Where clause in Linq in List c#

Question

I have a struct like this:

struct Test
{
   string name;
   string family;
   public Test...
}

in my code I have a list of this struct:

List<Test> testList=new List<Test>();

I have a linq expression like this:

var list =testList.Select(n=>n.Name);

but how can I filter this selection by Family of the testList? something like this:

var list=testList.Select(n=>n.Name).Where(f=>f.Family=="");

this Where clause just apply on selected Names which are a list of string

Any ideas how to do that?

Answer 1

Just put the Where before the Select:

var list=testList.Where(f=>f.Family=="").Select(n=>n.Name);

In Linq you need to apply the filter before projecting (unless the filter applies to the results of the projection rather than the original collection).

Answer 2

Filter using Where before you select just one property using Select. That way, you still get the full object:

testList.Where(t => t.Family == "").Select(t => t.Name)

After all, Select will take the object and then only pass on whatever you return in the lambda. In this case, you only return a string, so you throw all the other information from the Test object away. And as such, the information you want to filter on isn’t available anymore.

If you switch that around, you can filter on the test object, and then return only that one string.