Why is AutoMapper enumerating an IEnumerable multiple times?

user_1986991 0
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We ran in to performance issues and tracked it down to AutoMapper enumerating twice over an IEnumerable. It's easily fixable on our side by simply feeding AutoMapper a List or Array, but I'm still curious if this is expected behavior.

The following minimal repro test fails::

[Test]
public void AutoMapper_Should_Not_Enumerate_Multiple_Times()
{
    var counter = 0;
    var values = Enumerable.Range(0, 3).Select(_ => counter++);
    var mapper = new Mapper(new MapperConfiguration(_ => { }));

    var dest= mapper.Map<int[]>(values);

    Assert.Equal(3, counter);
}

Result: Counter = 6, dest = [3,4,5], which means the IEnumerable is iterated twice and the second iteration is returned.

I tried debugging this in AutoMapper, but I only figured out that something was first calling Count() and then iterating:

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Is this expected behavior? How or where is an IEnumerable mapped to an Array and why is Count() called before iterating?

automapper
1 Answers

This is implemented by the ArrayMapper and the implementation is functionally this:

var count = source.Count();
var array = new TDestination[count];

int i = 0;
foreach (var item in source)
    array[i++] = newItemFunc(item, context);
return array;
user_110990
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