C# Virtual method call in constructor - how to refactor?
- by Cristi Diaconescu
I have an abstract class for database-agnostic cursor actions.
Derived from that, there are classes that implement the abstract methods for handling database-specific stuff.
The problem is, the base class ctor needs to call an abstract method - when the ctor is called, it needs to initialize the database-specific cursor.
I know why this shouldn't be done, I don't need that explanation!
This is my first implementation, that obviously doesn't work - it's the textbook "wrong way" of doing it.
The overridden method accesses a field from the derived class, which is not yet instantiated:
public abstract class CursorReader
{
private readonly int m_rowCount;
protected CursorReader()
{
m_rowCount = CreateCursor(sqlCmd); //virtual call !
}
protected abstract int CreateCursor(string sqlCmd);
}
public class SqlCursorReader : CursorReader
{
private SqlConnection m_sqlConnection;
public SqlCursorReader(string sqlCmd, SqlConnection sqlConnection)
{
m_sqlConnection = sqlConnection; //field initialized here
}
protected override int CreateCursor(string sqlCmd)
{
//uses not-yet-initialized member *m_sqlConnection*
//so this throws a NullReferenceException
var cursor = new CustomCursor(sqlCmd, m_sqlConnection);
return cursor.Count();
}
}
I will follow up with an answer on my attempts to fix this...
