dotnet/wpf · error · ArgumentException

SR.Format(SR.ValueInArrayIsNull, "properties")

Error message

SR.Format(SR.ValueInArrayIsNull, "properties")

What it means

IAmbientProvider.GetFirstAmbientValue validates that no entry in the 'properties' array is null before delegating to the inner XamlContext. Passing a null XamlMember in the properties collection throws ArgumentException with the ValueInArrayIsNull message naming 'properties'.

Solutions

  1. Filter or reject null entries before calling: properties.Where(p => p != null).ToArray().
  2. Check that each XamlType.GetMember(...) call succeeded (return value not null) when building the array.
  3. Guard with a validation loop and throw a clearer exception identifying which lookup failed.
  4. Catch ArgumentException and report the offending index to the caller for diagnosis.

Example fix

// before
var props = new[] { tagType.GetMember("Width"), tagType.GetMember("NoSuchProp") };
var value = ambientProvider.GetFirstAmbientValue(null, props); // throws
// after
var props = new[] { tagType.GetMember("Width"), tagType.GetMember("NoSuchProp") }
    .Where(p => p != null).ToArray();
var value = ambientProvider.GetFirstAmbientValue(null, props);
Defensive patterns

Strategy: validation

Validate before calling

if (properties.Any(p => p == null)) throw new ArgumentException("properties contains null members", nameof(properties));

Type guard

XamlMember[] NonNull(XamlMember[] src) => src.Where(p => p != null).ToArray();

Try / catch

try { return ambient.GetFirstAmbientValue(ceiling, props); }
catch (ArgumentException ex) { // null member in array — log props and rethrow
  throw new ArgumentException("Bad properties array", ex); }

Prevention

When it happens

Trigger: Calling ServiceProvider_GetFirstAmbientValue/IAmbientProvider.GetFirstAmbientValue(ceilingTypes, properties) where the properties array contains a null element, e.g. GetFirstAmbientValue(null, new[] { someType.GetMember("Prop"), null }).

Common situations: Building the member list dynamically from lookups that can return null (GetMember on a missing property) and passing the array straight through; collection-initializer typos; data-driven ambient value queries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/15e23191906cdacd. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Context/ServiceProviderContext.cs:139

        {
            get { return _xamlContext.BaseUri; }
            set { throw new InvalidOperationException(SR.MustNotCallSetter); }
        }
        #endregion

        #region IAmbientProvider Members
        AmbientPropertyValue IAmbientProvider.GetFirstAmbientValue(
                                                    IEnumerable<XamlType> ceilingTypes,
                                                    params XamlMember[] properties)
        {
            ArgumentNullException.ThrowIfNull(properties);

            foreach (var property in properties)
            {
                if (property is null)
                {
                    // we don't allow any property to be null
                    throw new ArgumentException(SR.Format(SR.ValueInArrayIsNull, "properties"));
                }
            }

            return _xamlContext.ServiceProvider_GetFirstAmbientValue(ceilingTypes, properties);
        }

        object IAmbientProvider.GetFirstAmbientValue(params XamlType[] types)
        {
            ArgumentNullException.ThrowIfNull(types);

            foreach (var type in types)
            {
                if (type is null)
                {
                    // we don't allow any type to be null
                    throw new ArgumentException(SR.Format(SR.ValueInArrayIsNull, "types"));
                }
            }

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