dotnet/wpf · error · InvalidOperationException

SR.PropertyIsInitializeOnly (formatted with "Uri", type…

Error message

SR.PropertyIsInitializeOnly (formatted with "Uri", type name)

What it means

XmlNamespaceMapping.Uri is initialize-only: it may only be assigned during the BeginInit/EndInit initialization window. Assigning it after initialization throws an InvalidOperationException formatted with the property name and type name, same pattern as Prefix.

Solutions

Define the mapping in XAML so initialization protocol is applied automatically.
In code, wrap assignments in ((ISupportInitialize)mapping).BeginInit() ... EndInit().
Use the constructor that accepts Prefix and Uri directly (public XmlNamespaceMapping(string prefix, Uri uri)) so initialization happens once.

Example fix

// before
var mapping = new XmlNamespaceMapping();
mapping.Uri = new Uri("http://www.w3.org/2005/Atom"); // throws
// after
var mapping = new XmlNamespaceMapping("atom", new Uri("http://www.w3.org/2005/Atom"));

Defensive patterns

Strategy: validation

Validate before calling

if (isInitialized) throw new InvalidOperationException("Uri cannot be set after initialization");
((ISupportInitialize)mapping).BeginInit();
mapping.Uri = uri;
((ISupportInitialize)mapping).EndInit();

Try / catch

try { mapping.Uri = uri; }
catch (InvalidOperationException ex) { log.Error("Uri is initialize-only; use the constructor or BeginInit/EndInit", ex); }

Prevention

Use the XmlNamespaceMapping(prefix, uri) constructor.
Set Uri only inside BeginInit/EndInit.
Never mutate mappings after the XmlDataProvider is initialized.

When it happens

Trigger: Setting mapping.Uri from code after construction outside BeginInit/EndInit, e.g. after the mapping has already been parsed from XAML or finalized.

Common situations: Programmatic construction of mappings without the ISupportInitialize protocol; attempting to update the namespace URI on a live binding after load.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/86783bca9eeef639. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Data/XmlNamespaceMapping.cs:65

                if (_prefix != null && _prefix != value)
                    throw new InvalidOperationException(SR.Format(SR.PropertyIsImmutable, "Prefix", this.GetType().Name));

                _prefix = value;
            }
        }

        /// <summary>
        /// The Uri to be used for this Namespace,
        /// can be declared as an attribute or as the
        /// TextContent of the XmlNamespaceMapping markup tag
        /// </summary>
        public Uri Uri
        {
            get { return _uri; }
            set
            {
                if (!_initializing)
                    throw new InvalidOperationException(SR.Format(SR.PropertyIsInitializeOnly, "Uri", this.GetType().Name));
                if (_uri != null && _uri != value)
                    throw new InvalidOperationException(SR.Format(SR.PropertyIsImmutable, "Uri", this.GetType().Name));

                _uri = value;
            }
        }

        /// <summary>
        /// Equality comparison by value
        /// </summary>
        public override bool Equals(object obj)
        {
            return (this == (obj as XmlNamespaceMapping));  // call the == operator override
        }

        /// <summary>
        /// Equality comparison by value
        /// </summary>

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