Conversion Operators:

The Conversion operators in LINQ are useful in converting the type of the elements in a sequence (collection). There are three types of conversion operators: As operators (AsEnumerable and AsQueryable), To operators (ToArray, ToDictionary, ToList and ToLookup), and Casting operators (Cast and OfType).

The following table lists all the conversion operators.

Method Description
AsEnumerable Returns the input sequence as IEnumerable<t>
AsQueryable Converts IEnumerable to IQueryable, to simulate a remote query provider
Cast Coverts a non-generic collection to a generic collection (IEnumerable to IEnumerable<T>)
OfType Filters a collection based on a specified type
ToArray Converts a collection to an array
ToDictionary Puts elements into a Dictionary based on key selector function
ToList Converts collection to List
ToLookup Groups elements into an Lookup<TKey,TElement>

AsEnumerable & AsQueryable:

The AsEnumerable and AsQueryable methods cast or convert a source object to IEnumerable<T> or IQueryable<T> respectively.

Consider the following example: (courtesy: Jon Skeet)

Example: AsEnumerable & AsQueryable operator in C#:
        
class Program
{

    static void ReportTypeProperties<T>(T obj)
    {
        Console.WriteLine("Compile-time type: {0}", typeof(T).Name);
        Console.WriteLine("Actual type: {0}", obj.GetType().Name);
    }

    static void Main(string[] args)
    {
        Student[] studentArray = { 
                new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
                new Student() { StudentID = 2, StudentName = "Steve",  Age = 21 } ,
                new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
                new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
                new Student() { StudentID = 5, StudentName = "Ron" , Age = 31 } ,
            };   
            
        ReportTypeProperties( studentArray);
        ReportTypeProperties(studentArray.AsEnumerable());
        ReportTypeProperties(studentArray.AsQueryable());   
    }
}      
Output:
Compile-time type: Student[]
Actual type: Student[]
Compile-time type: IEnumerable`1
Actual type: Student[]
Compile-time type: IQueryable`1
Actual type: EnumerableQuery`1

As you can see in the above example AsEnumerable and AsQueryable methods convert compile time type to IEnumerable and IQueryable respectively

Visit stackoverflow for detail information on AsEnumerable and AsQueryable method.

Cast:

Cast does the same thing as AsEnumerable<T>. It cast the source object into IEnumerable<T>.

Example: Cast operator in C#
        
class Program
{

    static void ReportTypeProperties<T>(T obj)
    {
        Console.WriteLine("Compile-time type: {0}", typeof(T).Name);
        Console.WriteLine("Actual type: {0}", obj.GetType().Name);
    }

    static void Main(string[] args)
    {
        Student[] studentArray = { 
                new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
                new Student() { StudentID = 2, StudentName = "Steve",  Age = 21 } ,
                new Student() { StudentID = 3, StudentName = "Bill",  Age = 25 } ,
                new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
                new Student() { StudentID = 5, StudentName = "Ron" , Age = 31 } ,
            };   
         
        ReportTypeProperties( studentArray);
        ReportTypeProperties(studentArray.Cast<Student>());
    }
}      
Output:
Compile-time type: Student[]
Actual type: Student[]
Compile-time type: IEnumerable`1
Actual type: Student[]
Compile-time type: IEnumerable`1
Actual type: Student[]
Compile-time type: IEnumerable`1
Actual type: Student[]

studentArray.Cast<Student>() is the same as (IEnumerable<Student>)studentArray but Cast<Student>() is more readable.

To Operators: ToArray(), ToList(), ToDictionary():

As the name suggests, ToArray(), ToList(), ToDictionary() method converts a source object into an array, List or Dictionary respectively.

To operators force the execution of the query. It forces the remote query provider to execute a query and get the result from the underlying data source e.g. SQL Server database.

Example: ToArray & ToList in C#
        
IList<string> strList = new List<string>() { 
                                            "One", 
                                            "Two", 
                                            "Three", 
                                            "Four", 
                                            "Three" 
                                            };

string[] strArray = strList.ToArray<string>();// converts List to Array

IList<string> list = strArray.ToList<string>(); // converts array into list

ToDictionary - Converts a Generic list to a generic dictionary:

Example: ToDictionary in C#:
        
IList<Student> studentList = new List<Student>() { 
                    new Student() { StudentID = 1, StudentName = "John", age = 18 } ,
                    new Student() { StudentID = 2, StudentName = "Steve",  age = 21 } ,
                    new Student() { StudentID = 3, StudentName = "Bill",  age = 18 } ,
                    new Student() { StudentID = 4, StudentName = "Ram" , age = 20 } ,
                    new Student() { StudentID = 5, StudentName = "Ron" , age = 21 } 
                };

//following converts list into dictionary where StudentId is a key
IDictionary<int, Student> studentDict = 
                                studentList.ToDictionary<Student, int>(s => s.StudentID); 

foreach(var key in studentDict.Keys)
	Console.WriteLine("Key: {0}, Value: {1}", 
                                key, (studentDict[key] as Student).StudentName);

Output:
Key: 1, Value: John
Key: 2, Value: Steve
Key: 3, Value: Bill
Key: 4, Value: Ram
Key: 5, Value: Ron

The following figure shows how studentDict in the above example contains a key-value pair, where key is a StudentID and the value is Student object.

LINQ-ToDictionary Operator