devlooped/moq · error · ArgumentNullException

value (Argument is null)

Error message

value (Argument is null)

What it means

Moq throws this ArgumentNullException when null is assigned to Mock<T>.DefaultValueProvider. A mock must always have a default value provider to produce return values for loose invocations, so null is rejected immediately.

Solutions

1. Assign a concrete provider such as DefaultValue.Empty, DefaultValue.Mock, or a custom DefaultValueProvider subclass.
2. If you intended to reset behavior, create a new mock rather than setting the provider to null.
3. Null-check the value returned by your provider factory before assigning it.

Example fix

// before
mock.DefaultValueProvider = GetProvider(); // may be null
// after
var provider = GetProvider();
mock.DefaultValueProvider = provider ?? DefaultValue.Empty;

var provider = GetProvider();
if (provider is null) provider = DefaultValue.Empty;
mock.DefaultValueProvider = provider;

bool IsValidProvider(DefaultValueProvider p) => p is not null;

try { mock.DefaultValueProvider = provider; } catch (ArgumentNullException) { mock.DefaultValueProvider = DefaultValue.Empty; }

When it happens

Trigger: mock.DefaultValueProvider = null; or passing a null provider variable/return value of a factory into the DefaultValueProvider setter (src/Moq/Mock`1.cs:264).

Common situations: A provider-factory method returning null on a missed configuration branch; conditional config code that nulls out the provider to 'reset' it instead of assigning Mock.Of<DefaultValueProvider> or a concrete provider.

Related errors

• Value cannot be null. (Parameter 'value')
• Value cannot be null. (Parameter 'value')
• Value cannot be null. (Parameter 'newExpression')
• callback (Argument is null)
• condition (Argument is null)

AI-assisted analysis of devlooped/moq@89a5be629c (2026-09-16). Data as JSON: /api/errors/ba77634f97122297. Report an issue: GitHub.

Appendix: source

Thrown at src/Moq/Mock`1.cs:264

                    throw new NotSupportedException(Resources.CallBaseCannotBeUsedWithDelegateMocks);
                }

                this.callBase = value;
            }
        }

        internal override object?[] ConstructorArguments => this.constructorArguments;

        internal override Dictionary<Type, object?> ConfiguredDefaultValues => this.configuredDefaultValues;

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

        internal override EventHandlerCollection EventHandlers => this.eventHandlers;

        internal override List<Type> AdditionalInterfaces => this.additionalInterfaces;

        internal override InvocationCollection MutableInvocations => this.invocations;

        internal override bool IsObjectInitialized => this.instance != null;

        /// <summary>
        ///   Exposes the mocked object instance.
        /// </summary>
        public virtual new T Object
        {
            get { return (T)base.Object; }
        }

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