Difference between `where` in the query object and `if` in the extension methods

I am studying LINQ.

I don’t know the difference between using if in extension method and using where in query object.

The Console.WriteLine() results are the same, is there any difference in speed?

If you think about it, there seems to be a difference in readability, but that’s my guess.

To be honest, I know it’s a useless curious, but I was so curious about it, so I wrote it.

We look forward to hearing from you.

Oh, if there is anything in the article that needs to be improved, please advise.

...

public static IEnumerable<Student> GetTeenAgerStudents(this IEnumerable<Student> source)
{
  foreach (Student std in source)
  {
    yield return std;
  }
}

...

var teenAgerStudents = from s in studentList.GetTeenAgerStudents()
                       where s.Age >= 10 && s.Age < 20
                       select s;

...

public static IEnumerable<Student> GetTeenAgerStudents(this IEnumerable<Student> source)
{
  foreach (Student std in source)
  {
    if (std.Age >= 10 && std.Age < 20)
          yield return std;
  }
}

...

var teenAgerStudents = from s in studentList.GetTeenAgerStudents()
                       select s;

The above code is referenced from https://www.tutorialsteacher.com/linq/linq-deferred-execution.

>Solution :

I prediction the exact speed difference between them. if is faster. But for legibility reasons, where is the most commonly used route. You can understand where like a sql query. Here, it filters some data from the data collection and reveals the ones that are suitable for you. Looks like T-Sql.