C# contains behaviour

Question

Can someone please explain why these two methods are returning different values?

List<CustomerSummary> summaries = new List<CustomerSummary>();

for (var i = 0; i < 10; i++)
{

var summary = new CustomerSummary() { ID = 1, Name = "foo", balance = 50.00 };

if (!summaries.Contains(summary))
    summaries.Add(summary);

}

-

List<CustomerSummary> summaries = new List<CustomerSummary>();

for (var i = 0; i < 10; i++)
{

 var summary = new CustomerSummary() { ID = 1, Name = "foo", balance = 50.00 };

 if (!summaries.Any(s=> s.ID == summary.ID))
     summaries.Add(summary);

}

The first method has 10 items in the list while the second has 1. Why does the first ( Contains() ) method never evaluate as true?

What I'm trying to ask is why are 2 objects of the same type with the same values for each property not evaluating as equivalent (in the first method)?

Answer 1

Your first block of code is creating a new instance inside the loop, and then immediately checking to see if that exact instance is already in the collection. It can't be - you just created it.

You might as well write it like this:

List<CustomerSummary> summaries = new List<CustomerSummary>();

for (var i = 0; i < 10; i++)
{
    var summary = new CustomerSummary { ID = 1, Name = "foo", balance = 50.00 };

    summaries.Add(summary);
}

Your second block of code is checking for the existence of any item in the collection with a matching ID. Since you've hard-coded the ID to be 1, you're only going to add an item the first time around. Every instance created after that will return false in the if statement, and will not be added.

You could change that behavior by changing the ID:

for (var i = 0; i < 10; i++)
{
    var summary = new CustomerSummary { ID = i, Name = "foo", balance = 50.00 };

    if (!summaries.Any(s=> s.ID == summary.ID))
        summaries.Add(summary);
}

Again, the if statement is no longer even necessary, since you know the ID can't possibly already exist in the collection, so you could just remove the check.