c# – Inject arguments in LINQ queries – Education Career Blog

Question 1

I’m trying to build a complex query with linq.

I have a C# method that takes in parameters some arguments:

public void function returnAList(string arg1, string arg2, 
    string arg3, string arg4);

I put the following code that lets me return a list from my database:

List<Person> listOfPersons = this.businessLayer.ReturnPersons();
var persons= from p in listOfPersons
  select new
  {
    FirstName = p.FirstName,
    LastName = p.LastName,
    Age = p.Age,
    Address = p.Address,
    PhoneNumber = p.PhoneNumber
  };

What I want, is to inject my args in the query so that I can obtain the following algorithm:

List<Person> listOfPersons = this.businessLayer.ReturnPersons();
    var persons= from p in listOfPersons 
    where (
      if (arg1 isNotNull)
        (p.FirstName == arg1)
      if (arg2 usNotNull)
        (p.LastName == arg2) 
      ...
     )
      select new
      {
        FirstName = p.FirstName,
        LastName = p.LastName,
        Age = p.Age,
        Address = p.Address,
        PhoneNumber = p.PhoneNumber
      };

Is there a way to do that?

Question 2

Typically you do like this:

IEnumrable<Person> listOfPersons = this.businessLayer.ReturnPersons();

if (arg1 isNotNull)
    listOfPersons = listOfPersons.Where(p=> p.FirstName == arg1);
if (arg2 usNotNull)
    listOfPersons = listOfPersons.Where(p=> p.LastName == arg2);
...

var persons= from p in listOfAPerson
  select new
  {
    FirstName = p.FirstName,
    LastName = p.LastName,
    Age = p.Age,
    Address = p.Address,
    PhoneNumber = p.PhoneNumber
  };

Question 3

var persons = from p in listOfPersons
where arg1 == null || p.FirstName == arg1
   && arg2 == null || p.LastName == arg2
   && ...
  select new
  {
    FirstName = p.FirstName,
    LastName = p.LastName,
    Age = p.Age,
    Address = p.Address,
    PhoneNumber = p.PhoneNumber
  };

Albin’s suggestion is also a good way to do it

Question 4

Difficult to do portably with the syntax you have.

If you make the function take arsg as params, you could do:

var persons= from p in listOfAPerson
  where args.Where(arg=>arg!=null).Contains(p.FirstName)
  select new
  {
    FirstName = p.FirstName,
    LastName = p.LastName,
    Age = p.Age,
    Address = p.Address,
    PhoneNumber = p.PhoneNumber
  };

You haven’t specified what your function arguments mean, however. What are arg3 and arg4? Is arg3 to be compared to a FirstName and arg4 to be compared to a LastName, or something else?

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