Unexpected Guid change during a Linq request

March 27, 2023

I would like to understand the following snippet:

class TestNewGuid
{
    public Guid RequestId { get; set; } = Guid.NewGuid();

    // any other fields
}

[Fact]
public void Test()
{
    var toInitAnEnumerable = new int[] {1};
    var requests = toInitAnEnumerable
        .Select(a => new TestNewGuid());

    var requestIds = requests
        .Select(a => a.RequestId)
        .ToHashSet(); // this calls the TestNewGuid's constructor
    
    Assert.Equal(requestIds.First(), requests.First().RequestId); // Always fails
}

I’m surprise by the fact that the RequestId changes.

I know that ToHashSet() will create a copy, but because I have the .Select(a => a.RequestId) before, I don’t understand why it creates a copy of TestNewGuid and not of the Guid.

Any idea?

>Solution :

Select is not evaluated when you call it, it is what is known as "deferred execution" – which is to say it only runs when a method which will materialize the enumerable is executed. There are 2 such methods in your code; First() in the assertion, and ToHashSet().

Both of these calls will cause your enumerable to be evaluated, and thus call your construction of TestNewGuid