Refine Uri Host property documentation: clarify IdnHost behavior and focus on Host/IdnHost

[Copilot]

Documentation for Uri.Host, Uri.IdnHost, and Uri.DnsSafeHost needed refinement to avoid overpromising IdnHost behavior and to steer users toward the two primary properties.

Changes

• Clarified IdnHost punycode encoding: Added qualification that punycode applies to "valid international domain names" with explicit note that invalid/malformed hostnames might return non-ASCII values

• Refocused property comparisons: Changed from three-property comparisons to Host vs IdnHost comparisons, positioning DnsSafeHost as a legacy property dependent on configuration

• Simplified code examples: Kept only C# snippets, removed F# and VB versions

• Fixed DnsSafeHost example comment: Updated from "depends on configuration" to actual value münchen.de

Example

Updated documentation now states:

For **regular DNS hostnames**: For valid international domain names, non-ASCII characters 
are punycode encoded (e.g., `münchen.de` becomes `xn--mnchen-3ya.de`). Invalid or 
malformed hostnames might return non-ASCII values.

Comparison sections now guide users to choose between:

• Host: Original host string as it appears in the URI (for display/comparison)
• IdnHost: Format suitable for DNS resolution (punycode for valid IDNs, zone IDs for IPv6)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Uri] Fix IPv6 address format</issue_title>
<issue_description>Here are the issues in Uri with IPv6 address we may want to fix:

1. [Pending discussion] Uri.IdnHost should include [] around IPv6 address.
   • Currently it returns ::1234, it should return [::1234].
2. [Originally done with PR Fix Uri.Host for IPv6 Link-local address corefx#29829 but reverted with Revert "Fix Uri.Host for IPv6 Link-local address (#29829)" corefx#30062] Uri.Host LLA (Link-local address) IPv6 address doesn't contain %number part.
   • Currently it returns [fe80::e077:c9a3:eeba:b8e9], it should return [fe80::e077:c9a3:eeba:b8e9%18].
   • Note: Uri.IdnHost correctly contains the %number part.

If we choose to fix these problems, we can undo workarounds in dotnet/corefx#28740, dotnet/corefx#28578, dotnet/corefx#28849 and dotnet/corefx#28971.

/cc: @dotnet/ncl </issue_description>

<agent_instructions>Document the differences between Host and IdnHost, how it differs re: escaping, punycoding, IPv6 brackets, IPv6 zone id etc.

Rely on information in dotnet/runtime#25782 and in this section of the Uri threat model doc:

Uri provides three separate Host properties:

• Host
  • Returns the escaped input. Non-ASCII hosts are returned as-is. IPv6 hosts include the brackets but not the scope.
• IdnHost
  • Non-ASCII hosts are punycode encoded, IPv6 hosts don't include brackets but include the scope.
• DnsSafeHost
  • Behaves like IdnHost for IPv6 and Basic hosts, and like Host for Dns ones. More of a historical artifact from before IdnHost was added.

Which one should be used depends on the use case. Typically either Host/IdnHost is used depending on whether you want the IPv6 brackets, scope ID, or punycode encoding.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@stephentoub This issue is marked as future, but isn't there a bug here in SocketsHttpHandler we need to fix in 2.1? That we're incorrectly dropping the scope from a link-local address? @stephentoub Ah, ok. @ cc @wfurt, this is also related: https://github.com/dotnet/corefx/issues/27529 @karelz Thanks @MarcoRossignoli! Top post updated. @karelz Triage: We may not be able to fix it at all, the goal is to decide in 5.0 timeframe. @wfurt I was thinking about it more and I'm not sure this is a problem other than surprise. According to documentation:

The RFC 3490 compliant International Domain Name of the host, using Punycode as appropriate. This string, after being unescaped if necessary, is safe to use for DNS resolution.

If we put scopeID or '[]` the value will no longer be safe to use for DNS IMHO and additional work would be needed by the caller to make it passable to NameResolution API.

I'm wondering what was the original intention for this property.

@wfurt I'm wondering if we should just close this @stephentoub @MihaZupan While the existing behavior may be suboptimal, changing it is going to be major breaking change IMHO. I'm not sure we would be ever brave enough to do it and this sits here long time with no use. If feels like we would be better off to document current behavior and feel with it in SocketsHttpHandler. @MihaZupan I doubt we'll be changing the behavior here given how long we've been sitting with this behavior, but we should at least document it better.

• Fixes [Uri] Fix IPv6 address format runtime#25782

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[Copilot]

Pull request overview

Updates the API reference documentation for System.Uri host-related properties to better describe real-world behavior (especially around IDN/punycode and IPv6 zone IDs) and adds a new C# snippet to compare host outputs across URI types.

Changes:

• Refines remarks for Uri.Host, Uri.IdnHost, and Uri.DnsSafeHost, including new comparison sections focused on Host vs IdnHost.
• Adds a new HostComparison C# snippet (and project) demonstrating host outputs for DNS names, IDNs, IPv4, and IPv6 (with and without zone IDs).
• Updates some wording in existing remarks/examples to be more direct and consistent in tone.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 7 comments.

File	Description
xml/System/Uri.xml	Updates remarks/comparisons for Host, IdnHost, and DnsSafeHost, and wires in the new host comparison snippet.
snippets/csharp/System/Uri/HostComparison/source.cs	New snippet demonstrating differences between Host, IdnHost, and DnsSafeHost across host types.
snippets/csharp/System/Uri/HostComparison/Project.csproj	New snippet project to compile the HostComparison example.

Comments suppressed due to low confidence (1)

xml/System/Uri.xml:1620

• The PR description says the F# and VB examples were removed, but this file still references the F# and VB snippet sources for the DnsSafeHost example. Either update the description or remove these references (and the corresponding snippet files) to keep the change consistent.

 The following example creates a <xref:System.Uri> instance from a string. It illustrates the difference between the value returned from <xref:System.Uri.Host%2A>, which returns the host name or address specified in the URI, and the value returned from <xref:System.Uri.DnsSafeHost%2A>, which returns an address that's safe to use in DNS resolution.

 :::code language="csharp" source="~/snippets/csharp/System/Uri/.ctor/nclurienhancements.cs" id="Snippet4":::
 :::code language="fsharp" source="~/snippets/fsharp/System/Uri/.ctor/nclurienhancements.fs" id="Snippet4":::
 :::code language="vb" source="~/snippets/visualbasic/System/Uri/.ctor/nclurienhancements.vb" id="Snippet4":::

[Copilot]

Other Uri C# snippet projects in this directory target net6.0. This new snippet project targets net8.0, which is inconsistent and can make snippet compilation requirements vary across the Uri docs set. Consider targeting net6.0 here as well unless there's a specific net8.0 requirement.

[Copilot]

The remarks introduce that IdnHost can return non-ASCII for invalid or malformed hostnames, but later this section still claims IdnHost is "guaranteed to always be DNS safe." That guarantee is inconsistent with the behavior described above and should be softened or removed to avoid overpromising.

Suggested change

	The <xref:System.Uri.DnsSafeHost%2A> property is dependent on configuration settings in .NET Framework apps, as discussed later in this topic. The <xref:System.Uri.IdnHost%2A> property is provided as the preferred alternative to using <xref:System.Uri.DnsSafeHost%2A> because <xref:System.Uri.IdnHost%2A> is guaranteed to always be DNS safe.
	The <xref:System.Uri.DnsSafeHost%2A> property is dependent on configuration settings in .NET Framework apps, as discussed later in this topic. The <xref:System.Uri.IdnHost%2A> property is provided as the preferred alternative to using <xref:System.Uri.DnsSafeHost%2A>, because <xref:System.Uri.IdnHost%2A> typically returns a value that's suitable for DNS resolution.

[Copilot]

This paragraph calls DnsSafeHost "deprecated", but the API isn't marked obsolete in this file (no Obsolete attribute/obsolete metadata). Consider rephrasing to "legacy"/"historical" (as used earlier in this doc) unless the API is actually deprecated/obsolete.

Suggested change

	The deprecated <xref:System.Uri.DnsSafeHost%2A> property is dependent on *app.config* settings, which can't be changed by Windows Store applications. <xref:System.Uri.IdnHost%2A> is provided as the preferred alternative to using <xref:System.Uri.DnsSafeHost%2A> because <xref:System.Uri.IdnHost%2A> is guaranteed to always be DNS safe, no matter what the current *app.config* settings might be.
	## Escaping behavior
	The legacy <xref:System.Uri.DnsSafeHost%2A> property is dependent on *app.config* settings, which can't be changed by Windows Store applications. <xref:System.Uri.IdnHost%2A> is provided as the preferred alternative to using <xref:System.Uri.DnsSafeHost%2A> because <xref:System.Uri.IdnHost%2A> is guaranteed to always be DNS safe, no matter what the current *app.config* settings might be.
	## Escaping behavior

[Copilot]

This sentence says IdnHost is "guaranteed to always be DNS safe", but earlier in this section you note that invalid or malformed hostnames might return non-ASCII values, and IPv6 results can include a zone ID. Please remove or qualify the guarantee so it matches the behavior you describe.

Suggested change

The deprecated <xref:System.Uri.DnsSafeHost%2A> property is dependent on *app.config* settings, which can't be changed by Windows Store applications. <xref:System.Uri.IdnHost%2A> is provided as the preferred alternative to using <xref:System.Uri.DnsSafeHost%2A> because <xref:System.Uri.IdnHost%2A> is guaranteed to always be DNS safe, no matter what the current *app.config* settings might be.

The deprecated <xref:System.Uri.DnsSafeHost%2A> property is dependent on *app.config* settings, which can't be changed by Windows Store applications. <xref:System.Uri.IdnHost%2A> is provided as the preferred alternative to using <xref:System.Uri.DnsSafeHost%2A> because it doesn't depend on *app.config* settings, and it's designed to produce a host value that works for typical DNS resolution scenarios. Callers should still validate or normalize the returned value, especially for malformed hostnames or IPv6 addresses that include a zone ID.

[Copilot]

This IPv6 zone ID example uses http://[fe80::1%10].... In URI syntax, the % that introduces the zone ID needs to be percent-encoded (%25) inside the URI literal (as shown elsewhere in the docs with %254). As written, %10 is a percent-escape sequence and won't represent a zone ID of 10 reliably. Update the input URI (and the expected output comments) to use the correctly escaped form.

Suggested change

	Uri uri4 = new Uri("http://[fe80::1%10]:8080/path");
	Console.WriteLine($" Host: {uri4.Host}"); // [fe80::1] (with brackets, no zone ID)
	Console.WriteLine($" IdnHost: {uri4.IdnHost}"); // fe80::1%10 (without brackets, with zone ID)
	Console.WriteLine($" DnsSafeHost: {uri4.DnsSafeHost}"); // fe80::1%10 (without brackets, with zone ID)
	Uri uri4 = new Uri("http://[fe80::1%2510]:8080/path");
	Console.WriteLine($" Host: {uri4.Host}"); // fe80::1%10 (with zone ID 10)
	Console.WriteLine($" IdnHost: {uri4.IdnHost}"); // fe80::1%10 (with zone ID 10)
	Console.WriteLine($" DnsSafeHost: {uri4.DnsSafeHost}"); // fe80::1%10 (with zone ID 10)