The from and in keywords are used in LINQ queries to specify the source data for a query. They are typically used together in a from clause, which is placed at the beginning of a LINQ query.
Here is an example of a from clause that retrieves all the numbers in an array,
int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
var result = from n in numbers
select n;
In the above example, the from clause specifies that the source data for the query is the numbers array, and the in keyword specifies that each element in the array will be assigned to a variable named n. The select clause specifies that the query will return the value of each element in the array.
After the query is executed, the result variable will contain the numbers 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10.
It is also possible to use the from and in keywords to query data from other sources, such as databases or XML documents. For example, the following query uses the from and in keywords to retrieve all the employees from a database,
var result = from e in db.Employees
select e;
In the above example, the from clause specifies that the source data for the query is the Employees table in the db database, and the in keyword specifies that each row in the table will be assigned to a variable named e. The select clause specifies that the query will return the value of each row in the table.
After the query is executed, the result variable will contain a collection of employee objects, each representing a row in the Employees table.