.net 基础知识大杂烩(3) ——循环语句

循环语句

C#	VB	输出
int i = 0; while (i <= 2) {     Console.WriteLine(i);     i++; }; // 这个分号可有可无	Dim i As Integer = 0 While i <= 2     Console.WriteLine(i)     i += 1 End While	0 1 2
int i = 0; do {     Console.WriteLine(i);     i++; } while (i <= 2); // 这个分号必须写	Dim i As Integer = 0 Do     Console.WriteLine(i)     i += 1 Loop While i <= 2	0 1 2
/	Dim i As Integer = 0 Do     Console.WriteLine(i)     i += 1 Loop Until i >= 3	0 1 2
/	Dim i As Integer = 0 Do While i <= 2     Console.WriteLine(i)     i += 1 Loop	0 1 2
/	Dim i As Integer = 0 Do Until i >= 3     Console.WriteLine(i)     i += 1 Loop	0 1 2
/	Do     Console.WriteLine("Hello") Loop	Hello Hello Hello ……无限循环
for (; ; ) {     Console.WriteLine("Hello"); }	/	Hello Hello Hello ……无限循环
for (int i = 2; i >= 0; i--) {     Console.WriteLine(i); }; // 这个分号可有可无	For i As Integer = 2 To 0 Step -1     Console.WriteLine(i) Next	2 1 0
int i = 0; do {     for (int j = 1; j <= 10; j++)     {         Console.WriteLine(i.ToString() + j.ToString());         if (j >= 2)         {             break;         }         if (i >= 3)         {             goto enddo;         }     }     i++; } while (true); enddo: ;	Dim i As Integer = 0 Do     For j As Integer = 1 To 10         Console.WriteLine(i & j)         If j >= 2 Then             Exit For         End If         If i >= 3 Then             Exit Do         End If     Next     i += 1 Loop	01 02 11 12 21 22 31
for (int i = 1; i <= 10; i++) {     if (i % 2 == 0)     {         continue;     }     Console.WriteLine(i); }	For i As Integer = 1 To 10     If (i Mod 2 = 0) Then         Continue For     End If     Console.WriteLine(i) Next	1 3 5 7 9
int[] a = new int[] { 2, 4, 6 }; foreach (int i in a) {     Console.WriteLine(i); }	Dim a() As Integer = New Integer() {2, 4, 6} For Each i As Integer In a     Console.WriteLine(i) Next	2 4 6

foreach 语句的本质

foreach是一个语法糖。

IList<int> a = new List<int>();
foreach (int i in a)
{
    Console.WriteLine(i);
}

会被编译器转换成

IList<int> a = new List<int>();
IEnumerator<int> e = a.GetEnumerator();
try
{
    while (e.MoveNext())
    {
        Console.WriteLine(e.Current);
    }
}
finally
{
    if (e != null)
        e.Dispose();
}