Issue with return value of XmlNamespaceManager.LookupPrefix

[andersjonsson]

Issue description

The docs says that the return value of XmlNamespaceManager.LookupPrefix returns The matching prefix. If there is no mapped prefix, the method returns String.Empty. If a null value is supplied, then null is returned.
My tests indicate that this method returns null if there is no mapped prefix.

Example:

XmlNamespaceManager mgr = new XmlNamespaceManager(new NameTable());

var prefix = mgr.LookupPrefix("http://www.microsoft.com");

if (prefix == null)
	Console.WriteLine("Prefix is null");
else
	Console.WriteLine("Prefix is not null");

This code outputs "Prefix is null". According to the docs the returned prefix should have been String.Empty.

Target framework

• .NET Core 5.0
• .NET Framework 4.8

dotnet --info output or About VS info

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Issue Details

---

Issue description

The docs says that the return value of XmlNamespaceManager.LookupPrefix returns The matching prefix. If there is no mapped prefix, the method returns String.Empty. If a null value is supplied, then null is returned.
My tests indicate that this method returns null if there is no mapped prefix.

Example:

XmlNamespaceManager mgr = new XmlNamespaceManager(new NameTable());

var prefix = mgr.LookupPrefix("http://www.microsoft.com");

if (prefix == null)
	Console.WriteLine("Prefix is null");
else
	Console.WriteLine("Prefix is not null");

This code outputs "Prefix is null". According to the docs the returned prefix should have been String.Empty.

Target framework

• .NET Core 5.0
• .NET Framework 4.8

dotnet --info output or About VS info

<replace>

Author:	andersjonsson
Assignees:	-
Labels:	Pri3, area-System.Xml, :watch: Not Triaged
Milestone:	-

[krwq]

Interesting, looking at the interface it implements the doc seems correct:
https://docs.microsoft.com/en-us/dotnet/api/system.xml.ixmlnamespaceresolver.lookupprefix?view=net-6.0

Perhaps we should match the doc