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Argument constraints

When configuring and asserting calls in FakeItEasy, the arguments of the call can be constrained so that only calls to the configured method where the arguments matches the constraint are selected.

Matching values exactly

Assume the following interface exists:

public interface IFoo
    void Bar(string s, int i);

Then the arguments to Bar can be constrained to limit call matching:

var foo = A.Fake<IFoo>();

A.CallTo(() => foo.Bar("hello", 17)).MustHaveHappened();

Then FakeItEasy will look only for calls made with the arguments "hello" and 17 - no other calls will match the rule.

When checking for argument equality, FakeItEasy uses the first applicable equality test from this list:

  • if both values are null, they are considered equal,
  • if one value is null and the other isn't, they are not considered equal,
  • the highest-priority custom argument equality comparer that can compare the example object's type, or
  • if both values are string, string.Equals (as an optimization),
  • if the example object's type implements System.Collections.IEnumerable, the values are considered equal if and only if the actual object's type also implements System.Collections.IEnumerable and System.Linq.Enumerable.SequenceEqual evaluates to true, or
  • Object.Equals

If this list is not satisfactory, you may have to use the That.Matches method described in Custom matching. Be particularly careful of types whose Equals methods perform reference equality rather than value equality. In that case, the objects have to be the same object in order to match, and this sometimes produces unexpected results. When in doubt, verify the type's Equals behavior manually.

Other matchers

Ignoring arguments

Suppose the value of the integer in the Bar call wasn't important, but the string was. Then the following constraint could be used:

A.CallTo(() => foo.Bar("hello", A<int>.Ignored)).MustHaveHappened();

Then any call will match, so long as the string value was "hello". The Ignored property can be used on any type.

An underscore (_) can be used as a shorthand for Ignored as well:

A.CallTo(() => foo.Bar("hello", A<int>._)).MustHaveHappened();

More convenience matchers

If more complicated constraints are needed, the That method can be used. There are a few built-in matchers:

Matcher Tests for
IsNull() null
IsNotNull() not null
IsEqualTo(other) object equality using object.Equals
IsEqualTo(other, equalityComparer) object equality using equalityComparer.Equals
IsSameAs(other) object identity - like object.ReferenceEquals
IsInstanceOf(type) an argument that can be assigned to a variable of type type
Contains(string) substring match with ordinal string comparison
Contains(string, comparisonType) substring match with the specified comparison type
StartsWith(string) substring at beginning of string with ordinal string comparison
StartsWith(string, comparisonType) substring at beginning of string with the specified comparison type
EndsWith(string) substring at end of string with ordinal string comparison
EndsWith(string, comparisonType) substring at end of string with the specified comparison type
IsNullOrEmpty() null or ""
IsEmpty() empty enumerable
Contains(item) item's presence in an enumerable
IsSameSequenceAs(enumerable) sequence equality, like System.Linq.Enumerable.SequenceEqual
IsSameSequenceAs(value1, value2, ...) sequence equality, like System.Linq.Enumerable.SequenceEqual
Not inverts the sense of the matcher

Custom matching

If none of the canned matchers are sufficient, you can provide a predicate to perform custom matching using That.Matches. Like in this rather contrived example:

A<string>.That.Matches(s => s.Length == 3 && s[1] == 'X');

FakeItEasy will evaluate the predicate against any supplied argument. The predicate can be supplied as an Expression<Func<T, bool>> or as a Func<T, bool>. FakeItEasy can generate a description of the matcher when an Expression is supplied (although you may supply your own as well), but you must supply a description when using a Func.

For another example of using That.Matches, see Jonathan Channon's Comparing object instances with FakeItEasy.

Always place Ignored and That inside A.CallTo

The Ignored (and _) and That matchers must be placed within the expression inside the A.CallTo call. This is because these special constraint methods do not return an actual matcher object. They tell FakeItEasy how to match the parameter via a special event that's fired when the constraint method is invoked. FakeItEasy only listens to the events in the context of an A.CallTo.

So, tempting as it might be to save one of the constraints away in a handy variable, don't do it.

Using correct grammar

FakeItEasy's API attempts to imitate the English language, so that call configurations and assertions read naturally. In that spirit, it's possible to use An<T> instead of A<T> for types whose name starts with a vowel sound. For instance:

An<Apple>.That.Matches(a => a.Color == "Red")

A<T> and An<T> are exact synonyms and can be used exactly the same way.

out parameters

The incoming argument value of out parameters is ignored when matching calls. The incoming value of an out parameter can't be seen by the method body anyhow, so there's no advantage to constraining by it.

For example, this test passes:

string configurationValue = "lollipop";
A.CallTo(() => aFakeDictionary.TryGetValue(theKey, out configurationValue))

string fetchedValue = "licorice";
var success = aFakeDictionary.TryGetValue(theKey, out fetchedValue);

Assert.That(success, Is.True);

See Implicitly Assigning out Parameter Values to learn how the initial configurationValue is used in this case.

ref parameters

Due to the limitations of working with ref parameters in C#, only exact-value matching is possible using argument constraints, and the argument value must be compared against a local variable or a field:

int someValue = 3;
A.CallTo(() => aFake.aMethod(ref someValue)).Returns(true);

To perform more sophisticated matching of ref parameter values in C#, use constraints that work on the entire call, such as WithAnyArguments or WhenArgumentsMatch, described below.

In addition to constraining by ref argument values, calls can be explicitly configured to assign outgoing ref argument values.

Overriding argument matchers

Sometimes individually constraining arguments isn't sufficient. In such a case, other methods may be used to determine which calls match the fake's configuration.

When using the following methods, any inline argument constraints are ignored, and only the special method is used to match the call. Some arguments have to be supplied in order to satisfy the compiler, but the values will not be used, so you can supply whatever values make the code easiest for you to read.

WithAnyArguments ensures that no argument constraints will be applied when matching calls:

A.CallTo(() => foo.Bar(null, 7)).WithAnyArguments().MustHaveHappened();

The example above will match any call to foo.Bar, regardless of the arguments. The Ignored property performs the same task, and is more flexible, but some people prefer the look of WithAnyArguments.

WhenArgumentsMatch accepts a predicate that operates on the entire collection of call arguments. For example, to have a Fake throw an exception when a call is made to Bar where the first arguments is a string representation of the second, use

A.CallTo(() => fake.Bar(null, 0))
    .WhenArgumentsMatch(args =>

Strongly typed overloads of WhenArgumentsMatch are also available for methods of up to 8 parameters (if a method has more parameters, use the variant described above):

A.CallTo(() => fake.Bar(null, 0))
    .WhenArgumentsMatch((string theString, int theInt) =>

Nested argument constraints

Note that an argument constraint cannot be "nested" in an argument; the constraint has to be the whole argument. For instance, the following call configurations are invalid and will throw an exception:

A.CallTo(() => fake.Foo(new Bar(A<int>._))).Returns(42);
A.CallTo(() => fake.Foo(new Bar { X = A<string>.That.Contains("hello") })).Returns(42);

To achieve the desired effect, you can do this instead:

A.CallTo(() => fake.Foo(A<Bar>._)).Returns(42);
A.CallTo(() => fake.Foo(A<Bar>.That.Matches(bar => bar.X.Contains("hello")))).Returns(42);