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     我们首先回故下Builder模式的意图： 将一个复杂对象的构建与它的表示分离，使得同样的构建过程可以创建不同的表示。[DP]
那我们C＃中switch语句块，从一个层面上来看，就是由几部分组成的，我们来实现一个SwitchBuilder类具有这种功能．那什么
是Fluent接口，可以先看这儿wiki．

     有的时候我们需要Enum类型处理复杂的类型，有一个描述字段，或还有更多其它的字段信息．我们创建一个使用static readonly
字段的类，看下面这是一个典型的实现：

  /// <summary>/// EnumType/// </summary>/// <remarks>http://wintersun.cnblogs.com</remarks>public class EnumType{public static readonly EnumType ONE = new EnumType(1, "Descr1");public static readonly EnumType TWO = new EnumType(2, "Descr2");public static readonly EnumType THREE = new EnumType(3, "Descr3");private readonly int id;private readonly string description;private EnumType(int id, string description){this.description = description;this.id = id;}public int Id{get { return id; }}public string Description{get { return description; }}#region Object's overridepublic override bool Equals(object obj){if (ReferenceEquals(null, obj)){return false;}if (ReferenceEquals(this, obj)){return true;}return obj.GetType() != typeof (EnumType) && ((EnumType) obj).Id == Id;}public override int GetHashCode(){return id.GetHashCode();}public override string ToString(){return string.Format("id:<{0}> and Descr:<{1}>", Id, Description);}

public interface IDo
{void Do();
}public interface IBody : IDo
{ICase Case(object obj);IDefault Default { get; }
}public interface ICase
{ICase Case(object obj);IBody Body(Action action);
}public interface IDefault
{IDo Body(Action action);
}public interface ISwitch
{ICase Switch(object obj);
}

再我们的具体类：

public class SwitchBuilder : ISwitch, IDefault, ICase, IBody
{private Action defaultAction;private object testObject;private IList<object> caseList;private readonly IDictionary<object, Action> caseActions = new Dictionary<object, Action>();private SwitchBuilder() {}public static ISwitch Create(){return new SwitchBuilder();}public ICase Switch(object obj){caseList = new List<object>();testObject = obj;return this;}public ICase Case(object obj){caseList.Add(obj);return this;}public IBody Body(Action action){foreach (var switchCase in caseList){caseActions.Add(switchCase, action);}caseList = new List<object>();return this;}public IDefault Default{get { return this; }}IDo IDefault.Body(Action action){defaultAction = action;return this;}public void Do(){foreach (KeyValuePair<object, Action> caseAction in caseActions){if (ReferenceEquals(caseAction.Key, testObject) || Equals(caseAction.Key, testObject)){caseAction.Value();return;}}if (defaultAction != null){defaultAction();}}
}

如何使用呢？看这个UnitTest

[Test]
public void CanCreateFluentSwitchBuilder()
{EnumType state = null;EnumType enumType = EnumType.THREE;SwitchBuilder.Create().Switch(enumType).Case(EnumType.ONE).Body(() =>{Console.WriteLine(EnumType.ONE);state = EnumType.ONE;}).Case(EnumType.TWO).Case(EnumType.THREE).Body(() =>{Console.WriteLine("->" + EnumType.TWO + EnumType.THREE);state = EnumType.TWO;}).Default.Body(() => Console.WriteLine("Def")).Do();Assert.AreEqual(state, EnumType.TWO);
}

那些接口的一个作用在于，保证我们的SwitchBuilder使用时，要先Create然后Switch…最后do.

结论

　　Builder模式与Fluent接口在很多种场景下并不是那么简单的API，有时还需要简单的验证．还有其它方式实现
Fluent接口模式，例如使用嵌套类．

Author: Petter Liu   http://wintersun.cnblogs.com 
Reference: source