Question

I have this function to repeat a sequence:

public static List<T> Repeat<T>(this IEnumerable<T> lst, int count)
{
    if (count < 0)
        throw new ArgumentOutOfRangeException("count");

    var ret = Enumerable.Empty<T>();

    for (var i = 0; i < count; i++)
        ret = ret.Concat(lst);

    return ret.ToList();
}

Now if I do:

var d = Enumerable.Range(1, 100);
var f = d.Select(t => new Person()).Repeat(10); 
int i = f.Distinct().Count();

I expect i to be 100, but its giving me 1000! My question strictly is why is this happening? Shouldn’t Linq be smart enough to figure out that it’s the first selected 100 persons I need to concatenate with variable ret? I’m getting a feeling that here the Concat is being given preference when it’s used with a Select when its executed at ret.ToList()..

Edit:

If I do this I get the correct result as expected:

var f = d.Select(t => new Person()).ToList().Repeat(10); 
int i = f.Distinct().Count(); //prints 100

Edit again:

I have not overridden Equals. I’m just trying to get 100 unique persons (by reference of course). My question is can someone elucidate to me why is Linq not doing the select operation first and then concatenation (of course at the time of execution)?

Answer 1

The problem is that unless you call ToList, the d.Select(t => new Person()) is re-enumerated each time the Repeat goes through the list, creating duplicate Persons. The technique is known as the deferred execution.

In general, LINQ does not assume that each time it enumerates a sequence it would get the same sequence, or even a sequence of the same length. If this effect is not desirable, you can always “materialize” the sequence inside your Repeat method by calling ToList right away, like this:

public static List<T> Repeat<T>(this IEnumerable<T> lstEnum, int count) {
    if (count < 0)
        throw new ArgumentOutOfRangeException("count");

    var lst = lstEnum.ToList(); // Enumerate only once
    var ret = Enumerable.Empty<T>();

    for (var i = 0; i < count; i++)
        ret = ret.Concat(lst);

    return ret.ToList();
}