SR.ParserFilterXmlReaderNoIndexAttributeAccess

Error message

SR.ParserFilterXmlReaderNoIndexAttributeAccess

What it means

FilteredXmlReader wraps an XmlReader during XAML parsing to filter uid/namespace attributes. It deliberately forbids index-based attribute access because indexes shift after filtering; GetAttribute(int) always throws InvalidOperationException to force name-based access instead.

Solutions

Rewrite attribute access to use GetAttribute(string localName) or GetAttribute(string name, string ns)
Use MoveToAttribute(string) / MoveToFirstAttribute + reader indexer by name instead of indexes
Obtain the underlying inner reader if index access is truly required (and accept unfiltered data)
Avoid wrapping FilteredXmlReader in code that strictly requires index-based attribute APIs

Example fix

// before
string v = filteredReader.GetAttribute(0); // throws
// after
string v = filteredReader.GetAttribute("Uid") ?? filteredReader.GetAttribute("x:Uid");

Defensive patterns

Strategy: type-guard

Validate before calling

if (reader is FilteredXmlReader) return (reader as FilteredXmlReader).GetAttribute(name); else return reader.GetAttribute(index);

Type guard

static bool SupportsIndexAttributeAccess(XmlReader r) => !(r is FilteredXmlReader);

Try / catch

try { v = reader.GetAttribute(i); } catch (InvalidOperationException ex) when (ex.Message.Contains("Index") || ex.Message.Contains("attribute")) { v = reader.GetAttribute(attributeNames[i]); }

Prevention

Never index into a FilteredXmlReader; use name-based GetAttribute/MoveToAttribute
Build attribute-name lists from MoveToFirstAttribute/MoveToNextAttribute traversal
Keep FilteredXmlReader details out of generic XmlReader utilities

When it happens

Trigger: Any call to the filtered reader's GetAttribute(attributeIndex) overload — e.g. generic XmlReader-processing code that iterates attributes by index over a FilteredXmlReader instance.

Common situations: Plugging FilteredXmlReader into frameworks/tools that assume standard XmlReader index semantics (XmlReadMode internals, diagnostics readers, custom parser extensions); upgrading code paths that used to receive a raw XmlReader.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/d99e8535a7f1fea6. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/FilteredXmlReader.cs:122

        {
            return GetAttribute( localName, namespaceUri ); // Defined elsewhere in this file.
        }
    }

    #endregion Public Properties
    
    //------------------------------------------------------
    //
    //  Public Methods
    //
    //------------------------------------------------------

    #region Public Methods

    public override string GetAttribute( int attributeIndex )
    {
        // Index-based acccess are not allowed.  See remark at top of this file.
        throw new InvalidOperationException(
            SR.ParserFilterXmlReaderNoIndexAttributeAccess);
    }

    public override string GetAttribute( string attributeName )
    {
        if( attributeName == uidQualifiedName )
        {
            return null;  // ...these aren't the attributes you're looking for...
        }
        else
        {
            return base.GetAttribute( attributeName );
        }
    }
    
    public override string GetAttribute( string localName, string namespaceUri )
    {
        if( localName == uidLocalName &&

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