How is an Enumerable converted to a Dictionary?

Question

I have the following code from MSDN sample:

if (sheetData.Elements<Row>().Where(r => r.RowIndex == rowIndex).Count() != 0)
{
    row = sheetData.Elements<Row>().Where(r => r.RowIndex == rowIndex).First();
...

Which I refactored as follows:

Dictionary<uint, Row> rowDic = sheetData.Elements<Row>().ToDictionary(r => r.RowIndex.Value);
if (rowDic[rowIndex].Count() != 0)
{
    row = rowDic[rowIndex];
...

Now, I sensed that if the Enumerable.ToDictionary<> method actually has to enumerate through all the data, then this would be as well redundant, but the MSDN documentation does not say anything about how this conversion takes place.

The alternative I'm thinking of using is:

var foundRow = sheetData.Elements<Row>().Where(r => r.RowIndex == rowIndex);
if (foundRow.Count() != 0)
{
    row = foundRow.First();
...

However, I would like to know from possibly previous experiences which would be faster and why.

Thanks.

*What* would be redundant? It's not really clear what you're asking. But `ToDictionary` *does* iterate through the whole input sequence, eagerly. — Jon Skeet, May 08 '13 at 13:48
If you want to know which of two things is faster then **run them** and you'll soon find out. — Eric Lippert, May 08 '13 at 15:01

Jon Skeet · Accepted Answer · 2013-05-08T13:54:51.780

5

The cleaner alternative is:

var row = sheetData.Elements<Row>()
                   .FirstOrDefault(r => r.RowIndex == rowIndex);
if (row != null)
{
    // Use row
}

That will only iterate through the sequence once, and it will stop as soon as it finds a match.

edited May 08 '13 at 13:54

answered May 08 '13 at 13:48

Jon Skeet

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Redundant .Where clause. Lambda can be moved to .FirstOrDefault. e.g. `.FirstOrDefault(r => r.RowIndex == rowIndex);` – spender May 08 '13 at 13:51
@spender: True - I always forget about that. (Ditto for Count). Nice catch. – Jon Skeet May 08 '13 at 13:55
Same. Resharper usually saves me. – spender May 08 '13 at 14:00

score 1 · Answer 2 · answered May 08 '13 at 13:52

Both the .Count() and ToDictionary methods have to enumerate all elements to obtain the result.

Here's the most efficient implementation:

var foundRow = sheetData.Elements<Row>().FirstOrDefault(r => r.RowIndex == rowIndex);
if (foundRow != null)
{
    row = foundRow;

...

How is an Enumerable converted to a Dictionary?

2 Answers2