Recursion is a technique used to define a problem in terms of the problem itself, usually in terms of a simpler version of the problem itself.
For example, the implementation of the generator for the n-th value of the Fibonacci sequence comes naturally from its mathematical definition, when recursion is used:
Function NthFibonacciNumber(ByVal n As Integer) As Integer
If n <= 1 Then
Return 1
Else
Return NthFibonacciNumber(n - 1) + NthFibonacciNumber(n - 2)
End If
End Function
As opposed to:
Function NthFibonacciNumber(ByVal n As Integer) As Integer
Dim previous As Integer = 0
Dim last As Integer = 1
For i = 0 To n - 1
Dim temp = previous
previous = last
last = last + temp
Next
Return last
End Function
The use of recursion is acceptable in methods, like the one above, where you can break out of it.
Function NthFibonacciNumber(ByVal n As Integer) As Integer
If n <= 1 Then
Return 1 ' Base case: stop the recursion
End If
' ...
End Function
It is also acceptable and makes sense in some type definitions:
Class Box
Inherits IComparable(Of Box)
Public Function CompareTo(ByVal other As Box?) As Integer
' Compare the two Box instances...
End Function
End Class
With types, some invalid recursive definitions are caught by the compiler:
Class C2(Of T) ' Error BC31447 C2(Of T) cannot reference itself in Inherits clause
Inherits C2(Of T)
End Class
Class C2(Of T)
Inherits C2(Of C2(Of T)) ' Error BC31447 C2(Of T) cannot reference itself in Inherits clause
End Class
In more complex scenarios, however, the code will compile but execution will result in a TypeLoadException if you try to instantiate the class.
Class C1(Of T)
End Class
Class C2(Of T) ' Noncompliant
Inherits C1(Of C2(Of C2(Of T)))
End Class
Dim c2 = New C2(Of Integer) ' This would result into a TypeLoadException