dotnet/wpf · error · ArgumentException

SR.CustomDictionaryNullItem

Error message

SR.CustomDictionaryNullItem

What it means

ValidateUri, used by the custom-dictionary Uri collection on Insert/Add/construction, throws ArgumentException(SR.CustomDictionaryNullItem) when a null Uri is supplied. The collection only accepts real, valid dictionary source Uris, and null entries cannot be stored.

Solutions

1. Guard with if (uri != null) before adding; skip or log null entries.
2. Validate that the value producing the Uri is non-empty and parse successfully with Uri.TryCreate before constructing the collection.
3. Fix the configuration/source so it does not emit empty dictionary entries.
4. Catch ArgumentException during bulk add to skip nulls, if acceptable.

Example fix

// before
Uri uri = new Uri(configPath); // or null
lexicons.Add(uri); // throws on null
// after
if (Uri.TryCreate(configPath, UriKind.Absolute, out var uri))
    lexicons.Add(uri);

if (uri != null)
    lexicons.Add(uri);

static bool IsValidDictionaryUri(Uri item) => item != null;

try
{
    lexicons.Add(item);
}
catch (ArgumentException)
{
    // null item supplied; skip and log
}

When it happens

Trigger: Calling Add/Insert on the custom dictionary Uri collection, or constructing CustomDictionarySources with a list containing a null Uri (e.g. from a null entry in configuration or a failed Uri.TryCreate).

Common situations: Building the dictionary list from config/app settings where a blank path yields null, or code that assumes Uri parsing always succeeds and passes the unassigned variable.

Related errors

• SR.CustomDictionaryItemAlreadyExists
• SR.CustomDictionarySourcesUnsupportedURI
• annotation component
• ArgumentException: path
• ArgumentException: relativeTo

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/26c753145a58b047. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/CustomDictionarySources.cs:338

        //  Private Methods
        //
        //------------------------------------------------------
        #region Private Methods

        /// <summary>
        /// Makes sure Uri is of supported kind. Throws exception for unsupported URI type.
        /// Supported URI types [please keep this list updated]
        /// 1. Local path
        /// 2. Relative path
        /// 3. UNC path
        /// 4. Pack URI
        /// </summary>
        /// <param name="item"></param>
        private static void ValidateUri(Uri item)
        {
            if (item == null)
            {
                throw new ArgumentException(SR.CustomDictionaryNullItem);
            }
            if (item.IsAbsoluteUri)
            {
                if (!(item.IsUnc || item.IsFile || MS.Internal.IO.Packaging.PackUriHelper.IsPackUri(item)))
                {
                    throw new NotSupportedException(SR.CustomDictionarySourcesUnsupportedURI);
                }
            }
        }
        #endregion Private Methods
        
        //------------------------------------------------------
        //
        //  Private Fields
        //
        //------------------------------------------------------
        #region Private Fields
        private readonly List<Uri> _uriList;

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