dotnet/wpf · error · ArgumentNullException

The propertyName argument cannot be null.

Error message

The propertyName argument cannot be null.

What it means

PropertyMap's translator setter (this[propertyName] indexer) validates that propertyName is not null or empty before applying the translator, throwing ArgumentNullException with SR.WFI_NullArgument ('The propertyName argument cannot be null.'). Property mapping keys must be real property names on the host control, so an empty key is rejected up front.

Solutions

1. Ensure the property name string is non-null and non-empty before indexing PropertyMap.
2. Skip or log empty entries when iterating a dynamic list of property names.
3. Use nameof() or verified property metadata to build names instead of user/config strings.
4. Note a null translator value is rejected separately (error 5318); fix both when populating maps from data.

Example fix

// before
map[entry.Name] = translator; // entry.Name may be null/empty
// after
if (!string.IsNullOrEmpty(entry.Name))
{
    map[entry.Name] = translator;
}

if (!string.IsNullOrEmpty(propertyName))
{
    host.PropertyMap[propertyName] = translator;
}
else
{
    logger.LogWarning("Skipped property mapping with empty name");
}

static bool IsValidPropertyName(string name) => !string.IsNullOrWhiteSpace(name);

try
{
    host.PropertyMap[propertyName] = translator;
}
catch (ArgumentNullException ex) when (ex.ParamName == "propertyName")
{
    logger.LogError(ex, "Cannot map property: name is null or empty");
}

When it happens

Trigger: Assigning elementHost.PropertyMap[null] = translator or [""] = translator; computing the property name from a variable that is null/empty (e.g. from reflection returning null, or uninitialized config).

Common situations: Dynamic property-name lists built from settings or resource files where entries are missing; string.Split producing empty tokens used as keys; loops over metadata where a name field is blank.

Related errors

• The translator argument cannot be null.
• A property mapping with the same name already exists.
• The exception argument cannot be null.
• annotation component
• ArgumentNullException

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/2154e448dcee615b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsFormsIntegration/System/Windows/Integration/PropertyMap.cs:82

        public PropertyTranslator this[String propertyName]
        {
            get
            {
                ThrowIfPropertyDoesntExistOnSource(propertyName);
                PropertyTranslator value;
                if (_wrappedDictionary.TryGetValue(propertyName, out value))
                {
                    return value;
                }
                return null;
            }
            set
            {
                //Run through our regular invalid property checks, then
                //apply the translator
                if (string.IsNullOrEmpty(propertyName))
                {
                    throw new ArgumentNullException(string.Format(CultureInfo.CurrentCulture, SR.WFI_NullArgument, "propertyName"));
                }
                if (value == null)
                {
                    throw new ArgumentNullException(string.Format(CultureInfo.CurrentCulture, SR.WFI_NullArgument, "translator"));
                }
                ThrowIfPropertyDoesntExistOnSource(propertyName);
                _wrappedDictionary[propertyName] = value;    //This will replace an existing mapping, unlike Add.
                Apply(propertyName);
            }
        }

        /// <summary>
        ///     Returns an ICollection of strings identifying all of the host control 
        ///     properties that have been mapped.  
        /// </summary>
        public ICollection Keys
        {
            get

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