C# 常用函式與方法速查手冊：DateTime、字串、編碼、集合、LINQ

DateTime 與時間處理

取得當下時間

DateTime now = DateTime.Now;           // 本地時間
DateTime utc = DateTime.UtcNow;        // UTC 時間 (推薦儲存/傳輸用)
DateTime today = DateTime.Today;       // 今日 00:00:00

元件存取

int year = now.Year;
int month = now.Month;
int day = now.Day;
int hour = now.Hour;
int minute = now.Minute;
int second = now.Second;
int ms = now.Millisecond;
DayOfWeek dow = now.DayOfWeek;         // 週幾
int doy = now.DayOfYear;               // 第幾天

格式化輸出 (ToString 格式字串)

日期運算

DateTime future = now.AddDays(30);
DateTime past = now.AddMonths(-3);
DateTime specific = new DateTime(2024, 12, 25);
TimeSpan diff = future - now;          // 差值
double totalHours = diff.TotalHours;
int days = diff.Days;

// 週期運算
DateTime firstDayOfMonth = new DateTime(now.Year, now.Month, 1);
DateTime lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddDays(-1);

解析字串

DateTime parsed = DateTime.Parse("2024-03-15");           // 依目前文化特性
DateTime parsed2 = DateTime.ParseExact("15/03/2024", "dd/MM/yyyy", CultureInfo.InvariantCulture);
bool success = DateTime.TryParse(input, out DateTime result); // 安全解析

字串操作核心大全

建立與連接

// 字串插值 (C# 6+, 效能佳、可讀)
string s = $"User: {name}, Age: {age}, Time: {DateTime.Now:HH:mm}";

// StringBuilder (大量連接/迴圈中)
var sb = new StringBuilder();
for (int i = 0; i < 100; i++) sb.AppendLine($"Line {i}");
string result = sb.ToString();

// string.Join (陣列/集合連接)
string csv = string.Join(",", new[] { "A", "B", "C" }); // "A,B,C"

查找與比較

string text = "Hello World";

// 索引查找
int idx = text.IndexOf("World");     // 6 (找不到回傳 -1)
int last = text.LastIndexOf("l");    // 9
bool starts = text.StartsWith("He"); // true
bool ends = text.EndsWith("ld");     // true
bool contains = text.Contains("lo"); // true

// 比較 (忽略大小寫)
bool eq = text.Equals("hello world", StringComparison.OrdinalIgnoreCase);
bool eq2 = string.Equals(text, "HELLO WORLD", StringComparison.OrdinalIgnoreCase);

// 排序比較
int cmp = string.Compare("A", "B", StringComparison.Ordinal); // -1

擷取與移除

string s = "Hello World";

string sub = s.Substring(6);       // "World" (從索引 6 到結尾)
string sub2 = s.Substring(6, 3);   // "Wor" (長度 3)

// 移除
string removed = s.Remove(5);      // "Hello" (索引 5 之後刪除)
string removed2 = s.Remove(5, 6);  // "Hello" (索引 5 刪除 6 字元)

// 取代
string replaced = s.Replace("World", "C#"); // "Hello C#"
string multi = s.Replace("l", "L");         // "HeLLo WorLd" (全部取代)

分割與修剪

string csv = "A,B,C, D ,E";
string[] parts = csv.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
// 結果: ["A", "B", "C", "D", "E"]

string padded = "  Hello  ";
string trimmed = padded.Trim();       // "Hello"
string trimStart = padded.TrimStart(); // "Hello  "
string trimEnd = padded.TrimEnd();     // "  Hello"

格式化數值/日期

int num = 12345;
num.ToString("N");    // "12,345.00" (千分位)
num.ToString("C");    // "NT$12,345.00" (貨幣)
num.ToString("X");    // "3039" (十六進位)
num.ToString("D8");   // "00012345" (補零)

double d = 0.1234;
d.ToString("P");      // "12.34%" (百分比)
d.ToString("E");      // "1.234000E-001" (科學記號)

編碼轉換

string text = "中文";
byte[] utf8 = Encoding.UTF8.GetBytes(text);
byte[] unicode = Encoding.Unicode.GetBytes(text);     // UTF-16 LE
byte[] bigEndian = Encoding.BigEndianUnicode.GetBytes(text);

string back = Encoding.UTF8.GetString(utf8); // 還原

// Base64
string b64 = Convert.ToBase64String(utf8);
byte[] decoded = Convert.FromBase64String(b64);

集合與 LINQ 必備技巧

List<T> 常用操作

var list = new List { 3, 1, 4, 1, 5 };

list.Add(9);
list.AddRange(new[] { 2, 6 });
list.Insert(0, 0);           // 索引 0 插入
list.Remove(1);              // 移除第一個 1
list.RemoveAt(0);            // 移除索引 0
list.RemoveAll(x => x > 5);  // 移除所有 > 5
list.Sort();                 // 就地排序
list.Reverse();              // 反轉
bool has = list.Contains(3);
int idx = list.IndexOf(4);
int count = list.Count;
int[] arr = list.ToArray();

Dictionary<TKey, TValue>

var dict = new Dictionary
{
    ["Apple"] = 1,
    ["Banana"] = 2
};

dict["Cherry"] = 3;          // 新增/更新
bool exists = dict.TryGetValue("Apple", out int val); // 安全取值
dict.Remove("Banana");
foreach (var kvp in dict) { /* kvp.Key, kvp.Value */ }
var keys = dict.Keys;
var values = dict.Values;

LINQ 核心查詢語法 (方法語法)

var numbers = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

// 篩選
var evens = numbers.Where(n => n % 2 == 0);           // 2,4,6,8,10

// 投影
var squares = numbers.Select(n => n * n);             // 1,4,9,16,25

// 排序
var desc = numbers.OrderByDescending(n => n);         // 10..1

// 分組
var grouped = numbers.GroupBy(n => n % 2);            // 兩群：奇/偶

// 聚合
int sum = numbers.Sum();
double avg = numbers.Average();
int max = numbers.Max();
int min = numbers.Min();

// 取前幾筆 / 跳過
var top3 = numbers.Take(3);                           // 1,2,3
var skip2 = numbers.Skip(2);                          // 3..10

// 判斷
bool any = numbers.Any(n => n > 5);                   // true
bool all = numbers.All(n => n > 0);                   // true

// 轉換
List list = numbers.ToList();
Dictionary dict = numbers.ToDictionary(n => n, n => n * 10);
HashSet set = numbers.ToHashSet();

// 連接 (Join)
var users = new[] { new { Id = 1, Name = "A" }, new { Id = 2, Name = "B" } };
var orders = new[] { new { UserId = 1, Amount = 100 }, new { UserId = 1, Amount = 200 } };
var q = users.Join(orders, u => u.Id, o => o.UserId, (u, o) => new { u.Name, o.Amount });

LINQ 查詢語法 (SQL 風格)

var result = from n in numbers
             where n % 2 == 0
             orderby n descending
             select n * n;
// 編譯器轉換為方法語法，效能相同

常用類別靜態方法速查

結語

熟練掌握上述 API，能解決 90% 以上的日常開發需求。建議：

1. 優先使用 LINQ 表達資料轉換邏輯，減少手寫迴圈
2. 字串大量操作用 StringBuilder，單行插值用 $""
3. 日期儲存用 UTC，顯示時轉本地時區
4. 編碼統一用 UTF-8，避免 BigEndianUnicode 除非協定要求
5. 善用 Try 系列方法 (TryParse, TryGetValue) 避免例外控制流程