C# 循环
`for`、`while`、`do-while`、`foreach`、以及跳转语句。
Key Points遍历集合用 `foreach`，需要下标或改元素用 `for`。
四种循环
C#
// for
for (int i = 0; i < 3; i++)
{
Console.Write(i + " ");
}
Console.WriteLine();
// while
int n = 3;
while (n > 0)
{
Console.Write(n-- + " ");
}
Console.WriteLine();
// do-while：至少执行一次
int k = 0;
do
{
Console.Write("执行 ");
} while (k++ < 1);
Console.WriteLine();
// foreach
foreach (var c in "abc")
{
Console.Write(c + " ");
}
Console.WriteLine();Output
0 1 2
3 2 1
执行 执行
a b c foreach 的细节
C#
var list = new List<int> { 10, 20, 30 };
foreach (var item in list)
{
Console.Write(item + " ");
}
Console.WriteLine();
// 遍历字典
var map = new Dictionary<string, int> { ["a"] = 1, ["b"] = 2 };
foreach (var (key, value) in map)
{
Console.Write($"{key}={value} ");
}
Console.WriteLine();
// 遍历时不能修改集合
try
{
foreach (var item in list)
{
list.Add(99);
}
}
catch (InvalidOperationException)
{
Console.WriteLine("遍历中修改集合会抛异常");
}Output
10 20 30
a=1 b=2
遍历中修改集合会抛异常Modifying a collection during
foreach throws InvalidOperationException. To delete while iterating, loop backwards with for, or copy via ToList() first.break 与 continue
C#
for (int i = 1; i <= 6; i++)
{
if (i % 2 == 0) continue; // 跳过偶数
if (i > 5) break; // 大于 5 就退出
Console.Write(i + " ");
}
Console.WriteLine();
// goto：跳出多层循环
for (int i = 0; i < 3; i++)
{
for (int j = 0; j < 3; j++)
{
if (i * j == 2)
{
Console.WriteLine($"找到 {i} {j}");
goto done;
}
}
}
done:
Console.WriteLine("结束");Output
1 3 5
找到 1 2
结束C# has no labelled break. To escape nested loops use goto, or extract the loop into a method and return — the latter usually reads better.
常见模式
C#
// 倒序
for (int i = 5; i > 0; i--) Console.Write(i + " ");
Console.WriteLine();
// 步长
for (int i = 0; i <= 10; i += 5) Console.Write(i + " ");
Console.WriteLine();
// 无限循环 + 条件退出
int count = 0;
while (true)
{
if (++count >= 3) break;
}
Console.WriteLine(count);
// LINQ 替代循环
var evens = Enumerable.Range(1, 10).Where(x => x % 2 == 0).ToList();
Console.WriteLine(string.Join(",", evens));Output
5 4 3 2 1
0 5 10
3
2,4,6,8,10For filter-map style work LINQ is shorter and clearer than a loop. On hot paths (millions per second) hand-written loops still win — LINQ pays delegate and iterator overhead.
四种循环
C#
// for
for (int i = 0; i < 3; i++)
{
Console.Write(i + " ");
}
Console.WriteLine();
// while
int n = 3;
while (n > 0)
{
Console.Write(n-- + " ");
}
Console.WriteLine();
// do-while：至少执行一次
int k = 0;
do
{
Console.Write("执行 ");
} while (k++ < 1);
Console.WriteLine();
// foreach
foreach (var c in "abc")
{
Console.Write(c + " ");
}
Console.WriteLine();Output
0 1 2
3 2 1
执行 执行
a b c foreach 的细节
C#
var list = new List<int> { 10, 20, 30 };
foreach (var item in list)
{
Console.Write(item + " ");
}
Console.WriteLine();
// 遍历字典
var map = new Dictionary<string, int> { ["a"] = 1, ["b"] = 2 };
foreach (var (key, value) in map)
{
Console.Write($"{key}={value} ");
}
Console.WriteLine();
// 遍历时不能修改集合
try
{
foreach (var item in list)
{
list.Add(99);
}
}
catch (InvalidOperationException)
{
Console.WriteLine("遍历中修改集合会抛异常");
}Output
10 20 30
a=1 b=2
遍历中修改集合会抛异常foreach 期间修改集合会抛 InvalidOperationException: Collection was modified。要边遍历边删，用 for 倒序循环，或先 ToList() 复制一份。break 与 continue
C#
for (int i = 1; i <= 6; i++)
{
if (i % 2 == 0) continue; // 跳过偶数
if (i > 5) break; // 大于 5 就退出
Console.Write(i + " ");
}
Console.WriteLine();
// goto：跳出多层循环
for (int i = 0; i < 3; i++)
{
for (int j = 0; j < 3; j++)
{
if (i * j == 2)
{
Console.WriteLine($"找到 {i} {j}");
goto done;
}
}
}
done:
Console.WriteLine("结束");Output
1 3 5
找到 1 2
结束C# 没有带标签的 break，跳出嵌套循环要么用 goto，要么把循环抽成方法用 return。后者通常更清晰。
常见模式
C#
// 倒序
for (int i = 5; i > 0; i--) Console.Write(i + " ");
Console.WriteLine();
// 步长
for (int i = 0; i <= 10; i += 5) Console.Write(i + " ");
Console.WriteLine();
// 无限循环 + 条件退出
int count = 0;
while (true)
{
if (++count >= 3) break;
}
Console.WriteLine(count);
// LINQ 替代循环
var evens = Enumerable.Range(1, 10).Where(x => x % 2 == 0).ToList();
Console.WriteLine(string.Join(",", evens));Output
5 4 3 2 1
0 5 10
3
2,4,6,8,10「遍历 + 过滤 + 转换」这类逻辑，LINQ 往往比手写循环更短更清楚。但热路径上（每秒百万次）手写循环仍更快——LINQ 有委托调用和迭代器开销。