Empty Sequence in LINQ

I recently faced an interview question related to LINQ.

What is the use of empty sequence?.He asked "if i suppose to ask you to use the one,where do you fit it?"

     public static IEnumerable<TResult> Empty<TResult>()
     {
       开发者_如何学Go yield break;
     }

I did not answer it.Help is appreciated.

If you had a loop that aggregated together different sets into a result set you can use it to initialize your result set variable and loop/accumulate. For example:

IEnumerable<string> results = Enumerable.Empty<string>();

for(....)
{
    IEnumerable<string> subset = GetSomeSubset(...);

    results = results.Union(subset);
}

Without Empty you'd have to have written a null check into your loop logic:

IEnumerable<string> results = null;

for(....)
{
    IEnumerable<string> subset = GetSomeSubset(...);

    if(results == null)
    {
        results = subset;
    }
    else
    {
        results = results.Union(subset);
    }
}

It doesn't just have to be a loop scenario and it doesn't have to be Union (could be any aggregate function), but that's one of the more common examples.

You can use this when you want to quickly create an IEnumerable<T> this way you don't have to create a reference to a new List<T> and take advantage of the yield keyword.

List<string[]> namesList =
    new List<string[]> { names1, names2, names3 };

// Only include arrays that have four or more elements
IEnumerable<string> allNames =
    namesList.Aggregate(Enumerable.Empty<string>(),
    (current, next) => next.Length > 3 ? current.Union(next) : current);

Note the use of Union because it is not a List you can not call Add method, but you could call Union on an IEnumerable