a function or subroutine via ByVal it is still a reference it is not a copy.