Default configurations

AutoFixture comes with a set of default builders so that most common scenarios are supported out-of-the-box.

These builders can be overridden with ad-hoc customizations or when building an anonymous variable.


The default builder for numbers is the RandomNumericSequenceGenerator.

It supports the following types: Byte, Decimal, Double, Int16, Int32, Int64, SByte, Single, UInt16, UInt32, UInt64.

Unique numbers are generated randomly from the set [1, 255]. Once these are used up they are then be generated from the set [256, 65 535]. And finally from the set [65 536, 2 147 483 647].

When all numbers within the final set have been used AutoFixture will start again from the first set.


The default builder for chars is the RandomCharSequenceGenerator.

It generates random characters from the printable ASCII character set (! (33) to ~ (126)).


The default builder for strings is an instance of StringGenerator.

It returns randomly generated Guid as string using the format xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx.

Dates and times

The default builder for DateTime is the RandomDateTimeSequenceGenerator.

It generates a date and a time between 2 years prior and 2 years after the current date.

var dateTime = fixture.Create<DateTime>();
Assert.That(dateTime, Is.GreaterOrEqualThan(DateTime.Today.AddYears(-2)));
Assert.That(dateTime, Is.LessOrEqualThan(DateTime.Today.AddYears(2)));


The EnumGenerator is the default builder for enumerations.

It returns all the values of the enumeration in order. When all values are used, it starts from the first value again.


AutoFixture has a built-in support for many collection types: Dictionary, SortedDictionary, SortedList, Collection, List, HashSet, SortedSet, ObservableCollection, and Array.

Also, AutoFixture natively supports common collection interfaces like IDictionary, IReadOnlyDictionary, ICollection, IReadOnlyCollection, IList, IReadOnlyList, ISet, and IEnumerable. When requested, AutoFixture will return a type implementing the requested interface.

Tuple and ValueTuple

AutoFixture is able to generate out-of-the-box anonymous variables of both Tuple and ValueTuple types.

var tuple = fixture.Create<Tuple<string, int>>();
var valueTuple = fixture.Create<ValueTuple<string, int>>();

In the case of ValueTuple, it also supports the simplified syntax.

var valueTuple = fixture.Create<(string, int)>();


The default builder for Uri is the UriGenerator.

The UriGenerator delegates the scheme creation to a UriSchemeGenerator configured to return http by default.


The default builder for the MailAddress type is the MailAddressGenerator.

This generator splits the email address into two parts that can be independently customized:

  • a EmailAddressLocalPart, generated via a EmailAddressLocalPartGenerator

  • a DomainName, generated via a DomainNameGenerator

Fixed values

For some types, the default configuration of AutoFixture is to return well-known values.

Here is a list of these cases:

  • IPAddress is registered with the value IPAddress.Loopback

  • Encoding is registered with the value Encoding.UTF8

  • CultureInfo is registered with the value CultureInfo.InvariantCulture