Function Return Issue? - vb.net

Question

In my VB.Net program I have found two (2) warnings shows the following messages:
Function doesn't return a value on all code paths. A null reference exception could occur at run time when the result is used
help me where is the problem and to fix it. The following snapshot is where the warning error indicates:
The Firs Warning near 'End Function'
'Executes SQL commands to the system database
Public Function ExecSQL(ByVal sql As String)
Dim com As New MySqlCommand(sql)
Try
RefreshConnection()
com.Connection = con
com.ExecuteNonQuery()
Catch ex As Exception
Return ex
MsgBox(ex.Message, MsgBoxStyle.Critical, "Error")
End Try
End Function
The Second Waring near 'End Function'
'Get the value of an specific field in a given sql string
Public Function GetField(ByVal sql As String, ByVal field As String)
Try
RefreshConnection()
Dim com As New MySqlCommand(sql, con)
Dim dReader As MySqlDataReader = com.ExecuteReader
While dReader.Read
GetField = dReader(field).ToString
End While
dReader.Close()
Catch ex As Exception
Return ex
MsgBox(ex.Message, MsgBoxStyle.Critical, "Error")
End Try
End Function

Answer 1

This occurs because in the exception handler you actually return the exception object ex. You're not returning anything if no exception occurs. You could explicitly add Return Nothing just above Catch ex as Exception, but I guess from the message that VB is doing that automatically for you...
By the way: What's the point returning the exception to the caller? Put the message box into the calling method and wrap your call in Try-Catch there. A method that's obviously not meant to interact with the user shouldn't show messages at all. And then you can just pass the exception up until you end up the presentation layer.
Long story short: No message boxes in business logic. User information only in presentation layer.
After reading your second method: Things are even worse here! If the data reader has rows, you set the result of the method to the last (!) result you found (this is a WTF in itself, but another story). If an error occurs, the result is an object of type Exception, so the calling code even needs to make sure to decide whether the result of the function call is actually a field value or an exception!
This is really bad design my friend.
You asked for fixed code:
'Executes SQL commands to the system database
Public Sub ExecSQL(ByVal sql As String)
Dim com As New MySqlCommand(sql)
RefreshConnection()
com.Connection = con
com.ExecuteNonQuery()
End Sub
'Get the value of an specific field in a given sql string
Public Function GetField(ByVal sql As String, ByVal field As String)
RefreshConnection()
Dim com As New MySqlCommand(sql, con)
Dim dReader As MySqlDataReader = com.ExecuteReader
GetField = Nothing
While dReader.Read
GetField = dReader(field).ToString
End While
dReader.Close()
End Function

Answer 2

There are several flaws here. In the interest of brevity, I'll just post some improved code:
'Executes SQL commands to the system database
Private Function ExecSQL(ByVal sql As String, ByVal params As IEnumerable(Of MySqlParameter))
Using cn As New MySqlConnection(GetConnectionString()), _
com As New MySqlCommand(sql, cn)
For Each param As MySqlParameter in params
com.Parameters.Add(param)
Next param
cn.Open()
com.ExecuteNonQuery()
End Using
End Function
'Get the value of an specific field in a given sql string
Private Function GetField(Of T)(ByVal sql As String, ByVal params As IEnumerable(Of MySqlParameter), ByVal field As String) As T
Using cn As New MySqlConnection(GetConnectionString())
com As New MySqlCommand(sql, cn)
For Each param As MySqlParameter In params
com.Parameters.Add(param)
Next param
cn.Open()
Using rdr As MySqlDataReader = com.ExecuteReader()
While rdr.Read()
Return CType(rdr(field), T)
End While
End Using
End Using
Return Nothing
End Function