LINQ in C# - Examples - Conversion Operators (Cast, OfType) and AsEnumerable, DefaultIfEmpty

• The conversion operators provide a simple and convenient way of converting sequences to other collection types.

Cast

• The Cast operator is used to cast every element of an input sequence to an output sequence of the specified type. 
• Fundamentally, Cast() is implemented like this:

public IEnumerable<T> Cast<T>(this IEnumerable source)
{
  foreach(object o in source)
    yield return (T) o;
}

• As you can see from the implementation of Cast(), first in cast every element of the sequence as object o, as object as the base type of all the types in C#, then it simply try to cast the object type to corresponding type using explicit casting (T) o, if this casting fails it will throw InvalidCastException.
• So while casting of non-generic collection holding same type of object, then only we can use Cast() operator
• Example 1

object[] ItemsSet1 = new object[]{ "01", "02", "03"};

            string[] AfterCasting = ItemsSet1.Cast<string>().ToArray();
           
            foreach (var item in AfterCasting)
                Console.WriteLine(item);
//Output
01
02
03

• Example 2

 ArrayList ItemsSet1 = new ArrayList{ 4, 3, 2, 1};

            List<int> AfterCasting = ItemsSet1.Cast<int>().ToList();
           
            foreach (var item in AfterCasting)
                Console.WriteLine(item);
//Output
4
3
2
1

OfType

• If a non-generic collection contains different types of objects and we want to extract only one type of object from the collection then we can use OfType.
• For understanding how OfType works, refer the implementation below.

public IEnumerable<T> OfType<T>(this IEnumerable source)
{
  foreach(object o in source)
    if(o is T)
      yield return (T) o;
}

• Example 1

ArrayList ItemsSet1 = new ArrayList{ "Raja", 3, "Ravi", 1};

            List<int> AfterCasting = ItemsSet1.OfType<int>().ToList();
           
            foreach (var item in AfterCasting)
                Console.WriteLine(item);
//Output
3
1

AsEnumerable

• The AsEnumerable operator simply causes its input sequence of type IEnumerable to be returned as type IEnumerable. 
• Example - this example is using Query Expression Syntax

Northwind db = new Northwind(@"Data Source=.\SQLEXPRESS;Initial Catalog=Northwind");

var custs = from c in db.Customers
                  where c.City == "Rio de Janeiro"
                  select c;
foreach (var cust in custs)
           Console.WriteLine("{0}", cust.CompanyName);

DefaultIfEmpty

• Refer the example below

List<string> StudentNames = new List<string> { "Ram", "Raja", "Ravi"};

            //In below statement the assumption is that, there is atleast one name with length >5
            //so the method first will return the first name with length >5
            //But there is no name, so exception will be thrown at runtime of Type System.InvalidOperationException:

            string FoundName = StudentNames.Where(x => x.Length > 5).DefaultIfEmpty().First();
            if(FoundName!=null)
                Console.WriteLine("found Name {0}", FoundName);
            else
                Console.WriteLine("No name found");

• Fix is using DefaultIfEmpty Example 1 - returns default value of type string is null
  

List<string> StudentNames = new List<string> { "Ram", "Raja", "Ravi"};           

string FoundName = StudentNames.Where(x => x.Length > 5).DefaultIfEmpty().First();

            if(FoundName!=null)
                Console.WriteLine("found Name {0}", FoundName);
            else
                Console.WriteLine("No name found");

//Output
No name found

• Fix is using DefaultIfEmpty Example 2 with Default Specified

 List<string> StudentNames = new List<string> { "Ram", "Raja", "Ravi"};
           
 string FoundName = StudentNames.Where(x => x.Length > 5).
DefaultIfEmpty("Akshara").First();
     
            Console.WriteLine("found Name {0}", FoundName);
//Output
found Name Akshara