BC42105: Function '<procedurename>' doesn't return a value on all code paths - vb.net

Question

I tried all possible solutions and they didn't work. Any help would be appreciated.
Function '' doesn't return a value on all code paths is my error
Public Function Isprime(n2 As Long)
Dim n, i As Integer
Dim b As Boolean
Console.WriteLine("enter a no : \")
n = Console.ReadLine()
i = 2
b = True
While i < n
If n Mod i = 0 Then
b = False
End If
i = i + 1
End While
If b Then
Console.WriteLine("prime no")
Else
Console.WriteLine("not prime no\")
End If
Console.ReadLine()
End Function
Public Function PrimePairs(ByVal n As Long, ByVal n2 As Long) As Integer
Dim count As Integer = 0
Console.ReadLine()
If n Mod 2 = 0 Then
For i = 1 To (n / 2) + 1
n2 = n - i
If Isprime(i) And Isprime(n2) = True Then
count += 1
End If
Next
Else
n2 = n - 2
If Isprime(n2) = True Then
count = +1
End If
End If
Console.WriteLine(count)
Return n
End Function
End Module

Answer 1

You function does not return a value and is quite convoluted. It does I/O and does a prime test at the same time. Separate logic from I/O.
Public Function IsPrime(n As Long) As Boolean
n = Math.Abs(n) ' Allows to consider negative prime numbers
If n < 2 Then ' Disallows -1, 0, 1
Return False
End If
Dim i As Long
i = 2
While i < n ' Note that for n = 2 we don't enter the loop and thus return True.
If n Mod i = 0 Then
Return False
End If
i += 1
End While
Return True
End Function
Also, it is missing a return type. Always work with Option Explicit On for better code quality.
Note that you can return when you hit the first prime factor. There is no point in continuing the loop then.
But there are ways to optimize this. For example we could test the divisibility by 2 separately and then test only odd divisors and it is enough to test divisors up to the square root of n.
Public Function IsPrime(n As Long) As Boolean
n = Math.Abs(n)
If n = 2 Then
Return True
End If
If n < 2 Or n Mod 2 = 0 Then
Return False
End If
Dim i As Long = 3
Dim limit As Long = CLng(Math.Sqrt(n))
While i <= limit
If n Mod i = 0 Then
Return False
End If
i += 2
End While
Return True
End Function