Chapter 7. Extending LINQ

In closing, let's touch on the extensibility of LINQ, another of its elegant design characteristics. As you have seen, LINQ interoperates with a range of data sources, from object graphs to relational databases to XML documents. Let's look at a quick example of how we go about extending one of our own classes to natively support LINQ.

Our running demo has focused on doctors working at the fictitious University Hospital. You've seen many queries involving doctors, often using the Distinct operator to eliminate duplicates. For example, a query to yield the distinct doctors working in October 2006:

 DataSets.SchedulingDocs ds = FillDataSet(); var oct2006 = (                 from doc in ds.Doctors                 join call in ds.Calls                 on doc.Initials equals call.Initials                 where call.DateOfCall >= new DateTime(2006, 10, 1) &&                       call.DateOfCall <= new DateTime(2006, 10, 31)                 select doc               )               .Distinct(); 

Since we designed the data model, we know that a doctor's initials are unique. We can take advantage of this information to optimize the application of Distinct in the query above by providing our own implementation specific to our DataSet:

 using DS = DataSets.SchedulingDocs; public static class DataSetsExtension {   public static IEnumerable<DS.DoctorsRow> Distinct(this                                                     IEnumerable<DS.DoctorsRow> src)   {     // optimize by using a hashtable with doctors' initials:     System.Collections.Hashtable ht = new System.Collections.Hashtable();     foreach(var doc in src)  // for each doctor in the original sequence:     {        if (ht[doc.Initials] == null)  // first time, so remember and return:        {           ht.Add(doc.Initials, doc.Initials);           yield return doc;        }        else  // we've yielded this doc already, so skip:           continue;     }   } }//class 

For sequences of type DoctorsRowi.e., doctors in our DataSetthis implementation will be used in place of the standard LINQ implementation of Distinct. Thus, for any query of the form,

 var query = from doc in ds.Doctors             .             .             select doc; 

the application of Distinct will invoke our optimized implementation. That's it!