Comprehensive guide to advanced AutoFixture customization techniques. Use when you need to customize AutoFixture builders or define test-data generation rules for special types. Covers automatic integration with DataAnnotations, implementing ISpecimenBuilder, and priority/ordering management. Includes DateTime and numeric-range builders, generic design patterns, and fluent extension methods. Keywords: autofixture customization, autofixture customize, autofixture custom, specimen builder, ISpecimenBuilder, RandomDateTimeSequenceGenerator, NumericRangeBuilder, DataAnnotations autofixture, fixture.Customizations, Insert(0), custom builder, NoSpecimen, generic builder
本技能涵蓋 AutoFixture 的進階自訂化功能，讓您能根據業務需求精確控制測試資料的生成邏輯。
[StringLength]、[Range] 等驗證屬性.With() 配合 Random.Shared 動態產生隨機值Insert(0) vs Add() 的差異<PackageReference Include="AutoFixture" Version="4.18.1" />
<PackageReference Include="AutoFixture.Xunit2" Version="4.18.1" />
AutoFixture 能自動識別 System.ComponentModel.DataAnnotations 的驗證屬性：
public class Person
{
public Guid Id { get; set; }
[StringLength(10)]
public string Name { get; set; } = string.Empty;
[Range(10, 80)]
public int Age { get; set; }
public DateTime CreateTime { get; set; }
}
[Fact]
public void AutoFixture_應能識別DataAnnotations()
{
var fixture = new Fixture();
var person = fixture.Create<Person>();
person.Name.Length.Should().Be(10); // StringLength(10)
person.Age.Should().BeInRange(10, 80); // Range(10, 80)
}
// ❌ 固定值：只執行一次，所有物件相同值
.With(x => x.Age, Random.Shared.Next(30, 50))
// ✅ 動態值：每個物件都重新計算
.With(x => x.Age, () => Random.Shared.Next(30, 50))
| 特性 | new Random() | Random.Shared |
| ---------- | -------------------------- | -------------------- |
| 實例化方式 | 每次建立新實例 | 全域共用單一實例 |
| 執行緒安全 | 不是 | 是 |
| 效能 | 多次建立有負擔，可能重複值 | 效能更佳，避免重複值 |
涵蓋 RandomRangedDateTimeBuilder（精確控制特定 DateTime 屬性）、ImprovedRandomRangedNumericSequenceBuilder（改進版數值範圍建構器）、泛型化 NumericRangeBuilder<TValue>（支援 int/long/decimal/double/float 等多種型別），以及流暢介面擴充方法 AddRandomRange。每個建構器皆附完整實作與使用範例。
完整 ISpecimenBuilder 實作範例請參考 references/specimen-builder-examples.md
AutoFixture 內建的 RangeAttributeRelay、NumericSequenceGenerator 可能比自訂建構器有更高優先順序：
// ❌ 可能失效：被內建建構器攔截
fixture.Customizations.Add(new MyNumericBuilder(30, 50, "Age"));
// ✅ 正確：確保最高優先順序
fixture.Customizations.Insert(0, new MyNumericBuilder(30, 50, "Age"));
| 型別 | 內建建構器 | 優先順序影響 |
| ---------- | ------------------------------------------------- | -------------------------- |
| int | RangeAttributeRelay、NumericSequenceGenerator | 會被攔截，需用 Insert(0) |
| DateTime | 無特定建構器 | 不會被攔截，Add() 即可 |
Insert(0)Add() 一定生效請參考 templates 資料夾中的範例檔案：
本技能內容提煉自「老派軟體工程師的測試修練 - 30 天挑戰」系列文章：
npx skills add kevintsengtw/dotnet-testing-autofixture-自定义下载完整 Skill 目录，包含 SKILL.md 及所有相关文件
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