we have many projection operators in linq like: Select(),where()....are Extension methods takes delegates as
input parameters
var numbers = new[] { 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 };
var values= numbers.Where((n, index) => (index % 2) != 0).ToList();
foreach(int item in values)
{
Console.WriteLine(item.Tostring())
}
Here: numbers.Where(Fun<int,bool>,predicate)
based on source(numbers).so
delegate points to integers.
another example is
var names = new[] { "app",
"balu", "charan",
"dhanush" };
IEnumerable<string> stringvalues = names.Where(s => s.Contains("charan")).ToList();
foreach
(string item in
stringvalues)
{
Console.WriteLine(item.ToString());
}
Here: names.Where(Fun<string,bool>,predicate);
As well in case of custom types like Students
List<Students> val = new List<Students>();
val.Add(new Students {
Name = "aanand", Age = 22,City="Hyd" });
val.Add(new Students {
Name = "balu", Age = 24,City="Pune" });
val.Add(new Students {
Name = "charan", Age = 19,City="Bangalore" });
val.Add(new Students { Name = "dinesh",
Age = 19, City="Hyd"});
or
// List<Students> val = new List<Students>(IEnumerable ier)
IEnumerable<Students> t = val.Where((Students, s1) =>
(Students.City == "Hyd"));----2nd
predicate
--àt.Where(Fun<Student,int,bool>);
IEnumerable<Students> t = val.Where(Students => (Students.City == "Hyd"));
---
1st predicate
--àt.Where(Fun<Student,bool>,predicate);
And
1st predicate
(Students.City == "Hyd")-àreturn True/False.
Early loading/eagar loading:
index % 2 != 0
true
index % 2 != 0
false
index % 2 != 0
true
index % 2 != 0
false
index % 2 != 0
true
if students belongs to Hyd returns true other
wise false.
2nd predicate
Val.Where((students,s1)=>(Students.City==”Hyd”));
if students belongs to Hyd returns true other
wise false.
foreach (Students item in
t)
{
Console.WriteLine(item.Name+" "
+item.Age+" "+item.City);
}
Output:
1,3,8,21,55---intarray
Charan-----stringarray
Anand 22 Hyd
Dinesh 19 Hyd
s.Contains("charan")
false
s.Contains("charan")
false
s.Contains("charan")
true
s.Contains("charan")
false
s.Contains("charan")
The name 's' does not exist
in the current context
above 2 are eagar/early loading.
Select
Predicate:
List<Students> val = new
List<Students>();
val.Add(new
Students { Name = "aanand",
Age = 22, City = "Hyd" });
val.Add(new
Students { Name = "balu",
Age = 24, City = "Pune" });
val.Add(new
Students { Name = "charan",
Age = 19, City = "Bangalore" });
val.Add(new
Students { Name = "dinesh",
Age = 19, City = "Hyd" });
IEnumerable<Students>
stuvalues = val.Select(r
=> r);
foreach
(Students item in
stuvalues)
{
Console.WriteLine(item.City);
}
2nd
predicate:
IEnumerable<string>
stuvalues = val.Select(r => r.City);
or
IEnumerable<string>
stuvalues = val.Select<Students, string>(r
=> r.City);
foreach (string item in
stuvalues)
{
Console.WriteLine(item.City);
}
but
Lazy loading/deffered loading
Students.City=="Hyd"
false
Students.City=="Hyd"
True
Both perdicates: where and select
IEnumerable<Students> stuvalues = val.Where((Students, s1) =>
(Students.City == "Hyd")).Select(r => r);
or
IEnumerable<Students> stuvalues = val.Where((Students, s1) => (Students.City == "Hyd"));//.Select(r => r);
foreach
(Students item in
stuvalues)
{
Console.WriteLine(item.City);
}
IEnumerable<string> stuvalues =
val.Where((Students, s1) => (Students.City == "Hyd")).Select(r => r.City);
foreach
(string item in
stuvalues)
{
Console.WriteLine(item);
}
Note: projection operator can be where/select.but the return type is should math
both the sites
we can know the return type of Linq query while showing
like
before writing select projection in linq query
after writing select projection in linq query
