How does the compiler use 'yield return' to build a class

Question

In specific, if I say:

    public static IEnumerable<String> Data()
    {
        String connectionString = "...";
        using (SqlConnection connection = new SqlConnection(connectionString))
        {
            connection.Open();

            IDataReader reader = new SqlCommand("", connection).ExecuteReader();

            while (reader.Read())
                yield return String.Format("Have a beer {0} {1}!", reader["First_Name"], reader["Last_Name"]);

            connection.Close();
        }
    }

How does the compiler go about generating a concrete enumerable class out of this?

Answer 1

It builds a state machine, basically:

• It creates a private nested class, with instance variables corresponding to the local variables in your method, and a state variable to keep track of where it's got to
• The state machine implements IEnumerable<T> and IEnumerator<T> - the MoveNext() method gets to the right bit of the logic (based on state) and sets an instance variable to keep track of the last-yielded value (the Current property)
• The compiler creates a "skeleton" method with the same signature as your original, which creates an instance of the state machine

See my article on the topic for more details. Also note that async/await in C# 5 is built with a lot of the same ideas (although there are various implementation differences).