dotnet/wpf · error · InvalidOperationException

SR.PropertyIsImmutable (formatted with "Prefix", type name)

Error message

SR.PropertyIsImmutable (formatted with "Prefix", type name)

What it means

Once XmlNamespaceMapping.Prefix has been set (during initialization), it cannot be changed: a second assignment with a different value throws an InvalidOperationException with SR.PropertyIsImmutable. This keeps the mapping stable while the XML data binding is active.

Solutions

Create a new XmlNamespaceMapping instance instead of mutating an existing one.
Ensure the same Prefix value is assigned only once per instance.
If the prefix must change, rebuild the mapping and re-add it to the collection.

Example fix

// before
mapping.Prefix = "rss";
mapping.Prefix = "atom"; // throws
// after
var atomMapping = new XmlNamespaceMapping();
((ISupportInitialize)atomMapping).BeginInit();
atomMapping.Prefix = "atom";
atomMapping.Uri = new Uri("http://www.w3.org/2005/Atom");
((ISupportInitialize)atomMapping).EndInit();

Defensive patterns

Strategy: validation

Validate before calling

if (existing.Prefix != null && existing.Prefix != newPrefix)
    throw new InvalidOperationException("Prefix is immutable; create a new mapping");

Try / catch

try { mapping.Prefix = newPrefix; }
catch (InvalidOperationException ex) { log.Error("Prefix already set; create a new XmlNamespaceMapping", ex); }

Prevention

Treat Prefix as write-once; never reuse mapping instances across namespaces.
Create a fresh XmlNamespaceMapping per prefix/uri pair.
Centralize mapping creation in one helper method.

When it happens

Trigger: Inside a BeginInit scope, assigning Prefix a value when _prefix is already non-null and different from the new value (e.g. reusing and mutating a parsed mapping object).

Common situations: Reusing a single XmlNamespaceMapping instance across several XmlNamespaceMappingCollection entries with different prefixes; attempting to hot-swap the prefix on a live binding.

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/8c3067d530101f22. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Data/XmlNamespaceMapping.cs:48

        /// </summary>
        public XmlNamespaceMapping(string prefix, Uri uri)
        {
            _prefix = prefix;
            _uri = uri;
        }

        /// <summary>
        /// The prefix to be used for this Namespace
        /// </summary>
        public string Prefix
        {
            get { return _prefix; }
            set
            {
                if (!_initializing)
                    throw new InvalidOperationException(SR.Format(SR.PropertyIsInitializeOnly, "Prefix", this.GetType().Name));
                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)

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