SampleComputedIndexField.cs

public class MyComputedIndexField : IComputedIndexField
{
  public object ComputeFieldValue(IIndexable indexable)
  {
    // Check that we have something to index
    if (indexable == null)
      return null;

    // Check that it is a Sitecore indexable item
    var indexableItem = indexable as SitecoreIndexableItem;
    if (indexableItem == null)
      return null;
    
    // We're in business, let's grab the Sitecore item being indexed, and do stuff!    
    Item sitecoreItem = indexableItem.Item;

    // Sneaky computations goes here...
    
    // Finally we return the value, that we need to store in the search index
    return "The value stored in the index";
  }
  
  public string FieldName { get; set; }
  public string ReturnType { get; set; }
}