I solved my problem in a different way:

public abstract class baseLookupForm
{
public abstract void OpenEditForm();

public void btnModify_Clicked(...)
{
this.OpenEditForm();
}

}


public class CodeLookupForm : baseLookupForm
{
private override void OpenEditForm()
{
CodeEditForm oCode = new CodeEditForm();
oCode.ShowDialog();
}
}