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Fluent APIs in C#

Everybody wants to use fluent APIs but writing them is tedious. With this library providing fluent APIs for your classes becomes a breeze. Simply annotate them with attributes and the source code for the fluent API will be generated. The fluent API library leverages incremental source code generation at development time and your IDE will offer you the corresponding code completion immediately.

The generated code follows the builder design pattern and allows you to construct objects step by step. This approach avoids big constructors and results in very readable code.

Accompanying blog post: www.m31coding.com>blog>fluent-api

Features

Builder code generation controlled by attributes
Stepwise object construction
Special handling for boolean, collection, and nullable members
Nested fluent APIs via lambda methods
Custom builder methods
Optional (skippable) builder methods
Forking and branching capabilities
Support for returning arbitrary types
Support for inheritance, generics, and partial classes

Installing via NuGet

Install the latest version of the package M31.FluentApi via your IDE or use the package manager console:

PM> Install-Package M31.FluentApi

A package reference will be added to your csproj file. Moreover, since this library provides code via source code generation, consumers of your project don't need the reference to M31.FluentApi. Therefore, it is recommended to use the PrivateAssets metadata tag:

<PackageReference Include="M31.FluentApi" Version="1.9.0" PrivateAssets="all"/>

If you would like to examine the generated code, you may emit it by adding the following lines to your csproj file:

<PropertyGroup>
    <CompilerGeneratedFilesOutputPath>$(BaseIntermediateOutputPath)Generated</CompilerGeneratedFilesOutputPath>
    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
</PropertyGroup>

The code can then be found in the obj/Generated folder.

Usage

If you use this library for the first time I recommend that you read the storybook:

Moreover, you may find several Fluent API examples and their usage in the example project:

Here is an example from the introduction to the basics:

[FluentApi]
public class Student
{
    [FluentMember(0, "Named", 0)]
    public string FirstName { get; private set; }

    [FluentMember(0, "Named", 1)]
    public string LastName { get; private set; }

    [FluentMember(1, "OfAge")]
    public int Age { get; private set; }

    [FluentMethod(1)]
    private void BornOn(DateOnly dateOfBirth)
    {
        DateOnly today = DateOnly.FromDateTime(DateTime.Today);
        int age = today.Year - dateOfBirth.Year;
        if (dateOfBirth > today.AddYears(-age)) age--;
        Age = age;
    }

    [FluentMember(2, "InSemester")]
    [FluentDefault("WhoStartsUniversity")]
    public int Semester { get; private set; } = 0;

    [FluentMember(3, "LivingIn")]
    [FluentDefault("LivingInBoston")]
    [FluentNullable("InUnknownCity")]
    public string? City { get; private set; } = "Boston";

    [FluentPredicate(4, "WhoIsHappy", "WhoIsSad")]
    [FluentNullable("WithUnknownMood")]
    public bool? IsHappy { get; private set; }

    [FluentCollection(5, "Friend", "WhoseFriendsAre", "WhoseFriendIs", "WhoHasNoFriends")]
    public IReadOnlyCollection<string> Friends { get; private set; }
 }

fluent-api-usage

You may have a look at the generated code for this example: CreateStudent.g.cs. Note that if you use private members or properties with a private set accessor, as it is the case in this example, the generated code will use reflection to set the properties.

Attributes

The attributes FluentApi and FluentMember are all you need in order to get started.

The attributes FluentPredicate and FluentCollection can be used instead of the FluentMember attribute if the decorated member is a boolean or a collection, respectively.

FluentDefault and FluentNullable can be used in combination with these attributes to set a default value or null, respectively.

The FluentMethod attribute is used for custom builder method implementations.

The control attribute FluentSkippable allows builder methods to be optional, while FluentContinueWith indicates a jump to the specified builder step. FluentBreak stops the builder, and FluentReturn allows returning arbitrary types and values within the generated API.

FluentApi

FluentApi(string builderClassName = "Create{Name}")

Use this attribute for your class / struct / record. The optional parameter allows you to specify the name of the builder class that will be generated. Within the argument the template {Name} can be used, which will be replaced by the name of your decorated type.

[FluentApi]
public class Student

You can create instances by statically accessing the generated CreateStudent class:

Student alice = CreateStudent.WithFirstName("Alice")...

Alternatively, you can call InitialStep to get a new builder instance:

ICreateStudent createStudent = CreateStudent.InitialStep();
Student alice = createStudent.WithFirstName("Alice")...

FluentMember

FluentMember(int builderStep, string method = "With{Name}", int parameterPosition = 0)

Use this attribute for fields and properties of your class. They can be private but properties must have a set accessor. The builderStep parameter specifies the step in which the member can be set. With the method parameter you can specify the name of the builder method.

[FluentMember(0)]
public string FirstName { get; private set; }

...WithFirstName("Alice")...

If two FluentMember attributes with the same builder step and equal method names are specified, a compound method will be created, which is a builder method that sets multiple properties at once. For compounds the position of the parameters can be controlled by the parameter parameterPosition.

[FluentMember(0, "Named", 0)]
public string FirstName { get; private set; }

[FluentMember(0, "Named", 1)]
public string LastName { get; private set; }

...Named("Alice", "King")...

If the decorated member has its own Fluent API, an additional lambda method is generated, e.g.

[FluentMember(1)]
public Address Address { get; private set; }

...WithAddress(a => a.WithHouseNumber("108").WithStreet("5th Avenue").InCity("New York"))...

FluentPredicate

FluentPredicate(int builderStep, string method = "{Name}", string negatedMethod = "Not{Name}")

Can be used instead of the FluentMember attribute if the decorated member is of type bool. This attribute generates three methods, one for setting the value of the member to true, one for setting it to false, and one for passing the boolean value.

[FluentPredicate(4, "WhoIsHappy", "WhoIsSad")]
public bool IsHappy { get; private set; }

...WhoIsHappy()...
...WhoIsSad()...
...WhoIsHappy(true)...

FluentCollection

FluentCollection(
    int builderStep,
    string singularName,
    string withItems = "With{Name}",
    string withItem = "With{SingularName}",
    string withZeroItems = "WithZero{Name}")

Can be used instead of the FluentMember attribute if the decorated member is a collection. This attribute generates methods for setting multiple items, one item and zero items. The supported collection types can be seen in the source file CollectionInference.cs.

[FluentCollection(5, "Friend", "WhoseFriendsAre", "WhoseFriendIs", "WhoHasNoFriends")]
public IReadOnlyCollection<string> Friends { get; private set; }

...WhoseFriendsAre(new string[] { "Bob", "Carol", "Eve" })...
...WhoseFriendsAre("Bob", "Carol", "Eve")...
...WhoseFriendIs("Alice")...
...WhoHasNoFriends()...

If the element type of the decorated member has its own Fluent API, additional lambda methods are generated, e.g.

[FluentCollection(1, "Address")]
public IReadOnlyCollection<Address> Addresses { get; private set; }

...WithAddresses(
    a => a.WithHouseNumber("108").WithStreet("5th Avenue").InCity("New York"),
    a => a.WithHouseNumber("42").WithStreet("Maple Ave").InCity("Boston"))...
...WithAddress(a => a.WithHouseNumber("82").WithStreet("Friedrichstraße").InCity("Berlin"))...

FluentDefault

FluentDefault(string method = "WithDefault{Name}")

Can be used for fields and properties in addition to other attributes. When the generated builder method is called the member will keep its initial value.

[FluentMember(3, "LivingIn")]
[FluentDefault("LivingInBoston")]
[FluentNullable("InUnknownCity")]
public string? City { get; private set; } = "Boston";

...LivingInBoston()... // City is not changed

FluentNullable

FluentNullable(string method = "Without{Name}")

Can be used for fields and properties in addition to other attributes. Generates a builder method that sets the member to null.

[FluentMember(3, "LivingIn")]
[FluentDefault("LivingInBoston")]
[FluentNullable("InUnknownCity")]
public string? City { get; private set; } = "Boston";

...InUnknownCity()... // City is set to null

FluentMethod

FluentMethod(int builderStep, string method = "{Name}")

Use this attribute on methods to provide a custom implementation for setting values or triggering additional behavior. The decorated method must return void.

[FluentMethod(1)]
private void BornOn(DateOnly dateOfBirth)
{
    DateOnly today = DateOnly.FromDateTime(DateTime.Today);
    int age = today.Year - dateOfBirth.Year;
    if (dateOfBirth > today.AddYears(-age)) age--;
    Age = age;
}

...BornOn(new DateOnly(2003, 6, 24))...

FluentSkippable

FluentSkippable()

Can be used at all steps on fields, properties, and methods to create an optional builder method. The generated API will offer the method but it does not have to be called.

[FluentMember(0)]
public string FirstName { get; private set; }

[FluentMember(1)]
[FluentSkippable]
public string? MiddleName { get; private set; }

[FluentMember(2)]
public string LastName { get; private set; }

...WithFirstName("Bob").WithLastName("Bishop")...
...WithFirstName("Alice").WithMiddleName("Sophia").WithLastName("King")...

FluentContinueWith

FluentContinueWith(int builderStep)

Can be used at all steps on fields, properties, and methods to jump to a specific builder step. Useful for branching.

[FluentMethod(3)]
[FluentContinueWith(7)]
private void WhoIsADigitalNomad()
{
    IsDigitalNomad = true;
}

// ...

[FluentMethod(7)]
private void LivingInCity(string city)
{
    City = city;
}

...WhoIsADigitalNomad().LivingInCity("Berlin")...

FluentBreak

FluentBreak()

Can be used at all steps on fields, properties, and methods to stop the builder. Only relevant for non-linear APIs that make use of FluentContinueWith.

[FluentMethod(3)]
[FluentBreak]
private void WhoseAddressIsUnknown()
{
}

...WhoseAddressIsUnknown();

FluentReturn

Allows the builder to respect the return value of the decorated method, enabling the return of arbitrary types and values within the generated API. If a void method is decorated with this attribute, the builder method will also return void.

[FluentMethod(1)]
[FluentReturn]
public string ToJson()
{
    return JsonSerializer.Serialize(this);
}

string serialized = ...ToJson();

Miscellaneous

Forks

To create forks specify builder methods at the same builder step. The resulting API offers all specified methods at this step but only one can be called:

[FluentMember(1, "OfAge")]
public int Age { get; private set; }

[FluentMethod(1)]
private void BornOn(DateOnly dateOfBirth)
{
    DateOnly today = DateOnly.FromDateTime(DateTime.Today);
    int age = today.Year - dateOfBirth.Year;
    if (dateOfBirth > today.AddYears(-age)) age--;
    Age = age;
}

...OfAge(22)...
...BornOn(new DateOnly(2002, 8, 3))...

Lambda pattern

Instances of Fluent API classes can be created and passed into methods of other classes using the lambda pattern. For example, given a University class that needs to be augmented with an AddStudent method, the following code demonstrates the lambda pattern:

public void AddStudent(Func<CreateStudent.ICreateStudent, Student> createStudent)
{
    Student student = createStudent(CreateStudent.InitialStep());
    students.Add(student);
}

university.AddStudent(s => s.Named("Alice", "King").OfAge(22)...);

Note that if you want to set a member of a Fluent API class, you can simply use FluentMember or FluentCollection instead of the pattern above.

Problems with the IDE

Since code generation is potentially triggered with every single key stroke, there are sometimes situations where the code completion index of the IDE does not keep up with all the changes.

In particular, if your IDE visually indicates that there are errors in your code but it compiles and runs just fine, try the following things:

Rebuild the project or the whole solution
Unload and reload the project
Close and reopen the IDE
Remove the .vs folder (Visual Studio) or the .idea folder (Rider)

Support and Contribution

This library is free. If you find it valuable and wish to express your support, please leave a star. You are kindly invited to contribute. If you see the possibility for enhancement, please create a GitHub issue and you will receive timely feedback.

Happy coding!