C#: yield return within a foreach fails - body cannot be an iterator block

Question

Consider this bit of obfuscated code. The intention is to create a new object on the fly via the anonymous constructor and yield return it. The goal is to avoid having to maintain a local collection just to simply return it.

public static List<DesktopComputer> BuildComputerAssets()
{           
    List<string> idTags = GetComputerIdTags();

    foreach (var pcTag in idTags)
    {
        yield return new DesktopComputer() {AssetTag= pcTag
                                          , Description = "PC " + pcTag
                                          , AcquireDate = DateTime.Now
                                           };
    }            
}

Unfortunately, this bit of code produces an exception:

Error 28 The body of 'Foo.BuildComputerAssets()' cannot be an iterator block because 'System.Collections.Generic.List' is not an iterator interface type

Questions

• What does this error message mean?
• How can I avoid this error and use yield return properly?

Answer 1

You can only use yield return in a function that returns an IEnumerable or an IEnumerator, not a List<T>.

You need to change your function to return an IEnumerable<DesktopComputer>.

Alternatively, you can rewrite the function to use List<T>.ConvertAll:

return GetComputerIdTags().ConvertAll(pcTag => 
    new DesktopComputer() {
        AssetTag    = pcTag,
        Description = "PC " + pcTag,
        AcquireDate = DateTime.Now
    });

Answer 2

Your method signature is wrong. It should be:

public static IEnumerable<DesktopComputer> BuildComputerAssets()

Answer 3

yield only works on Iterator types:

The yield statement can only appear inside an iterator block

Iterators are defined as

The return type of an iterator must be IEnumerable, IEnumerator, IEnumerable<T>, or IEnumerator<T>.

IList and IList<T> do implement IEnumerable/IEnumerable<T>, but every caller to an enumerator expects one of the four types above and none else.

Answer 4

You could also implement the same functionality using a LINQ query (in C# 3.0+). This is less efficient than using ConvertAll method, but it is more general. Later, you may also need to use other LINQ features such as filtering:

return (from pcTag in GetComputerIdTags()
        select new DesktopComputer() { 
          AssetTag    = pcTag, 
          Description = "PC " + pcTag, 
          AcquireDate = DateTime.Now 
        }).ToList();

The ToList method converts the result from IEnumerable<T> to List<T>. I personally don't like ConvertAll, because it does the same thing as LINQ. But because it was added earlier, it cannot be used with LINQ (it should have been called Select).