Linq 表达式未按预期工作

Question

I want to select some values from a collection of strings, I wrote it one using LINQ and one using a foreach statement.

With the foreach version I get a list of about 300 entries.

List<string> res = new List<String>();
foreach (var l in anchors)
{
      if (l.Attributes["href"] != null)
      {
          res.Add(l.Attributes["href"].Value);
      }
}

With the LINQ version I get null :

IEnumerable<string> res2 = anchors.Select(l => l?.Attributes["href"]?.Value);

Answer 1

With the linq, you're getting null values as well and adding it to your enumerable. It's not identical to your foreach. Change it to:

IList<string> res2 = anchors.Where(l=>l.Attributes["href"] != null).Select(l => l.Attributes["href"].Value).ToList();

Answer 2

The .? syntax returns null if the item it is applied to returns null . In this case, null is added to the output.

With the check if (l.Attributes["href"] != null) it is not added to the output.

To mimic that in LINQ, add a Where clause.