PrismLibrary/Prism · error · ArgumentNullException

ArgumentNullException(nameof(behaviorType))

Error message

ArgumentNullException(nameof(behaviorType))

What it means

Validation guard in RegionBehaviorFactory.AddIfMissing: the factory resolves the behavior instance from behaviorType via the container; a null Type cannot be instantiated or registered, so ArgumentNullException naming 'behaviorType' is thrown after the null-key check.

Solutions

1. Pass typeof(MyBehavior) directly instead of Type.GetType on a string
2. If using Type.GetType, check for null and include the assembly-qualified name
3. Ensure the assembly containing the behavior is referenced and loaded

Example fix

// before
var t = Type.GetType("MyBehavior"); // null
factory.AddIfMissing(key, t);
// after
factory.AddIfMissing(key, typeof(MyBehavior));

var t = Type.GetType(name);
if (t == null) throw new InvalidOperationException($"Type {name} not found");
factory.AddIfMissing(key, t);

bool ResolvableType(string n) => Type.GetType(n, throwOnError: false) != null;

try { factory.AddIfMissing(key, t); } catch (ArgumentNullException ex) when (ex.ParamName == "behaviorType") { /* resolve type */ }

When it happens

Trigger: Calling AddIfMissing(key, null) — e.g. typeof resolution from a string via Type.GetType that returned null because the assembly-qualified name was wrong.

Common situations: Type.GetType with a wrong or incomplete assembly-qualified name (returns null silently); loading behavior types from config; assembly not referenced/loaded in a plugin scenario.

Related errors

• ArgumentNullException(nameof(key))
• ArgumentNullException(nameof(regionBehavior))
• ArgumentNullException(nameof(behaviorKey))
• Layout 's Children property is not empty. This control is…
• regionName

AI-assisted analysis of PrismLibrary/Prism@358118cd64 (2026-09-15). Data as JSON: /api/errors/03f16bd334525b96. Report an issue: GitHub.

Appendix: source

Thrown at src/Prism.Core/Navigation/Regions/RegionBehaviorFactory.cs:42

        {
            _container = container;
        }

        /// <summary>
        /// Adds a particular type of RegionBehavior if it was not already registered. The <paramref name="behaviorKey"/> string is used to check if the behavior is already present
        /// </summary>
        /// <param name="behaviorKey">The behavior key that's used to find if a certain behavior is already added.</param>
        /// <param name="behaviorType">Type of the behavior to add.</param>
        public void AddIfMissing(string behaviorKey, Type behaviorType)
        {
            if (behaviorKey == null)
            {
                throw new ArgumentNullException(nameof(behaviorKey));
            }

            if (behaviorType == null)
            {
                throw new ArgumentNullException(nameof(behaviorType));
            }

            // Check if the type is a IRegionBehavior
            if (!typeof(IRegionBehavior).IsAssignableFrom(behaviorType))
            {
                throw new ArgumentException(
                    string.Format(Thread.CurrentThread.CurrentCulture, Resources.CanOnlyAddTypesThatInheritIFromRegionBehavior, behaviorType.Name), nameof(behaviorType));
            }

            // Only add the behaviorKey if it doesn't already exists.
            if (_registeredBehaviors.ContainsKey(behaviorKey))
            {
                return;
            }

            _registeredBehaviors.Add(behaviorKey, behaviorType);
        }

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