What causes "ArgumentNullException(nameof(value))" in dotnet/wpf?

Assign a non-null string, or create a new TypeExtension instance instead of nulling properties.

dotnet/wpf · error · ArgumentNullException

ArgumentNullException(nameof(value))

Error message

ArgumentNullException(nameof(value))

What it means

The TypeExtension.TypeName setter throws ArgumentNullException when assigned null. Assigning TypeName is meant to switch the extension to name-based resolution; null would leave the extension in an inconsistent state, so the library rejects it.

Solutions

Assign a non-null string, or create a new TypeExtension instance instead of nulling properties.
Guard: if (value != null) ext.TypeName = value;
Use the Type property with a real Type when the name is unknown.
Wrap in try/catch ArgumentNullException if null assignment is possible in legacy code paths.

Example fix

// before
ext.TypeName = GetTypeName() ; // GetTypeName() may return null
// after
var name = GetTypeName();
if (name != null) ext.TypeName = name;

Defensive patterns

Strategy: type-guard

Validate before calling

if (name is null) throw new ArgumentException("TypeName cannot be null.");
ext.TypeName = name;

Type guard

static bool IsNonEmpty(string? s) => !string.IsNullOrEmpty(s);

Try / catch

try { ext.TypeName = value; }
catch (ArgumentNullException) { ext.TypeName = string.Empty; log.Warn("Null TypeName ignored."); }

Prevention

Never reset markup-extension properties to null; recreate the instance instead.
Enable nullable reference types so null assignments surface at compile time.

When it happens

Trigger: Executing typeExtension.TypeName = null; on any TypeExtension instance.

Common situations: Code that clears markup-extension settings in bulk with null values; reflection-based serializers or designers assigning null for an absent attribute value.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/7229b8d52dc71b97. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Windows/Markup/TypeExtension.cs:95

            {
                throw new InvalidOperationException(SR.Format(SR.MarkupExtensionTypeNameBad, _typeName));
            }

            return _type;
        }

        /// <summary>
        /// The typename represented by this markup extension. The name has the format
        /// Prefix:Typename, where Prefix is the xml namespace prefix for this type.
        /// </summary>
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        public string TypeName
        {
            get => _typeName;
            set
            {
                // Reset the type so ProvideValue does not use the existing type.
                _typeName = value ?? throw new ArgumentNullException(nameof(value));
                _type = null;
            }
        }

        /// <summary>
        /// The type represented by this markup extension.
        /// </summary>
        [DefaultValue(null)]
        [ConstructorArgument("type")]
        public Type Type
        {
            get => _type;
            set
            {
                // Reset the type name so ProvideValue does not use the existing type name.
                _type = value ?? throw new ArgumentNullException(nameof(value));
                _typeName = null;
            }

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