我可以在LINQ表达式中键入要执行的代码吗？

Question

A very simple question. I just happen to be practicing LINQ now, I want to know if I can type the condition in "where" part of LINQ? for example

from a in myPerson
where a.Age > 15
select a;

and then I get to type my own condition in run-time. If I want to get the age greater than 25 this time, or I want to filter it not by age but by Names now. So I will just type

where a.Name == "John"

I wonder if it can be done.

Thanks in any help!

Answer 1

Instead of hardcoding your value in the condition, use a variable. So your condition can be handled with different values during runtime

var age = //put your code for retrieving age
from a in myPerson
where a.Age > age 
select a;

Answer 2

You can compose IQueryables. Just append additional Where , OrderBy etc clauses as needed:

// passed as parameters
int age;
string name;
bool sortByName;

var persons = _dbContext.Person;

// always filter by Age
var result = persons.Where(p => p.Age > age);

// additionally filter by Name if some condition is met
if (age > 25) {
    result = result.Where(p => p.Name == name);
}

// sort depending on parameter
if (sortByName) {
    result = result.OrderBy(p => p.Name);
}
else {
    result = result.OrderBy(p => p.Age);
}

// query will be executed when you enumerate the IQueryable
return result.ToArray();

Answer 3

Make a IQueryable

var ageConditiion = 15;
var originQuery = from a in myPerson select a;
var result1 = originQuery.Where(a => a.Age > ageConditiion).ToList();
var result2 = originQuery.Where(a => a.Name == "John").ToList();

Queryable wont send any request to database.