C# 字符串（String）
不可变性、常用方法、`StringBuilder`、以及比较与格式化。
Key Points`string` 不可变，每次「修改」都产生新对象；频繁拼接用 `StringBuilder`。
不可变性
C#
string s = "hello";
string t = s.ToUpper(); // 产生新字符串，s 不变
Console.WriteLine(s);
Console.WriteLine(t);
Console.WriteLine(object.ReferenceEquals(s, t));
// 每次 += 都新建一个字符串
string r = "";
for (int i = 0; i < 3; i++)
{
r += i;
}
Console.WriteLine(r);Output
hello
HELLO
False
012Concatenating with += N times creates N temporary strings — O(n²). One of the classic C# performance traps.
StringBuilder
C#
using System.Text;
var sb = new StringBuilder();
for (int i = 0; i < 5; i++)
{
sb.Append(i).Append(',');
}
Console.WriteLine(sb.ToString());
sb.Insert(0, "数字：");
sb.Replace(",", " ");
Console.WriteLine(sb.ToString().TrimEnd());
Console.WriteLine(sb.Length);Output
0,1,2,3,4,
数字：0 1 2 3 4
14StringBuilder keeps a mutable buffer; Append allocates nothing new. Rule of thumb: switch once you concatenate more than 5–10 times.
常用方法
C#
string s = " Hello, World ";
Console.WriteLine($"[{s.Trim()}]");
Console.WriteLine(s.Trim().ToUpper());
Console.WriteLine(s.Trim().ToLower());
Console.WriteLine(s.Trim().Length);
Console.WriteLine(s.Contains("World"));
Console.WriteLine(s.Trim().StartsWith("Hello"));
Console.WriteLine(s.Trim().Replace("World", "C#"));
Console.WriteLine(s.Trim().Substring(0, 5));
Console.WriteLine(string.Join("|", s.Trim().Split(", ")));Output
[Hello, World]
HELLO, WORLD
hello, world
12
True
True
Hello, C#
Hello
Hello|World| Method | Effect |
|---|---|
Trim / TrimStart / TrimEnd | 去空白 |
Split | 按分隔符拆成数组 |
Join | 把数组拼成字符串 |
Substring | 取子串 |
Replace | 替换 |
IndexOf / LastIndexOf | 查找位置，找不到返回 -1 |
PadLeft / PadRight | 补齐 |
IsNullOrEmpty / IsNullOrWhiteSpace | 判空 |
比较
C#
string a = "Hello";
string b = "hello";
Console.WriteLine(a == b); // False，区分大小写
Console.WriteLine(string.Equals(a, b, StringComparison.OrdinalIgnoreCase));
Console.WriteLine(a.CompareTo(b));
Console.WriteLine(string.Compare(a, b, StringComparison.OrdinalIgnoreCase));Output
False
True
-1
0== on string compares values (it is overloaded) while object.ReferenceEquals compares references. For sorting and lookups, passing StringComparison.Ordinal is much faster than culture-aware comparison.插值与格式化
C#
var name = "张三";
var score = 92.567;
Console.WriteLine($"姓名：{name}，分数：{score:F1}");
Console.WriteLine($"补零：{42:D5}");
Console.WriteLine($"十六进制：{255:X}");
Console.WriteLine($"百分比：{0.856:P1}");
Console.WriteLine($"金额：{1234.5m:C}");
Console.WriteLine($"对齐：|{name,-6}|{name,6}|");Output
姓名：张三，分数：92.6
补零：00042
十六进制：FF
百分比：85.6%
金额：￥1,234.50
对齐：|张三 | 张三|In {value:format} the part after the colon is the format: F1 one decimal, D5 five digits zero-padded, X hex, P1 percent, C currency. A comma plus a signed number sets width and alignment.
不可变性
C#
string s = "hello";
string t = s.ToUpper(); // 产生新字符串，s 不变
Console.WriteLine(s);
Console.WriteLine(t);
Console.WriteLine(object.ReferenceEquals(s, t));
// 每次 += 都新建一个字符串
string r = "";
for (int i = 0; i < 3; i++)
{
r += i;
}
Console.WriteLine(r);Output
hello
HELLO
False
012循环里 += 拼接 N 次会产生 N 个临时字符串，复杂度 O(n²)。这是 C# 里最经典的性能陷阱之一。
StringBuilder
C#
using System.Text;
var sb = new StringBuilder();
for (int i = 0; i < 5; i++)
{
sb.Append(i).Append(',');
}
Console.WriteLine(sb.ToString());
sb.Insert(0, "数字：");
sb.Replace(",", " ");
Console.WriteLine(sb.ToString().TrimEnd());
Console.WriteLine(sb.Length);Output
0,1,2,3,4,
数字：0 1 2 3 4
14StringBuilder 内部维护一个可变缓冲区，Append 不分配新对象。经验值：拼接次数超过 5~10 次就换 StringBuilder。
常用方法
C#
string s = " Hello, World ";
Console.WriteLine($"[{s.Trim()}]");
Console.WriteLine(s.Trim().ToUpper());
Console.WriteLine(s.Trim().ToLower());
Console.WriteLine(s.Trim().Length);
Console.WriteLine(s.Contains("World"));
Console.WriteLine(s.Trim().StartsWith("Hello"));
Console.WriteLine(s.Trim().Replace("World", "C#"));
Console.WriteLine(s.Trim().Substring(0, 5));
Console.WriteLine(string.Join("|", s.Trim().Split(", ")));Output
[Hello, World]
HELLO, WORLD
hello, world
12
True
True
Hello, C#
Hello
Hello|World| 方法 | 作用 |
|---|---|
Trim / TrimStart / TrimEnd | 去空白 |
Split | 按分隔符拆成数组 |
Join | 把数组拼成字符串 |
Substring | 取子串 |
Replace | 替换 |
IndexOf / LastIndexOf | 查找位置，找不到返回 -1 |
PadLeft / PadRight | 补齐 |
IsNullOrEmpty / IsNullOrWhiteSpace | 判空 |
比较
C#
string a = "Hello";
string b = "hello";
Console.WriteLine(a == b); // False，区分大小写
Console.WriteLine(string.Equals(a, b, StringComparison.OrdinalIgnoreCase));
Console.WriteLine(a.CompareTo(b));
Console.WriteLine(string.Compare(a, b, StringComparison.OrdinalIgnoreCase));Output
False
True
-1
0== 对 string 是值比较（因为 string 重载了 ==），但 object.ReferenceEquals 是引用比较。排序和查找时显式传 StringComparison.Ordinal 比默认的区域敏感比较快得多。插值与格式化
C#
var name = "张三";
var score = 92.567;
Console.WriteLine($"姓名：{name}，分数：{score:F1}");
Console.WriteLine($"补零：{42:D5}");
Console.WriteLine($"十六进制：{255:X}");
Console.WriteLine($"百分比：{0.856:P1}");
Console.WriteLine($"金额：{1234.5m:C}");
Console.WriteLine($"对齐：|{name,-6}|{name,6}|");Output
姓名：张三，分数：92.6
补零：00042
十六进制：FF
百分比：85.6%
金额：￥1,234.50
对齐：|张三 | 张三|{值:格式} 里冒号后是格式串：F1 一位小数、D5 五位补零、X 十六进制、P1 百分比一位、C 货币。逗号后的正负数是宽度和对齐方向。