# V3220. The result of the LINQ method with deferred execution is never used. The method will not be executed.

The analyzer has detected that the result of the LINQ method with deferred execution is not executed.

The example:

    public static void PrintNames(List<string> names, char startLetter)
    {
      int it = 0;
      var filteredNames = names.Where(name =>
                          {
                            ++it;
                            Console.WriteLine($"{it}) {name}");
                            return name.StartsWith(startLetter);
                          });

      Console.WriteLine($"Type: '{filteredNames.GetType()}'");
      Console.WriteLine(String.Join(',', names));
    }

A delegate that both applies a filter condition and prints elements along with their positions from the original collection to the console is passed to the `Where` method.

However, neither the output of names with positions nor filtering will occur. Instead of `filteredNames`, `names` is passed to the`String.Join` method. As a result, the filtered collection is not printed to the console, and filtering does not occur at all. This behavior occurs because when `Where` is called, filtering is not executed immediately but is deferred. So, the delegate code will not be executed until the collection is iterated over.

You can learn more about deferred execution here.

Note that after calling `Where` via the `GetType` method, the collection type is printed. When this method is called, iteration over the collection is not executed, which means that the delegate passed to `Where` will not operate either.

To fix this error, pass `filteredNames` instead of `names` to `String.Join`:

    public static void PrintNames(List<string> names, char startLetter)
    {
      int it = 0;
      var filteredNames = names.Where(name =>
                          {
                            ++it;
                            Console.WriteLine($"{it}) {name}");
                            return name.StartsWith(startLetter);
                          });

      Console.WriteLine($"Type: '{filteredNames.GetType()}'");
      Console.WriteLine(String.Join(',', filteredNames));
    }

When `String.Join` is called, `filteredNames` is iterated over, triggering the collection filtering. So, the delegate code will be executed, and the elements with positions as well as the filtered collection are printed to the console.

This diagnostic is classified as:

CWE-563

You can look at examples of errors detected by the V3220 diagnostic.