dotnet/wpf · error · NotSupportedException

SR.CustomDictionarySourcesUnsupportedURI

Error message

SR.CustomDictionarySourcesUnsupportedURI

What it means

ValidateUri accepts only UNC, file, or pack Uris as spell-checking dictionary sources; any other absolute Uri scheme throws NotSupportedException(SR.CustomDictionarySourcesUnsupportedURI). HTTP/HTTPS and other remote schemes are not supported because the speller reads dictionary files synchronously from disk or pack resources.

Solutions

1. Download the dictionary to a local file first, then add a file:// Uri pointing to the local copy.
2. Convert the path to a valid file or UNC Uri (new Uri(@"C:\dicts\custom.lex", UriKind.Absolute) or \\server\share\dict.lex).
3. Embed the dictionary in the application and reference it via a pack Uri (pack://application:,,,/Dictionaries/custom.lex).
4. Validate the scheme before adding: allow only file, unc (path starting with \\\\), or pack.

Example fix

// before
lexicons.Add(new Uri("https://intranet/dicts/custom.lex")); // throws
// after
var local = Path.Combine(localDictDir, "custom.lex");
await DownloadDictAsync(new Uri("https://intranet/dicts/custom.lex"), local);
lexicons.Add(new Uri(local));

bool supported = uri != null && (!uri.IsAbsoluteUri || uri.IsUnc || uri.IsFile || uri.Scheme == "pack");

static bool IsSupportedDictionaryUri(Uri uri) => uri != null && (!uri.IsAbsoluteUri || uri.IsUnc || uri.IsFile || MS.Internal.IO.Packaging.PackUriHelper.IsPackUri(uri));

try
{
    lexicons.Add(uri);
}
catch (NotSupportedException)
{
    // unsupported scheme (http/https); download locally and retry with file Uri
}

When it happens

Trigger: Adding a dictionary Uri with an unsupported scheme (http://, https://, ftp://) via Add/Insert/constructor; only IsUnc, IsFile, or pack Uris pass the check.

Common situations: Pointing SpellCheck.CustomDictionaries at a dictionary hosted on a web server, storing dictionary paths in config as URLs instead of file paths, or users entering a URL in a settings UI.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• SR.CustomDictionaryItemAlreadyExists
• SR.CustomDictionaryNullItem
• ArgumentException: path
• ArgumentException: relativeTo
• ArgumentNullException: path

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/4d081589090a66c1. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/CustomDictionarySources.cs:344

        /// 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;
        private readonly TextBoxBase _owner;
        #endregion Private Fields



    }

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