devlooped/moq · error · ArgumentNullException

value (Argument is null)

Error message

value (Argument is null)

What it means

MockFactory.DefaultValueProvider is a non-nullable property, so its setter rejects null with ArgumentNullException(paramName: 'value'). Assigning null would leave the factory without a default value provider, which Moq treats as a programming error.

Solutions

1. Only assign a real DefaultValueProvider instance (DefaultValueProvider.Empty, .Mock, or a custom provider)
2. If you want 'no customization', assign DefaultValueProvider.Empty rather than null
3. Null-check the source value before copying it into the property

Example fix

// before
factory.DefaultValueProvider = maybeNull;
// after
factory.DefaultValueProvider = maybeNull ?? DefaultValueProvider.Empty;

if (provider is null)
    provider = DefaultValueProvider.Empty;
factory.DefaultValueProvider = provider;

try
{
    factory.DefaultValueProvider = provider;
}
catch (ArgumentNullException ex)
{
    // ex.ParamName == "value"
    factory.DefaultValueProvider = DefaultValueProvider.Empty;
}

When it happens

Trigger: Executing factory.DefaultValueProvider = null, typically via a property copy from a nullable source, reflection, or a config object whose field is null.

Common situations: Copying settings between MockFactory instances where the source was never initialized; data binding that pushes null into the property.

Related errors

• callback (Argument is null)
• value (Argument is out of range)
• Unsupported expression
• Could not determine the correct positions for all argument…
• The return type of the last member shown above is not…

AI-assisted analysis of devlooped/moq@89a5be629c (2026-09-16). Data as JSON: /api/errors/23803c4f02d2b385. Report an issue: GitHub.

Appendix: source

Thrown at src/Moq/Obsolete/MockFactory.cs:161

            set
            {
                this.DefaultValueProvider = value switch
                {
                    DefaultValue.Empty => DefaultValueProvider.Empty,
                    DefaultValue.Mock => DefaultValueProvider.Mock,
                    _ => throw new ArgumentOutOfRangeException(nameof(value)),
                };
            }
        }

        /// <summary>
        /// Gets or sets the <see cref="Moq.DefaultValueProvider"/> instance that will be used
        /// e. g. to produce default return values for unexpected invocations.
        /// </summary>
        public DefaultValueProvider DefaultValueProvider
        {
            get => this.defaultValueProvider;
            set => this.defaultValueProvider = value ?? throw new ArgumentNullException(nameof(value));
        }

        /// <summary>
        /// Gets the mocks that have been created by this factory and 
        /// that will get verified together.
        /// </summary>
        protected internal IEnumerable<Mock> Mocks { get { return mocks; } }

        /// <summary>
        /// A set of switches that influence how mocks created by this factory will operate.
        /// You can opt in or out of certain features via this property.
        /// </summary>
        public Switches Switches
        {
            get => this.switches;
            set => this.switches = value;
        }

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