dotnet/wpf · error · ArgumentException

SR.ParserKeysAreStrings (dictionary keys must be strings)

Error message

SR.ParserKeysAreStrings (dictionary keys must be strings)

What it means

The object indexer this[object prefix] throws this ArgumentException when the key is not a string. XmlnsDictionary is keyed by XML namespace prefixes, which are strings; the IDictionary object-based accessor enforces this by rejecting non-string keys (and null values on set) with the ParserKeysAreStrings message.

Solutions

Cast the key to string (or call key.ToString()) before indexing
Verify key type with 'is string' before lookup

Example fix

// before
var ns = dictionary[key]; // key is object

// after
var ns = key is string s ? dictionary[s] : null;

Defensive patterns

Strategy: type-guard

Validate before calling

if (key is string p) { var ns = dictionary[p]; }

Type guard

bool IsStringKey(object key) => key is string;

Try / catch

try { var ns = dictionary[key]; }
catch (ArgumentException) { /* key was not a string */ }

Prevention

Always index XmlnsDictionary with string keys
Convert Uri/int keys to strings before indexing
Restrict generic dictionary-walking code from touching XmlnsDictionary untyped

When it happens

Trigger: Reading dictionary[nonStringKey], e.g. dictionary[42] or dictionary[someUri].

Common situations: Generic code iterating IDictionary keys of mixed types; using a Uri object as the key instead of its string representation.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/ed03db11acf2b57a. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XmlnsDictionary.cs:470

        /// <summary>
        /// A property indexing into the dictionary by XML prefix
        /// </summary>
        public string this[string prefix]
        {
            get {   return LookupNamespace(prefix); }
            set {   AddNamespace(prefix, value as string);}
        }

        /// <summary>
        /// A property indexing into the dictionary by XML prefix (supports objects to satisfy IDictionary spec)
        /// </summary>
        public object this[object prefix]
        {
            get 
            {
                if (!(prefix is string))
                {
                    throw new ArgumentException(SR.ParserKeysAreStrings);
                }
                return LookupNamespace((string)prefix); 
            }
            set 
            {   
                if (!(prefix is string) || !(value is string))
                {
                    throw new ArgumentException(SR.ParserKeysAreStrings);
                }
                AddNamespace((string)prefix, (string)value); 
            }
        }

        /// <summary>
        /// An ICollection of all keys in the dictionary
        /// </summary>
        public ICollection Keys
        {

View on GitHub (pinned to 81131a70a4)