dotnet/wpf · error · XamlSchemaException

XamlSchemaException(SR.Format(SR.UsableDuringInitializationO…

Error message

XamlSchemaException(SR.Format(SR.UsableDuringInitializationOnME, this))

What it means

XAML forbids markup extensions from being marked UsableDuringInitialization, because an extension must be fully constructed from its constructor/property inputs before providing its value; being usable during initialization is contradictory. When a type reports UsableDuringInitialization == true and IsMarkupExtension == true, XamlSchemaException UsableDuringInitializationOnME is thrown during the cached bit lookup.

Solutions

Remove the [UsableDuringInitialization(true)] attribute from the markup extension class
Set UsableDuringInitialization(false) explicitly on the type
If the attribute comes from a base class, exclude it via attribute filtering or restructure inheritance

Example fix

// before
[UsableDuringInitialization(true)]
public class MyExtension : MarkupExtension { ... }
// after
public class MyExtension : MarkupExtension { ... } // attribute removed

Defensive patterns

Strategy: validation

Validate before calling

bool bad = typeof(MyExtension).IsDefined(typeof(UsableDuringInitializationAttribute), false)
    && typeof(System.Xaml.Markup.MarkupExtension).IsAssignableFrom(typeof(MyExtension));

Try / catch

try { var bit = xamlType.UsableDuringInitialization; }
catch (XamlSchemaException ex) { /* remove offending attribute from the extension */ }

Prevention

Never apply UsableDuringInitialization(true) to MarkupExtension-derived classes
Audit attribute copying when porting WPF classes to System.Xaml
Run XAML type resolution tests over all custom markup extensions

When it happens

Trigger: Applying [UsableDuringInitialization(true)] to a class deriving from MarkupExtension, or LookupUsableDuringInitialization discovering such an attribute while resolving the type's boolean bits during XAML parsing or type lookup.

Common situations: Custom markup extensions that copy attribute sets from regular objects; WPF-era attributes carried over when porting a class to System.Xaml; reflection-based metadata incorrectly reporting the attribute.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/cafe38ed6cecb8d9. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlType.cs:1764

                case BoolTypeBits.NameScope:
                    bit = LookupIsNameScope();
                    break;

                case BoolTypeBits.Public:
                    bit = LookupIsPublic();
                    break;

                case BoolTypeBits.TrimSurroundingWhitespace:
                    bit = LookupTrimSurroundingWhitespace();
                    break;

                case BoolTypeBits.UsableDuringInitialization:
                    bit = LookupUsableDuringInitialization();
                    if (bit && IsMarkupExtension)
                    {
                        // MarkupExtension cannot be used during initialization.
                        string err = SR.Format(SR.UsableDuringInitializationOnME, this);
                        throw new XamlSchemaException(err);
                    }

                    break;

                case BoolTypeBits.WhitespaceSignificantCollection:
                    bit = LookupIsWhitespaceSignificantCollection();
                    break;

                case BoolTypeBits.XmlData:
                    bit = LookupIsXData();
                    break;

                case BoolTypeBits.Ambient:
                    bit = LookupIsAmbient();
                    break;

                default:
                    Debug.Fail("Enum out of range");

View on GitHub (pinned to 81131a70a4)