LINQ in C# - Part 2

Query Operator

• query operators are the methods that form the LINQ patterns.
  
• In simple terms query operators are nothing but methods that operate on an object whose type implements the IEnumerable<T> interface or the IQueryable<T> interface. For example Select(), Where(), GroupBy(), ToList(), ToArray()..etc.
• These Query Operators are defined using Extension methods
• Based on the Query Operators used in the LINQ the timing of their execution differs, that is  depending on whether they return a singleton value(Immediate Execution) or a sequence of values(Deferred Execution).

Query Expression Syntax

• A query expression is a more readable form of expressing a query than its query operators equivalent.
• Example 

string sentence = "the quick brown fox jumps over the lazy dog";  
// Split the string into individual words to create a collection.  
string[] words = sentence.Split(' ');  
  
// Using query expression syntax.  
var query = from word in words  
            group word.ToUpper() by word.Length into gr  
            orderby gr.Key  
            select new { Length = gr.Key, Words = gr };  
  
// Using method-based query syntax.  
var query2 = words.  
    GroupBy(w => w.Length, w => w.ToUpper()).  
    Select(g => new { Length = g.Key, Words = g }).  
    OrderBy(o => o.Length);  
  
foreach (var obj in query)  
{  
    Console.WriteLine("Words of length {0}:", obj.Length);  
    foreach (string word in obj.Words)  
        Console.WriteLine(word);  
}  
  
// This code example produces the following output:  
//  
// Words of length 3:  
// THE  
// FOX  
// THE  
// DOG  
// Words of length 4:  
// OVER  
// LAZY  
// Words of length 5:  
// QUICK  
// BROWN  
// JUMPS

Deferred Execution Types

• Deferred Execution can be further sub-divided into two types based on the Query Operators used(when more than one data source used)
• Streaming
• Streaming operators do not have to read all the source data before they yield elements. 
• At the time of execution, a streaming operator performs its operation on each source element as it is read and yields the element if appropriate. 
• A streaming operator continues to read source elements until a result element can be produced. This means that more than one source element might be read to produce one result element.
• Non-Streaming 
• Non-streaming operators must read all the source data before they can yield a result element. Operations such as sorting or grouping fall into this category.
• At the time of execution, non-streaming query operators read all the source data, put it into a data structure, perform the operation, and yield the resulting elements.