Friday, 31 May 2019

What is the yield keyword used for in C#?



In the How Can I Expose Only a Fragment of IList<> question one of the answers had the following code snippet:



IEnumerable FilteredList()
{
    foreach(object item in FullList)

    {
        if(IsItemInPartialList(item))
            yield return item;
    }
}


What does the yield keyword do there?  I've seen it referenced in a couple places, and one other question, but I haven't quite figured out what it actually does.  I'm used to thinking of yield in the sense of one thread yielding to another, but that doesn't seem relevant here.

    
 
Answer
 

The yield keyword actually does quite a lot here.



The function returns an object that implements the IEnumerable interface. If a calling function starts foreaching over this object, the function is called again until it "yields". This is syntactic sugar introduced in C# 2.0. In earlier versions you had to create your own IEnumerable and IEnumerator objects to do stuff like this.


The easiest way understand code like this is to type-in an example, set some breakpoints and see what happens. Try stepping through this example:


public void Consumer()
{
    foreach(int i in Integers())
    {
        Console.WriteLine(i.ToString());

    }
}

public IEnumerable Integers()
{
    yield return 1;
    yield return 2;
    yield return 4;
    yield return 8;
    yield return 16;

    yield return 16777216;
}


When you step through the example, you'll find the first call to Integers() returns 1. The second call returns 2 and the line yield return 1 is not executed again.


Here is a real-life example:


public IEnumerable Read(string sql, Func make, params object[] parms)
{

    using (var connection = CreateConnection())
    {
        using (var command = CreateCommand(CommandType.Text, sql, connection, parms))
        {
            command.CommandTimeout = dataBaseSettings.ReadCommandTimeout;
            using (var reader = command.ExecuteReader())
            {
                while (reader.Read())
                {
                    yield return make(reader);

                }
            }
        }
    }
}

    










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