dotnet/wpf · error · InvalidEnumArgumentException

The value of argument 'unit

Error message

The value of argument 'unit' ({0}) is invalid for Enum type 'TextUnit'.

What it means

ITextRangeProvider.ExpandToEnclosingUnit throws InvalidEnumArgumentException when the TextUnit argument is not one of the defined values (Character/Word/Line/Paragraph/Page/Document). The switch has no handler for unknown unit values.

Solutions

1. Validate that Enum.IsDefined(typeof(TextUnit), value) before calling ExpandToEnclosingUnit.
2. Only pass literal TextUnit enum members, never raw ints.
3. Catch InvalidEnumArgumentException at the boundary and map unknown units to a supported one (e.g. Document).
4. Upgrade/reference the matching UIAutomation assemblies so the TextUnit enum values line up.

Example fix

// before
var unit = (TextUnit)rawInt;
range.ExpandToEnclosingUnit(unit); // InvalidEnumArgumentException
// after
var unit = Enum.IsDefined(typeof(TextUnit), rawInt) ? (TextUnit)rawInt : TextUnit.Document;
range.ExpandToEnclosingUnit(unit);

if (!Enum.IsDefined(typeof(TextUnit), value)) throw new ArgumentOutOfRangeException(nameof(value));

static bool IsValidTextUnit(int v) => v >= (int)TextUnit.Character && v <= (int)TextUnit.Document && Enum.IsDefined(typeof(TextUnit), v);

try { range.ExpandToEnclosingUnit(unit); }
catch (InvalidEnumArgumentException) { range.ExpandToEnclosingUnit(TextUnit.Document); }

When it happens

Trigger: Calling ExpandToEnclosingUnit (or code paths that call it) with a TextUnit value cast from an unvalidated int, or a TextUnit from a newer/other framework version not recognized by this switch.

Common situations: Casting raw int attribute values from config or interop to TextUnit; handling a protocol payload whose unit field is out of the enum's range; copying constants from documentation that don't match this API's enum.

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

• Win32 edit control cannot have a child element.
• Animation_UnrecognizedHandoffBehavior
• Character offset not valid in TextRange.
• InvalidEnumArgumentException for value (TextTrimming)
• InvalidEnumArgumentException("mode", (int)mode…

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/26e78e2a4df286a2. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/UIAutomation/UIAutomationClientSideProviders/MS/Internal/AutomationProxies/WindowsEditBoxRange.cs:174

                        // move start left until we reach a paragraph boundary.
                        for (; !AtParagraphBoundary(text, Start); Start--);

                        // move end right until we reach a paragraph boundary (different from Start).
                        End = Math.Min(Math.Max(End, Start + 1), text.Length);
                        for (; !AtParagraphBoundary(text, End); End++);
                    } 
                    break;

                case TextUnit.Format:
                case TextUnit.Page:
                case TextUnit.Document:
                    MoveTo(0, _provider.GetTextLength());
                    break;

                //break;
                default:
                    throw new System.ComponentModel.InvalidEnumArgumentException("unit", (int)unit, typeof(TextUnit));
            }
        }

        ITextRangeProvider ITextRangeProvider.FindAttribute(int attributeId, object val, bool backwards)
        {
            AutomationTextAttribute attribute = AutomationTextAttribute.LookupById(attributeId);
            // generic controls are plain text so if the attribute matches then it matches over the whole range.

            // To workaround the conversion that Marshaling of COM-interop did.
            object targetAttribute = GetAttributeValue(attribute);
            if (targetAttribute is Enum)
            {
                targetAttribute = (int)targetAttribute;
            }

            return val.Equals(targetAttribute) ? new WindowsEditBoxRange(_provider, Start, End) : null;
        }

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