Undocumented behavior changes of Uri.TryCreate between net6 and net8

[letientai299]

Description

Uri.IsLoopback returns true for https://LOCALHOST in net6, but false in net8.

This should be documented as a net8 breaking change. I also check and found no related item in net7 breaking changes page.

Reproduction Steps

Create a simple console app project with this file structure

.
├── Loopback.6.csproj
├── Loopback.8.csproj
└── Program.cs

Content of Program.cs

using System;

const string s = "https://LOCALHOST";
Uri.TryCreate(s, UriKind.Absolute, out var u);
Console.WriteLine(u?.IsLoopback);

Content of Loopback.6.csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
  </PropertyGroup>
</Project>

Content of Loopback.8.csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
  </PropertyGroup>
</Project>

Then, execute the program with each runtime

$ dotnet restore Loopback.8.csproj -v quiet && dotnet run --project Loopback.8.csproj --quiet
False

$ dotnet restore Loopback.6.csproj -v quiet && dotnet run --project Loopback.6.csproj --quiet
True

Expected behavior

N/A

Actual behavior

N/A

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

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[MihaZupan]

This was an unintended regression in .NET 8.
It's been fixed in .NET 9: #105008.

[letientai299]

I see. We have manually fixed it, but I think the docs should still be updated to inform those users who want to stay on LTS releases.