用于在兼容的引用类型之间执行转换。例如: 复制代码 string s = someObject as string; if (s != null) { // someObject is a string. } 备注 as 运算符类似于强制转换操作;但是,如果转换不可行,as 会返回 null 而不是引发异常。更严格地说,这种形式的表达式expression as type等效于 expression is type ? (type)expression : (type)null只是 expression 只被计算一次。注意,as 运算符只执行引用转换和装箱转换。as 运算符无法执行其他转换,如用户定义的转换,这类转换应使用 cast 表达式来执行。// cs_keyword_as.cs // The as operator. using System; class Class1 { }class Class2 { }class MainClass { static void Main() { object[] objArray = new object[6]; objArray[0] = new Class1(); objArray[1] = new Class2(); objArray[2] = "hello"; objArray[3] = 123; objArray[4] = 123.4; objArray[5] = null; for (int i = 0; i < objArray.Length; ++i) { string s = objArray[i] as string; Console.Write("{0}:", i); if (s != null) { Console.WriteLine("'" + s + "'"); } else { Console.WriteLine("not a string"); } } } }
输出 0:not a string 1:not a string 2:'hello' 3:not a string 4:not a string 5:not a string
(BackgroundWorker)sender这是强制转换,如果sender为null或类型不能转为BackgroundWorker就会异常。 sender as BackgroundWorker,如果sender能转成BackgroundWorker则转,不能转或sender为null就返回null。
as 运算符类似于强制转换,所不同的是,当转换失败时,运算符将产生空,而不是引发异常。 as等同于: expression is type ? (type)expression : (type)null as 运算符只执行引用转换和装箱转换。 as 运算符无法执行其他转换,如用户定义的转换,这类转换应使用 cast 表达式来执行这是MSDN的解释,现在你该明白为什么了吧
sender as BackgroundWorker比较好,(BackgroundWorker)sender有可能抛出InvalidCast异常。BackgroundWorker worker = sender as BackgroundWorker; 等价于以下代码,但效率更高: BackgroundWorker worker = null; if(sender is BackgroundWorker) { worker = (BackgroundWorker)sender; }
string s = someObject as string;
if (s != null)
{
// someObject is a string.
} 备注
as 运算符类似于强制转换操作;但是,如果转换不可行,as 会返回 null 而不是引发异常。更严格地说,这种形式的表达式expression as type等效于 expression is type ? (type)expression : (type)null只是 expression 只被计算一次。注意,as 运算符只执行引用转换和装箱转换。as 运算符无法执行其他转换,如用户定义的转换,这类转换应使用 cast 表达式来执行。// cs_keyword_as.cs
// The as operator.
using System;
class Class1
{
}class Class2
{
}class MainClass
{
static void Main()
{
object[] objArray = new object[6];
objArray[0] = new Class1();
objArray[1] = new Class2();
objArray[2] = "hello";
objArray[3] = 123;
objArray[4] = 123.4;
objArray[5] = null; for (int i = 0; i < objArray.Length; ++i)
{
string s = objArray[i] as string;
Console.Write("{0}:", i);
if (s != null)
{
Console.WriteLine("'" + s + "'");
}
else
{
Console.WriteLine("not a string");
}
}
}
}
0:not a string
1:not a string
2:'hello'
3:not a string
4:not a string
5:not a string
sender as BackgroundWorker,如果sender能转成BackgroundWorker则转,不能转或sender为null就返回null。
as等同于:
expression is type ? (type)expression : (type)null
as 运算符只执行引用转换和装箱转换。
as 运算符无法执行其他转换,如用户定义的转换,这类转换应使用 cast 表达式来执行这是MSDN的解释,现在你该明白为什么了吧
等价于以下代码,但效率更高:
BackgroundWorker worker = null;
if(sender is BackgroundWorker)
{
worker = (BackgroundWorker)sender;
}