Weird performance results using Dictionary<>

It seems that Dictionary<,> performance is affected by the size of the item being stored (which seems bizarre).

Here's my simple class:

public class MyObject
{
    public Guid Key { get; set; }
}

And two simple tests:

private long _Iterations = 1000000;

[TestMethod]
public void ShouldTestDefaultConstructorPerformance()
{
    for (var i = 0; i < _Iterations; i++)
    {
        var obj = new MyObject() { Key = Guid.NewGuid() };
    }
}

[TestMethod]
public void ShouldTestDefaultGuidDictionaryPerformance()
{
    var dict = new Dictionary<Guid, MyObject>();
    for (var i = 0; i < _Iterations; i++)
    {
        var obj = new MyObject() { Key = Guid.NewGuid() };
        dict.Add(obj.Key, obj);
    }
}

Initially I get the following timings:

ShouldTestDefaultConstructorPerformance    : 00:00:00.580
ShouldTestDefaultGuidDictionaryPerformance : 00:00:01.238

Now, I'll change MyObject class:

public class MyObject
{
    public Guid Key { get; set; }

    private Dictionary<string, string> _Property0 = new Dictionary<string, string>();
    private Dictionary<string, string> _Property1 = new Dictionary<string, string>();
    private Dictionary<string, string> _Property2 = new Dictionary<string, string>();
    private Dictionary<string, string> _Property3 = new Dictionary<string, string>();
    private Dictionary<string, string> _Property4 = new Dictionary<string, string>();
    private Dictionary<string, string> _Property5 = new Dictionary<string, string>();
    private Dictionary<string, string> _Property6 = new Dictionary<string, string>();
    private Dictionary<string, string> _Property7 = new Dictionary<string, string>();
    private Dictionary<string, string> _Property8 = new Dictionary<string, st开发者_运维问答ring>();
    private Dictionary<string, string> _Property9 = new Dictionary<string, string>();
}

And run the tests again:

ShouldTestDefaultConstructorPerformance    : 00:00:01.333
ShouldTestDefaultGuidDictionaryPerformance : 00:00:07.556

In the second test, the object construction takes 1.72x longer, but adding to the Dictionary takes 6.11x longer. I expected the tests to take longer, but why does the Dictionary take so much longer to add larger objects?

I think people need to read questions more carefully instead of rushing to post an answer. If you look at his sample code carefully (BOTH tests), the difference between having a MyObject with a Guid and MyObject with a Guid and 10 Dict's is under a second (object construction) for his loop. However the add dictionary accounts for at least 5 more seconds.

I think my answer would be: use a profiler and work out which bit is actually taking longer.

That'd probably highlight the instantiation. Maybe :)

I would think that var obj = new MyObject() { Key = Guid.NewGuid() }; this line actually takes much longer and not Add() of dictionary. Did you make measuring inside methods provided?

Each object you add to the dictionary is given a special unique identifier to speed up its searching and retrieval in memory. This special unique identifier (called a hash) is computed by analysing the entire content of the object. The larger the object, the slower it will be to compute the hash.

If you are interested in the fine details of how it works, check out this example from a university course: http://www.ccs.neu.edu/home/sbratus/com1101/hash-dict.html