C#•3y ago

Where does LinQ store its IEnumerable selections?

Say I have the following sample code

char[] upperCase0 = { 'A', 'B', 'C', 'D' };
string[] upperCase = { "A", "B", "C", "D" };
string[] lowerCase = { "c","b","a"};

var iterB = from up0 in upperCase0
            from up in upperCase
            from low in lowerCase
            where Char.ToLower(up0).ToString() == low && low== up.ToLower()
            select new { up0, up, low };

char[] upperCase0 = { 'A', 'B', 'C', 'D' };
string[] upperCase = { "A", "B", "C", "D" };
string[] lowerCase = { "c","b","a"};

var iterB = from up0 in upperCase0
            from up in upperCase
            from low in lowerCase
            where Char.ToLower(up0).ToString() == low && low== up.ToLower()
            select new { up0, up, low };

List<Person> personList = new List<Person>()
{ 
    new Person() { Name="John", Age=32, Kids=2, Gender='M'},
    new Person() { Name="Ana", Age=28, Kids=3, Gender='F'},
    new Person() { Name="Mary", Age=42, Kids=2, Gender='F'},
    new Person() { Name="Jason", Age=33, Kids=1, Gender='M'},
    new Person() { Name="Mike", Age=22, Kids=0, Gender='M'}
};

var iterC = personList
    .Where(p => p.Age > 30)
    .OrderByDescending(p => p.Age)
    .Select(p => p.Name);

List<Person> personList = new List<Person>()
{ 
    new Person() { Name="John", Age=32, Kids=2, Gender='M'},
    new Person() { Name="Ana", Age=28, Kids=3, Gender='F'},
    new Person() { Name="Mary", Age=42, Kids=2, Gender='F'},
    new Person() { Name="Jason", Age=33, Kids=1, Gender='M'},
    new Person() { Name="Mike", Age=22, Kids=0, Gender='M'}
};

var iterC = personList
    .Where(p => p.Age > 30)
    .OrderByDescending(p => p.Age)
    .Select(p => p.Name);

Looking at the debugger, you can see that both iterB and iterC are null. Where exactly does LinQ store its queried data?

6 Replies

Thinker•3y ago

Linq doesn't materialize any data until you explicitly request it So it's not stored anywhere, it's generated on the fly when you iterate the IEnumerable. A Linq query is merely a hierarchy of instructions on how to transform an underlying enumerable. Essentially a Linq query returns a structure like this

Original collection: IEnumerable
Transformation: lambda expression
Get enumerator: Enumerate the original collection and apply the transformation onto every element

Original collection: IEnumerable
Transformation: lambda expression
Get enumerator: Enumerate the original collection and apply the transformation onto every element

That original collection may be an IEnumerable generated by another Linq query, thus you can chain them.

canton7•3y ago

Don't get too hung up on linq. Where for instance is essentially as simply as:

public static void Where<T>(this IEnumberable<T> input, Func<T, bool> predicate)
{
    foreach (T item in input)
    {
        if (predicate(item))
        {
            yield return item;
        }
    }
}

public static void Where<T>(this IEnumberable<T> input, Func<T, bool> predicate)
{
    foreach (T item in input)
    {
        if (predicate(item))
        {
            yield return item;
        }
    }
}

(There's some extra error handling and optimizations, but that's essentially it)

Thinker•3y ago

yep And note the special syntax here, yield again doesn't materialize anything until it's enumerated.

canton7•3y ago

Yeah, if you're not comfortable with yield return and enumerators/etc, I'd recommend getting comfortable with those before starting on linq

surwrenOP•3y ago

I'm more worried about troubleshooting Because the data should be located somewhere in memory, otherwise how is my

foreach

foreach

method even outputting anything

canton7•3y ago

The next element in the sequence is generated on the next iteration of your foreach loop That's why I said to get comfortable with yield return first

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Where does LinQ store its IEnumerable selections?