dotnet/wpf · error · InvalidOperationException

SR.MustNotCallSetter

Error message

SR.MustNotCallSetter

What it means

ServiceProviderContext exposes IUriContext.BaseUri with a get-only implementation; calling the setter throws InvalidOperationException. The context intentionally forbids changing the base URI through this service provider view — BaseUri must be set on the underlying XamlParser/AmbientProvider context, not via the service provider handed to markup extensions and type converters.

Solutions

1. Read BaseUri and combine it with the relative path via new Uri(baseUri, relativeUri) instead of assigning to it.
2. Set the base URI where it is owned: pass it to XamlXmlReader/XamlXmlReaderSettings.BaseUri (or the parser setup) rather than the service provider.
3. If runtime adjustment is required, implement your own IUriContext wrapper or supply the resolved URI to your markup extension as a property.
4. Catch InvalidOperationException around the setter only as a last resort; treat it as a contract violation.

Example fix

// before
var uriContext = (IUriContext)serviceProvider.GetService(typeof(IUriContext));
uriContext.BaseUri = new Uri("pack://application:,,,/MyAssembly/");
var full = uriContext.BaseUri;
// after
var uriContext = (IUriContext)serviceProvider.GetService(typeof(IUriContext));
var resolved = uriContext.BaseUri != null
    ? new Uri(uriContext.BaseUri, relativeUri)
    : relativeUri;

var uriContext = serviceProvider.GetService(typeof(IUriContext)) as IUriContext;
if (uriContext == null) throw new InvalidOperationException("No IUriContext available");

bool CanSetBaseUri(object sp) => sp is not IUriContext u || u.GetType().GetProperty(nameof(IUriContext.BaseUri))?.CanWrite == true;

try { uriContext.BaseUri = newUri; }
catch (InvalidOperationException) { // read-only view: combine URIs instead
  resolved = uriContext.BaseUri != null ? new Uri(uriContext.BaseUri, relative) : relative; }

When it happens

Trigger: Code obtains an IUriContext from a XAML service provider (IXamlSchemaContextProvider/IServiceProvider.GetService(typeof(IUriContext))) and assigns to its BaseUri property, e.g. ((IUriContext)serviceProvider).BaseUri = new Uri(...).

Common situations: Markup extensions (MarkupExtension/TypeConverter ProvideValue) trying to rewrite the base URI to resolve relative URIs; ported code that previously ran in a framework where IUriContext.BaseUri was settable; generic library code that sets BaseUri defensively before reading it.

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

• SR.MustNotCallSetter
• Animation_Invalid_DefaultValue
• Cannot remove signature from read-only file.
• Image_EncoderNoColorContext
• Image_EncoderNoGlobalMetadata

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/68047e910a7c8341. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Context/ServiceProviderContext.cs:123

        PropertyDescriptor ITypeDescriptorContext.PropertyDescriptor
        {
            get { return null; }
        }
        #endregion

        #region IXamlTypeResolver Members
        Type IXamlTypeResolver.Resolve(string qName)
        {
            return _xamlContext.ServiceProvider_Resolve(qName);
        }
        #endregion

        #region IUriContext Members
        Uri IUriContext.BaseUri
        {
            get { return _xamlContext.BaseUri; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter); }
        }
        #endregion

        #region IAmbientProvider Members
        AmbientPropertyValue IAmbientProvider.GetFirstAmbientValue(
                                                    IEnumerable<XamlType> ceilingTypes,
                                                    params XamlMember[] properties)
        {
            ArgumentNullException.ThrowIfNull(properties);

            foreach (var property in properties)
            {
                if (property is null)
                {
                    // we don't allow any property to be null
                    throw new ArgumentException(SR.Format(SR.ValueInArrayIsNull, "properties"));
                }
            }

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