PrismLibrary/Prism · error · ArgumentNullException

Value cannot be null. (Parameter 'type')

Error message

Value cannot be null. (Parameter 'type')

What it means

In the ModuleInfo(string, string, params string[]) constructor, ModuleType is assigned as Type.GetType(type) ?? throw new ArgumentNullException(nameof(type)), so a null type string — or a type string that Type.GetType cannot resolve — raises ArgumentNullException('type').

Solutions

1. Pass typeof(FooModule).AssemblyQualifiedName as the type argument
2. If using a short name, resolve the Type yourself with a fallback search across loaded assemblies before constructing ModuleInfo
3. Ensure the module assembly is referenced and loaded

Example fix

// before
var info = new ModuleInfo("FooModule", "MyApp.Modules.FooModule"); // Type.GetType returns null
// after
var info = new ModuleInfo("FooModule", typeof(MyApp.Modules.FooModule).AssemblyQualifiedName);

var moduleType = Type.GetType(typeName) ?? AppDomain.CurrentDomain.GetAssemblies()
    .Select(a => a.GetType(typeName)).FirstOrDefault(t => t is not null);
if (moduleType is null)
    throw new InvalidOperationException($"Module type '{typeName}' not found");
var info = new ModuleInfo(name, moduleType.AssemblyQualifiedName);

bool IsResolvableType(string name) => Type.GetType(name, throwOnError: false) is not null;

try
{
    var info = new ModuleInfo(name, typeName);
}
catch (ArgumentNullException ex) when (ex.ParamName == "type")
{
    logger.LogError(ex, "Type '{TypeName}' unresolved — use AssemblyQualifiedName and ensure the assembly is loaded", typeName);
    throw;
}

When it happens

Trigger: new ModuleInfo("Foo", null); or Type.GetType("FooModule") failing because the name lacks the assembly-qualified name and the module lives in another assembly, so GetType returns null.

Common situations: Configuration storing only the class name instead of AssemblyQualifiedName; module assembly not referenced/loaded; typos in the type string; assemblies not resolvable by Type.GetType in the current context.

Related errors

• Value cannot be null. (Parameter 'name')
• Value cannot be null. (Parameter 'dependsOn')
• Value cannot be null. (Parameter 'moduleInitializer')
• Value cannot be null. (Parameter 'moduleCatalog')
• configureSegment

AI-assisted analysis of PrismLibrary/Prism@358118cd64 (2026-09-15). Data as JSON: /api/errors/c70e5c2d3f18fddd. Report an issue: GitHub.

Appendix: source

Thrown at src/Maui/Prism.Maui/Modularity/ModuleInfo.cs:38

    public ModuleInfo()
    {
    }

    /// <summary>
    /// Initializes a new instance of <see cref="ModuleInfo"/>.
    /// </summary>
    /// <param name="name">The module's name.</param>
    /// <param name="type">The module <see cref="Type"/>'s AssemblyQualifiedName.</param>
    /// <param name="dependsOn">The modules this instance depends on.</param>
    /// <exception cref="ArgumentNullException">An <see cref="ArgumentNullException"/> is thrown if <paramref name="dependsOn"/> is <see langword="null"/>.</exception>
    public ModuleInfo(string name, string type, params string[] dependsOn)
    {
        if (string.IsNullOrWhiteSpace(name))
            throw new ArgumentNullException(nameof(name));
        if (dependsOn == null)
            throw new ArgumentNullException(nameof(dependsOn));

        ModuleType = Type.GetType(type) ?? throw new ArgumentNullException(nameof(type));
        ModuleName = name;
        foreach (string dependency in dependsOn)
        {
            if (!DependsOn.Contains(dependency))
            {
                DependsOn.Add(dependency);
            }
        }
    }

    /// <summary>
    /// Initializes a new instance of <see cref="ModuleInfo"/>.
    /// </summary>
    /// <param name="name">The module's name.</param>
    /// <param name="type">The module's type.</param>
    public ModuleInfo(string name, string type)
        : this(name, type, Array.Empty<string>())
    {

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