dotnet/wpf · error · ArgumentException

SR.CollectionCannotContainNulls

Error message

SR.CollectionCannotContainNulls

What it means

The XamlDirective constructor validates that the xamlNamespaces collection contains no null entries before storing it as a read-only list. A null namespace string would make directive resolution ambiguous, so ArgumentException(SR.CollectionCannotContainNulls, nameof(xamlNamespaces)) is thrown eagerly.

Solutions

1. Sanitize the collection before constructing: remove or replace null entries with the correct namespace string.
2. Use string.Empty for an empty namespace if that was the intent (e.g. CLR default namespace).
3. Add a guard that throws with a clear message identifying which entry is null so callers can fix the source data.

Example fix

// before
var directive = new XamlDirective(namespacesFromConfig, "Key", XamlLanguage.String, null, AllowedMemberLocations.Any);
// namespacesFromConfig contains null
// after
var cleanNamespaces = namespacesFromConfig.Where(ns => ns is not null).ToList();
var directive = new XamlDirective(cleanNamespaces, "Key", XamlLanguage.String, null, AllowedMemberLocations.Any);

if (xamlNamespaces is not null && xamlNamespaces.Any(ns => ns is null)) throw new ArgumentException("xamlNamespaces contains null entries");

static bool HasNoNulls(IEnumerable<string?> src) => src is not null && !src.Any(ns => ns is null);

try { new XamlDirective(namespaces, name, type, invoker, loc); }
catch (ArgumentException ex) when (ex.ParamName == "xamlNamespaces") { /* sanitize list and retry */ }

When it happens

Trigger: Calling `new XamlDirective(xamlNamespaces, name, ...)` where the IEnumerable<string> passed as xamlNamespaces contains a null element, e.g. `new XamlDirective(new[] { null }, "MyDirective", ...)` or a list built with `list.Add(null)`.

Common situations: Programmatically building a namespace list where entries come from parsing or configuration and a missing namespace string yields null instead of empty; interop code converting non-.NET strings to XAML namespace strings.

Related errors

• ArgumentOutOfRangeException (timeout was Duration.Automatic)
• Collection_BadRank
• Collection_CopyTo_ArrayCannotBeMultidimensional
• Collection_CopyTo_ArrayCannotBeMultidimensional
• Collection_CopyTo_IndexGreaterThanOrEqualToArrayLength

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/e5463a467f4df1a8. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Schema/XamlDirective.cs:44

            }
#endif
            _xamlNamespaces = immutableXamlNamespaces;
            _allowedLocation = allowedLocation;
        }

        public XamlDirective(IEnumerable<string> xamlNamespaces, string name, XamlType xamlType,
            XamlValueConverter<TypeConverter> typeConverter, AllowedMemberLocations allowedLocation)
            : base(name, new MemberReflector(xamlType, typeConverter))
        {
            ArgumentNullException.ThrowIfNull(xamlType);
            ArgumentNullException.ThrowIfNull(xamlNamespaces);

            List<string> nsList = new List<string>(xamlNamespaces);
            foreach (string ns in nsList)
            {
                if (ns is null)
                {
                    throw new ArgumentException(SR.CollectionCannotContainNulls, nameof(xamlNamespaces));
                }
            }

            _xamlNamespaces = nsList.AsReadOnly();
            _allowedLocation = allowedLocation;
        }

        public XamlDirective(string xamlNamespace, string name)
            :base(name, null)
        {
            ArgumentNullException.ThrowIfNull(xamlNamespace);

            _xamlNamespaces = new ReadOnlyCollection<string>(new string[] { xamlNamespace });
            _allowedLocation = AllowedMemberLocations.Any;
        }

        public AllowedMemberLocations AllowedLocation { get { return _allowedLocation; } }

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