dotnet/wpf · error

SR.ParserProvideValueCantSetUri

Error message

SR.ParserProvideValueCantSetUri

What it means

When a markup extension's ProvideValue runs, the service provider exposes IUriContext, but its BaseUri setter deliberately throws NotSupportedException: markup extensions must not set the BaseUri during ProvideValue; only the parser owns it. The getter returns the context's BaseUri.

Solutions

  1. Do not set BaseUri inside ProvideValue; read it and resolve your URI against it instead
  2. Compute absolute URIs by combining the read-only BaseUri with your relative path
  3. Store desired state in the markup extension's constructor/properties rather than mutating services

Example fix

// before
var uriContext = (IUriContext)serviceProvider.GetService(typeof(IUriContext));
uriContext.BaseUri = new Uri("pack://application:,,,/MyAssembly");
// after
var uriContext = (IUriContext)serviceProvider.GetService(typeof(IUriContext));
var resolved = new Uri(uriContext.BaseUri, relativePath); // read-only, combine instead of assigning
Defensive patterns

Strategy: try-catch

Validate before calling

var uriCtx = serviceProvider.GetService(typeof(IUriContext)) as IUriContext;
// never assign uriCtx.BaseUri inside ProvideValue

Try / catch

try { ProvideValueCore(sp); }
catch (NotSupportedException ex) when (ex.Message.Contains("BaseUri")) { /* stop mutating BaseUri; read instead */ }

Prevention

When it happens

Trigger: Inside a custom MarkupExtension.ProvideValue implementation, casting IServiceProvider.GetService(typeof(IUriContext)) to IUriContext and assigning to BaseUri, or code that mutates BaseUri while the parser is invoking ProvideValue.

Common situations: Custom markup extensions that try to resolve relative URIs by rewriting BaseUri during ProvideValue; code migrated from a static-resource context where BaseUri appeared writable.

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/5e82b367a83aee61. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/ProvideValueServiceProvider.cs:72

        }

        // IProvideValueTarget implementation
        
        object IProvideValueTarget.TargetObject
        {
            get { return _targetObject; }
        }
        object IProvideValueTarget.TargetProperty
        {
            get { return _targetProperty; }
        }

        // IUriContext implementation
        
        Uri IUriContext.BaseUri
        {
            get { return _context.BaseUri; }
            set { throw new NotSupportedException(SR.ParserProvideValueCantSetUri); }
        }

        bool IFreezeFreezables.FreezeFreezables
        {
            get
            {
                return _context.FreezeFreezables;
            }
        }

        bool IFreezeFreezables.TryFreeze(string value, Freezable freezable)
        {
            return _context.TryCacheFreezable(value, freezable);
        }

        Freezable IFreezeFreezables.TryGetFreezable(string value)
        {
            return _context.TryGetFreezable(value);

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