Public Shared Function IsNullToString(ByVal _PossibleNullObject As Object, Optional ByVal _IfNullReturnString As String = "") As String
            If Not IsDBNull(_PossibleNullObject) Then
                Return _PossibleNullObject.ToString
            Else
                Return _IfNullReturnString
            End If
End Function

This code converts database NULL field values to a string. The first argument represents the field from the database (maybe a Datarow field) and the second argument is the optional string that should be returned if a NULL value is encountered. By default, the function returns an empty string value.

Convert database NULL value fields to any string

VB.NET