[SOLVED] Why are these hashcodes equals?

Why are these hashcodes equals?

This test is failing :

var hashCode = new 
{
    CustomerId = 3354,
    ServiceId = 3,
    CmsThematicId = (int?)605,
    StartDate = (DateTime?)new DateTime(2013, 1, 5),
    EndDate = (DateTime?)new DateTime(2013, 1, 6)
}.GetHashCode();
var hashCode2 = new
{
    CustomerId = 1210,
    ServiceId = 3,
    CmsThematicId = (int?)591,
    StartDate = (DateTime?)new DateTime(2013, 3, 31),
    EndDate = (DateTime?)new DateTime(2013, 4, 1)
}.GetHashCode();
Assert.AreNotEqual(hashCode, hashCode2);

Can you tell me why ?

Solution

Jim suggested me (in the chat room) to store my parameters so when i display my parameters, select the not used ones, then when someone registers I flag it as used. But it's a big PITA to generate all the parameters.

So my solution is to build a int64 hashcode like this

const long i = -1521134295;    
return -i * (-i * (-i * (-i * -117147284 + customerId.GetHashCode()) + serviceId.GetHashCode()) + cmsThematicId.GetHashCode()) + startDate.GetHashCode();

I removed the end date because Its value was depending on serviceId and startDate so I shouldn't have add this to the hashcode in the firstplace. I copy/pasted it from a decompilation of the generated class. I got not colision if I do a test with 300 000 differents combinations.