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開發專案中善用輔助方法：讓單元測試更簡潔易讀

編輯：關於.NET

前言

專案開發過程中，字串處理、代碼檢驗、格式輸出等重複邏輯層出不窮。當發現自己反覆撰寫相似代碼時，便該考慮將其提取為輔助方法（或類庫、框架）。除了遵循 DRY（Don't Repeat Yourself）原則，輔助方法更能讓代碼更易撰寫、結構更清晰、可讀性更佳——且只要動手去做，收益立竿見影。

實例：冗長的單元測試

以 ASP.NET MVC HomeController.Index 的單元測試為例，驗證回傳 ViewResult、預設視圖/母版頁名稱、Model 型別與屬性值等五項條件，原始代碼超過十幾行：

[TestMethod]
public void IndexTest()
{
    UserIdentity identity = new UserIdentity();
    Mock<HomeController> mockController = new Mock<HomeController>() { CallBase = true };
    mockController.Setup(c => c.Identity).Returns(identity);

    var result = mockController.Object.Index() as ViewResult;
    if (result == null)
        throw new Exception("result is expected to be ViewResult but not.");

    Assert.AreEqual("", result.ViewName, "the view name is expected to be the default one but '{0}'", result.ViewName);
    Assert.AreEqual("", result.MasterName, "the master name is expected to be the default one but '{0}'", result.MasterName);

    var model = result.ViewData.Model as IndexModel;
    if (model == null)
        throw new Exception("model is expected to be IndexModel but not.");

    Assert.AreEqual(identity, model.Identity);
    Assert.AreEqual("Welcome to ASP.NET MVC!", model.Message);
}

若每個測試方法都重複這段邏輯，極其乏味且難以維護。

引入輔助方法後的測試代碼

定義擴展方法 Is<T> 與 IsView 封裝斷言邏輯，測試瞬間精簡為：

[TestMethod]
public void IndexTest()
{
    UserIdentity identity = new UserIdentity();
    Mock<HomeController> mockController = new Mock<HomeController>() { CallBase = true };
    mockController.Setup(c => c.Identity).Returns(identity);

    var result = mockController.Object.Index().Is<ViewResult>().IsView(null, null);
    var model = result.ViewData.Model.Is<IndexModel>();

    Assert.AreEqual(identity, model.Identity);
    Assert.AreEqual("Welcome to ASP.NET MVC!", model.Message);
}

代碼行數大幅減少，且幾乎全靠 IntelliSense 提示完成，編寫體驗順暢。

輔助方法實作

public static class AssertHelpers
{
    public static T Is<T>(this object result)
    {
        Assert.IsTrue(
            result is T,
            "actionResult is expected to be '{0}' but '{1}'", typeof(T), result.GetType());
        return (T)result;
    }

    public static T IsView<T>(this T result, string viewName, string masterName) where T : ViewResult
    {
        viewName = viewName ?? "";
        masterName = masterName ?? "";

        Assert.IsTrue(
            String.Equals(viewName, result.ViewName, StringComparison.InvariantCultureIgnoreCase),
            "The view name is expected to be {0} but {1}",
            viewName == "" ? "the default one" : "'" + viewName + "'",
            result.ViewName == "" ? "the default one" : "'" + result.ViewName + "'");

        Assert.IsTrue(
            String.Equals(masterName, result.MasterName, StringComparison.InvariantCultureIgnoreCase),
            "The master name is expected to be {0} but {1}",
            masterName == "" ? "the default one" : "'" + masterName + "'",
            result.MasterName == "" ? "the default one" : "'" + result.MasterName + "'");

        return result;
    }
}

此處運用 C# 3.0 擴展方法 特性，無侵入性地為既有型別添加行為。