The best way to understand how “LinQ to objects” (in future just linq) works is to create a method by yourself. We will write the AwesomeSelect method which has the same functionality as real linq Select method. If you are interested in then welcome under the cut.

AwesomeSelect

Select method is an extension of IEnumerable, so let’s open our favourite editor (e.g. VIM) and create our AwesomeSelect extension.

public static class EnumerableAwesomeExtensions
{
public static IEnumerable<To> AwesomeSelect<From, To>(
this IEnumerable<From> source,
Func<From, To> map)
{
return new AwesomeSelectSequence<From, To>(source, map);
}
}

This method takes IEnumerable source and function which maps values of source type into To type. And returns IEnumerable represented by custom select sequence.

public class AwesomeSelectSequence<From, To> : IEnumerable<To>
{
private readonly IEnumerable<From> _values;
private readonly Func<From, To> _map;

public AwesomeSelectSequence(
IEnumerable<From> values,
Func<From, To> map)
{
_values = values;
_map = map;
}

public IEnumerator<To> GetEnumerator()
{
var enumerator = _values.GetEnumerator();

return new AwesomeSelectEnumerator<From, To>(
enumerator,
_map);
}

IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}

This sequence as any IEnumerable object returns IEnumerator. In our case this Enumerator is custom and all the magic lives here. Let’s look at the following piece of code.

public class AwesomeSelectEnumerator<From, To> : IEnumerator<To>
{
private readonly IEnumerator<From> _enumerator;
private readonly Func<From, To> _map;

public AwesomeSelectEnumerator(
IEnumerator<From> enumerator,
Func<From, To> map)
{
_enumerator = enumerator;
_map = map;
}

public To Current
{
get
{
return _map(_enumerator.Current);
}
}

object IEnumerator.Current
{
get
{
return Current;
}
}

public bool MoveNext()
{
return _enumerator.MoveNext();
}

public void Reset()
{
_enumerator.Reset();
}

public void Dispose()
{
_enumerator.Dispose();
}
}

This class works over IEnumerator of our source except property Current. When client code accessing Current property we just apply our map function to Current element of source and return the result.

The result of our work is the following:

string[] emails = { "a", "ab", "abc", "abcd" };

IEnumerable<int> result = emails.AwesomeSelect(e => e.Length);

foreach (var r in result)
{
Console.WriteLine(r);
}

If you want to deep more you are welcome to read my next article.