Beware - GuidV7 Generation In High Throughput Environments Sorting Gotcha

One of the features introduced in .NET 9 was ab ability to generate RFC 9562 complaint Guid values, in the version 7 format.

These were meant to be sortable based on the timestamp.

They are generated like this:

Guid.CreateVersion7();

Let us generate a number of these, with a brief pause to simulate work:

var list = new List<Guid>();
for (var i = 0; i < 10; i++)
{
  var temp = Guid.CreateVersion7();
  Console.WriteLine(temp);
  list.Add(temp);
  Thread.Sleep(1);
}

Console.WriteLine();

As we generate each s, we add them to a List of Guid.

Once we are done, we sort this list and print the contents to the console.

list.Sort();

list.ForEach(x => Console.WriteLine(x));

Now let us remove the pause, so that our code looks like this:

var list = new List<Guid>();
for (var i = 0; i < 10; i++)
{
  var temp = Guid.CreateVersion7();
  Console.WriteLine(temp);
  list.Add(temp);
}

Console.WriteLine();

list.Sort();

list.ForEach(x => Console.WriteLine(x));

The results now are different!

We can see here that they sort differently.

To understand why we need to understand the structure of a V7 Guid.

Here I have illustrated the structure from left to right, with each section size in bits.

If the Guid generation happens to be at the same millisecond, sorting will switch to the random blocks, hence the difference in sorted output.

This means the sorted order will be different from the generated order.

TLDR

V7 Guids generated by Guid.CreateVersion7() sort differently if multiple happen to be generated in the same millisecond.

The code is in my GitHub.

Happy hacking!