devlooped/moq · error · ArgumentNullException

condition (Argument is null)

Error message

condition (Argument is null)

What it means

Moq throws this ArgumentNullException when a null condition delegate is passed to Mock<T>.When(Func<bool> condition). The condition is evaluated to decide whether a conditional setup applies, so a null predicate is invalid.

Solutions

1. Pass a real predicate: mock.When(() => someState).Setup(...).
2. Null-check condition factory results before passing them to When.
3. If no condition is needed, use plain Setup instead of When.

Example fix

// before
mock.When(null).Setup(m => m.Foo()).Returns(1);
// after
mock.When(() => isEnabled).Setup(m => m.Foo()).Returns(1);

if (condition is null) throw new ArgumentNullException(nameof(condition));

bool IsValidCondition(Func<bool> c) => c is not null;

try { mock.When(condition).Setup(...); } catch (ArgumentNullException) { mock.Setup(...); }

When it happens

Trigger: mock.When(null) — any call to When whose condition argument is null (src/Moq/Mock`1.cs:690).

Common situations: Storing the predicate in a variable that is never assigned or returned null from a condition-factory; refactoring that removed the lambda body but left a null literal.

Related errors

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

AI-assisted analysis of devlooped/moq@89a5be629c (2026-09-16). Data as JSON: /api/errors/f9a5aa5b2a2f18e3. Report an issue: GitHub.

Appendix: source

Thrown at src/Moq/Mock`1.cs:690

        #endregion

        #region When

        /// <summary>
        ///   Allows setting up a conditional setup.
        ///   Conditional setups are only matched by an invocation
        ///   when the specified condition evaluates to <see langword="true"/>
        ///   at the time when the invocation occurs.
        /// </summary>
        /// <param name="condition">
        ///   The condition that should be checked
        ///   when a setup is being matched against an invocation.
        /// </param>
        public ISetupConditionResult<T> When(Func<bool> condition)
        {
            if (condition == null)
            {
                throw new ArgumentNullException(nameof(condition));
            }

            return new WhenPhrase<T>(this, new Condition(condition));
        }

        #endregion

        #region Verify

        /// <summary>
        ///   Verifies that a specific invocation matching the given expression was performed on the mock.
        ///   Use in conjunction with the default <see cref="MockBehavior.Loose"/>.
        /// </summary>
        /// <param name="expression">Expression to verify.</param>
        /// <exception cref="MockException">The invocation was not performed on the mock.</exception>
        /// <example group="verification">
        ///   This example assumes that the mock has been used, and later we want to verify
        ///   that a given invocation with specific parameters was performed:

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