DetailLoaderOld.cs

public abstract class DetailLoader<T> : IDetailLoader<T> 
{
    public virtual void Load(string importPath, ProgressItem progressItem, Action<T> addToBatch) 
    { 
        var spreadsheet = SpreadsheetDocument.Load(importPath); 
        var rows = spreadsheet.Rows(); 

        progressItem.Start(rows.Count()); 

        var details = GetDetailItems(rows, progressItem); 
        foreach (var detail in details) 
            addToBatch(detail);

        _progressItem.EndWithSuccess(); 
    }

    public virtual IEnumerable<T> GetDetailItems(IEnumerable<Row> rows, ProgressItem progressItem) 
    { 
        foreach (var row in rows) 
        { 
            var detail = GetDetailItemFrom(row); 
            progressItem.IncrementProgress(): 
            yield return detail; 
        } 
    }

    // Implement for each concrete loader without needing the progress item. 
    public abstract T GetDetailItemFrom(Row row); 
}