Stop doing this

Pet peeve: I hate micro-management when it comes to code. Yes, it's faster in the short-term but it's a pain when the code has to be changed. (If you don't know that all code has to be changed, you haven't programmed long enough.)

In this particular case I have encountered code similar to:

  var sb = new StringBuilder();
  foreach (var p in list)
  {
    sb.Append(p.Name);
    sb.Append(" ");
    sb.Append(p.Quantity);
    sb.Append("*");
    sb.Append(p.Price);
    sb.Append("=");
    sb.Append(p.Value);
    sb.Append(",");
  }
  if (list.Any())
  {
    sb.Remove(sb.Length - 1, 1);
  }
  
  return sb.ToString();

(It's even worse when it's a for loop instead of foreach, but let's not go there.)

The problem with this code is that it's extremely easy to get lost in the details. "Oh, I should put spaces around the operations." "Oh, I should add a space after the comma, but then it has to be removed at the end of the method." All of these are perfectly valid requests, but their place is not here. The purpose of this method is simple: generate a comma-separated list of products. That's all it does so that's all it should say:

  return string.Join(",", list.Select(ProductAsString));

(It's even better if you can use the ToString method instead of ProductAsString, but that's not always possible.)

The ProductAsString method is one abstraction level lower; it's fine for it to care about the details of creating a string representation of a product:

  private string ProductAsString(Product p)
  {
    var sb = new StringBuilder();
    sb.Append(p.Name);
    sb.Append(" ");
    sb.Append(p.Quantity);
    sb.Append("*");
    sb.Append(p.Price);
    sb.Append("=");
    sb.Append(p.Value);
    
    return sb.ToString();
  }

Of course, unless the speed is really much better with a StringBuilder instead of the string.Format method - and unless that's actually measured to be relevant - you should simplify the ProductAsString method to:

  private string ProductAsString(Product p)
  {
    return string.Format("{0} {1}*{2}={3}", p.Name, p.Quantity, p.Price, p.Value);
  }

This is much more expressive than the previous version.

Finally, the place of the ProductAsString method is among the method of the Product class. (It shows a severe case of feature envy.) Again, the best place would be the ToString method; if that's not possible for any reason, at least make it a public method of the Product class.

Ok, enough with the ranting, back to work.