Can't use C# .NET 6 PriorityQueue in Unity

Question

I'm trying to use PriorityQueue in Unity with C#. The documentation says that it's supported in .NET 6 in namespace System.Collections.Generic.

I've tried:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using TMPro;

public class Test : Monobehaviour
{
    void Start()
    {
        var queue = new PriorityQueue<int, int>();
    }
}

But an error is thrown back:

The type or namespace name 'PriorityQueue<,>' could not be found (are
you missing a using directive or an assembly reference?)
[Assembly-CSharp]

I've checked the .NET version within VS Code:

Why doesn't it work in Unity?

Answer 1

The version reported by dotnet is completely unrelated to Unity's C#/.NET version (at least until Unity finally migrates from Mono and then moves to CoreCLR etc.). While the language supported is almost equal to C# 9.0, the actual library that is used is a loose superset of what Mono provides, and actually is roughly equal to .NET Framework 4.8/.NET Standard 2.0 ATM; since neither of those has PriorityQueue, it's simply not available. You can either:

• wait for the transition to .NET 6+ happen (be aware it may take a couple of years - it's still an ongoing effort in 2025, with CoreCLR support being announced as "to come" for ~Unity 7 betas :),
• use a 3rd party implementation,
• implement it yourself,
• copy-paste the code from the official source (minor tweaks will be required for it to work) or
• use this direct port from official C# lib (single-file drop-in, ported by me, open source, no string attached).