dotnet/wpf · error · ArgumentException

name

Error message

name

What it means

WindowsRichEditRange.GetAttributeValue throws ArgumentException when asked for a TextAttribute value with an unsupported TextUnit. The proxy only maps a known set of TextUnit units (Character, Format, Word, Line, Paragraph, Page, Document) to TOM units; anything else (e.g. TextUnit.Zoom, future enum values) falls through the switch's default case and the unit name itself is thrown as the message.

Solutions

1. Restrict TextUnit arguments to TextUnit.Character through TextUnit.Document when working with RichEdit providers.
2. Wrap GetAttributeValue in a try/catch for ArgumentException and treat it as 'unsupported attribute' rather than a fatal error.
3. Update to a newer .NET/WPF servicing release that may support the requested TextUnit.

Example fix

// before
object v = textRange.GetAttributeValue(TextPattern.ZoomAttribute);
// after
object v;
try { v = textRange.GetAttributeValue(attr); }
catch (ArgumentException) { v = TextPattern.MixedAttributeValue; /* unsupported */ }

bool isSupported = TextUnit.Character <= unit && unit <= TextUnit.Document; // RichEdit proxy supports these only

static bool IsSupportedTextUnit(TextUnit u) => u >= TextUnit.Character && u <= TextUnit.Document;

try { value = range.GetAttributeValue(unit); }
catch (ArgumentException) { value = AutomationElement.NotSupported; /* unsupported unit */ }

When it happens

Trigger: Calling TextRange.GetAttributeValue with an attribute that requires a TextUnit the RichEdit provider does not support, causing the switch in the TextUnit-to-TomUnit conversion helper to hit the default branch at WindowsRichEditRange.cs:1261.

Common situations: Automating legacy RichEdit controls via UIAutomation while requesting attributes across all TextUnit values; code written against a newer TextUnit enum (e.g. Zoom) that this WPF UIAutomation provider predates.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• E_INVALIDARG
• ArgumentOutOfRangeException (timeout was Duration.Automatic)
• Collection_BadRank
• Collection_CopyTo_ArrayCannotBeMultidimensional
• Collection_CopyTo_ArrayCannotBeMultidimensional

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/3eb1f946551d08f4. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/UIAutomation/UIAutomationClientSideProviders/MS/Internal/AutomationProxies/WindowsRichEditRange.cs:1261

            {
                case TextUnit.Character:
                    return TomUnit.tomCharacter;

                case TextUnit.Word:
                    return TomUnit.tomWord;

                case TextUnit.Line:
                    return TomUnit.tomLine;

                case TextUnit.Paragraph:
                    return TomUnit.tomParagraph;

                case TextUnit.Page:
                case TextUnit.Document:
                    return TomUnit.tomStory;

                default:
                    throw new ArgumentException(name);
            }
        }

        #endregion Private Methods

        //------------------------------------------------------
        //
        //  Static Methods
        //
        //------------------------------------------------------

        #region Static Methods

        // these helper functions make it convenient to walk through a range by unit as follows:
        //   int end = range.End;
        //   for (ITextRange subrange=FirstUnit(range); NextUnit(end, subrange, tomUnit.tomCharFormat);)...
        // and
        //   int start = range.Start;

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