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Asked: 2026-05-11T14:04:52+00:00

Fluent interfaces is a fairly hot topic these days. C# 3.0 has some nice

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‘Fluent interfaces’ is a fairly hot topic these days. C# 3.0 has some nice features (particularly extension methods) that help you make them.

FYI, a fluent API means that each method call returns something useful, often the same object you called the method on, so you can keep chaining things. Martin Fowler discusses it with a Java example here. The concept kooks something like this:

var myListOfPeople = new List<Person>();  var person = new Person(); person.SetFirstName('Douglas').SetLastName('Adams').SetAge(42).AddToList(myListOfPeople);

I have seen some incredibly useful fluent interfaces in C# (one example is the fluent approach for validating parameters found in an earlier StackOverflow question I had asked. It blew me away. It was able to give highly readable syntax for expressing parameter validation rules, and also, if there were no exceptions, it was able to avoid instantiating any objects! So for the ‘normal case’, there was very little overhead. This one tidbit taught me a huge amount in a short time. I want to find more things like that).

So, I’d like to learn more by looking at and discussing some excellent examples. So, what are some excellent fluent interfaces you’ve made or seen in C#, and what made them so valuable?

Thanks.

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  1. Kudos for the method parameter validation, you’ve given me a new idea for our fluent APIs. I’ve hated our precondition checks anyways…

    I’ve built a extensibility system for a new product in development, where you can fluently describe the commands available, the user interface elements and more. This runs on top of StructureMap and FluentNHibernate, which are nice APIs too.

    MenuBarController mb; // ... mb.Add(Resources.FileMenu, x => {   x.Executes(CommandNames.File);   x.Menu     .AddButton(Resources.FileNewCommandImage, Resources.FileNew, Resources.FileNewTip, y => y.Executes(CommandNames.FileNew))     .AddButton(null, Resources.FileOpen, Resources.FileOpenTip, y =>      {       y.Executes(CommandNames.FileOpen);       y.Menu         .AddButton(Resources.FileOpenFileCommandImage, Resources.OpenFromFile, Resources.OpenFromFileTop, z => z.Executes(CommandNames.FileOpenFile))         .AddButton(Resources.FileOpenRecordCommandImage, Resources.OpenRecord, Resources.OpenRecordTip, z => z.Executes(CommandNames.FileOpenRecord));      })      .AddSeperator()      .AddButton(null, Resources.FileClose, Resources.FileCloseTip, y => y.Executes(CommandNames.FileClose))      .AddSeperator();      // ... });

    And you can configure all commands available like this:

    Command(CommandNames.File)   .Is<DummyCommand>()   .AlwaysEnabled();  Command(CommandNames.FileNew)   .Bind(Shortcut.CtrlN)   .Is<FileNewCommand>()   .Enable(WorkspaceStatusProviderNames.DocumentFactoryRegistered);  Command(CommandNames.FileSave)   .Bind(Shortcut.CtrlS)   .Enable(WorkspaceStatusProviderNames.DocumentOpen)   .Is<FileSaveCommand>();  Command(CommandNames.FileSaveAs)   .Bind(Shortcut.CtrlShiftS)   .Enable(WorkspaceStatusProviderNames.DocumentOpen)   .Is<FileSaveAsCommand>();  Command(CommandNames.FileOpen)   .Is<FileOpenCommand>()   .Enable(WorkspaceStatusProviderNames.DocumentFactoryRegistered);  Command(CommandNames.FileOpenFile)   .Bind(Shortcut.CtrlO)   .Is<FileOpenFileCommand>()   .Enable(WorkspaceStatusProviderNames.DocumentFactoryRegistered);  Command(CommandNames.FileOpenRecord)   .Bind(Shortcut.CtrlShiftO)   .Is<FileOpenRecordCommand>()   .Enable(WorkspaceStatusProviderNames.DocumentFactoryRegistered);

    Our view configure their controls for the standard edit menu commands using a service given to them by the workspace, where they just tell it to observe them:

    Workspace   .Observe(control1)   .Observe(control2)

    If the user tabs to the controls, the workspace automatically gets an appropriate adapter for the control and provides undo/redo and clipboard operations.

    It has helped us reduce the setup code dramatically and make it even more readable.

    I forgot to tell about a library we’re using in our WinForms MVP model presenters to validate the views: FluentValidation. Really easy, really testable, really nice!

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