Document GetXamlType null Type exception

[Copilot]

XamlSchemaContext.GetXamlType(Type) currently throws NullReferenceException when called with a null Type, while the API docs listed ArgumentNullException.

• API docs
  • Updated the GetXamlType(Type) exception documentation to match observed runtime behavior.
  • Changed the null type exception from ArgumentNullException to NullReferenceException.

var context = new XamlSchemaContext();
context.GetXamlType((Type)null); // throws NullReferenceException

• Fixes Replace MSDN link (3) #205

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Internal previews

File	Preview link
xml/System.Xaml/XamlSchemaContext.xml	Preview published page

[Copilot]

Pull request overview

Updates the System.Xaml API reference documentation for XamlSchemaContext.GetXamlType(Type) to reflect the currently observed exception thrown when a null Type is provided.

Changes:

• Updated the documented null-argument exception for GetXamlType(Type) from ArgumentNullException to NullReferenceException.

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[wadepickett]

@gewarren: I tried looking up the exception we are trying to document here in the source but didn't find what was expected. The current .NET source code throws ArgumentNullException, not NullReferenceException, when GetXamlType(Type) is called with a null type. The premise of the PR might be incorrect and might need a closer look. Maybe this is for a future change not yet merged to the main branch? Could be I have this wrong. Here is a link to the source:
https://github.com/dotnet/wpf/blob/1346571efc19a83a90edf3abe9059d18f8412cdb/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/Baml2006/WpfSharedXamlSchemaContext.cs#L18-L20

[gewarren]

@gewarren: I tried looking up the exception we are trying to document here in the source but didn't find what was expected. The current .NET source code throws ArgumentNullException, not NullReferenceException, when GetXamlType(Type) is called with a null type. The premise of the PR might be incorrect and might need a closer look. Maybe this is for a future change not yet merged to the main branch? Could be I have this wrong. Here is a link to the source: https://github.com/dotnet/wpf/blob/1346571efc19a83a90edf3abe9059d18f8412cdb/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/Baml2006/WpfSharedXamlSchemaContext.cs#L18-L20

Thanks @wadepickett. That is a different, internal type, but I checked for modern .NET versions and yes, it does appear to throw ANE. @adegeo Do you know where the source code is for .NET Framework 4.7.2 for this method?

[adegeo]

Browsing the source reference in a .NET Framework project shows the following code:

    public virtual XamlType GetXamlType(Type type)
    {
        return GetXamlType(type, XamlLanguage.TypeAlias(type));
    }

    internal XamlType GetXamlType(Type type, string alias)
    {
        if (type == null)
        {
            throw new ArgumentNullException("type");
        }

        XamlType value = null;
        if (!MasterTypeList.TryGetValue(type, out value))
        {
            value = new XamlType(alias, type, this, null, null);
            value = TryAdd(MasterTypeList, type, value);
        }

        return value;
    }

I don't know where this NullReferenceException came from, but it's wrong.

[adegeo]

Apparently the call to XamlLanguage.TypeAlias(type)); with null crashes WPF because there is no null check in that method. This method isn't explicitly throwing an exception. Do we document that as a known bug (warning in text?) or as an exception?

Considering this is thrown 100% by the bug, I think it's safe to document as a .NET Framework and up until .NET 5. Looking at the dates, this was fixed a month before .NET 6 was released.