dotnet/wpf · error · InvalidOperationException

SR.PropertyMustHaveValue (formatted with "Prefix", type…

Error message

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

What it means

XmlNamespaceMapping's ISupportInitialize.EndInit validates that Prefix was actually set. If _prefix is still null when initialization finishes, it throws an InvalidOperationException stating the property must have a value. A mapping without a Prefix is meaningless for XML namespace resolution.

Solutions

Set both Prefix and Uri before calling EndInit().
Use the (string prefix, Uri uri) constructor so both are always supplied.
Add a null check on mapping values before the EndInit call.

Example fix

// before
mapping.BeginInit();
mapping.Uri = nsUri;
mapping.EndInit(); // throws
// after
mapping.BeginInit();
mapping.Prefix = "atom";
mapping.Uri = nsUri;
mapping.EndInit();

Defensive patterns

Strategy: validation

Validate before calling

if (string.IsNullOrEmpty(prefix)) throw new ArgumentException("Prefix required before EndInit", nameof(prefix));
((ISupportInitialize)mapping).BeginInit();
mapping.Prefix = prefix;
((ISupportInitialize)mapping).EndInit();

Try / catch

try { ((ISupportInitialize)mapping).EndInit(); }
catch (InvalidOperationException ex) { log.Error("Mapping incomplete: " + ex.Message, ex); }

Prevention

Always set both Prefix and Uri before EndInit.
Use the two-argument constructor to avoid partial initialization.
Review code paths that conditionally skip property assignment.

When it happens

Trigger: Calling EndInit() after setting only Uri (Prefix left null), or calling EndInit() without ever setting either property.

Common situations: Partial programmatic initialization — forgetting one of the two required properties; conditional code paths that skip the Prefix assignment.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/e56faed53a88d430. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Data/XmlNamespaceMapping.cs:137

                return unchecked(hash + _uri.GetHashCode());
            else
                return hash;
        }

#region ISupportInitialize

        /// <summary>Begin Initialization</summary>
        void ISupportInitialize.BeginInit()
        {
            _initializing = true;
        }

        /// <summary>End Initialization, verify that internal state is consistent</summary>
        void ISupportInitialize.EndInit()
        {
            if (_prefix == null)
            {
                throw new InvalidOperationException(SR.Format(SR.PropertyMustHaveValue, "Prefix", this.GetType().Name));
            }
            if (_uri == null)
            {
                throw new InvalidOperationException(SR.Format(SR.PropertyMustHaveValue, "Uri", this.GetType().Name));
            }

            _initializing = false;
        }

#endregion ISupportInitialize

        private string _prefix;
        private Uri _uri;
        private bool _initializing;
    }
}

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