Select and SelectMany are projection operators. Select operator is used to select value from a collection and SelectMany operator is used to select values from a collection of collection i.e. nested collection.
Select operator produce one result value for every source value while SelectMany produce a single result that contains a concatenated value for every source value. Actually, SelectMany operator flatten IEnumerable<IEnumerable<T>> to IEnumrable<T> i.e. list of list to list.
class Employee {
public string Name {
get;
set;
}
public List < string > Skills {
get;
set;
}
}
class Program {
static void Main(string[] args) {
List < Employee > employees = new List < Employee > ();
Employee emp1 = new Employee {
Name = "Deepak", Skills = new List < string > {
"C", "C++", "Java"
}
};
Employee emp2 = new Employee {
Name = "Karan", Skills = new List < string > {
"SQL Server", "C#", "ASP.NET"
}
};
Employee emp3 = new Employee {
Name = "Lalit", Skills = new List < string > {
"C#", "ASP.NET MVC", "Windows Azure", "SQL Server"
}
};
employees.Add(emp1);
employees.Add(emp2);
employees.Add(emp3);
// Query using Select()
IEnumerable < List < String >> resultSelect = employees.Select(e = > e.Skills);
Console.WriteLine("Using Select():\n");
// Two foreach loops are required to iterate through the results
// because the query returns a collection of arrays.
foreach(List < String > skillList in resultSelect) {
foreach(string skill in skillList) {
Console.WriteLine(skill);
}
Console.WriteLine();
}
// Query using SelectMany()
IEnumerable < string > resultSelectMany = employees.SelectMany(emp = > emp.Skills);
Console.WriteLine("Using SelectMany():");
// Only one foreach loop is required to iterate through the results
// since query returns a one-dimensional collection.
foreach(string skill in resultSelectMany) {
Console.WriteLine(skill);
}
Console.ReadKey();
}
}
/*Output :
Using Select():
C
C++
Java
SQL Server
C#
ASP.NET
C#
ASP.NET MVC
Windows Azure
SQL Server
Using SelectMany():
C
C++
Java
SQL Server
C#
ASP.NET
C#
ASP.NET MVC
Windows Azure
SQL Server */