在编程接口时，我发现我做了很多强制转换或对象类型转换。
这两种转换方法有什么区别吗?如果是的话，是否有成本差异，或者这对我的程序有什么影响?
public interface IMyInterface
{
void AMethod();
}
public class MyClass : IMyInterface
{
public void AMethod()
{
//Do work
}
// Other helper methods....
}
public class Implementation
{
IMyInterface _MyObj;
MyClass _myCls1;
MyClass _myCls2;
public Implementation()
{
_MyObj = new MyClass();
// What is the difference here:
_myCls1 = (MyClass)_MyObj;
_myCls2 = (_MyObj as MyClass);
}
}
另外，“一般来说”首选的方法是什么?
我的答案只是在我们不检查类型和转换后不检查null的情况下的速度。我在Jon Skeet的代码中添加了两个额外的测试:
using System;
using System.Diagnostics;
class Test
{
const int Size = 30000000;
static void Main()
{
object[] values = new object[Size];
for (int i = 0; i < Size; i++)
{
values[i] = "x";
}
FindLengthWithIsAndCast(values);
FindLengthWithIsAndAs(values);
FindLengthWithAsAndNullCheck(values);
FindLengthWithCast(values);
FindLengthWithAs(values);
Console.ReadLine();
}
static void FindLengthWithIsAndCast(object[] values)
{
Stopwatch sw = Stopwatch.StartNew();
int len = 0;
foreach (object o in values)
{
if (o is string)
{
string a = (string)o;
len += a.Length;
}
}
sw.Stop();
Console.WriteLine("Is and Cast: {0} : {1}", len,
(long)sw.ElapsedMilliseconds);
}
static void FindLengthWithIsAndAs(object[] values)
{
Stopwatch sw = Stopwatch.StartNew();
int len = 0;
foreach (object o in values)
{
if (o is string)
{
string a = o as string;
len += a.Length;
}
}
sw.Stop();
Console.WriteLine("Is and As: {0} : {1}", len,
(long)sw.ElapsedMilliseconds);
}
static void FindLengthWithAsAndNullCheck(object[] values)
{
Stopwatch sw = Stopwatch.StartNew();
int len = 0;
foreach (object o in values)
{
string a = o as string;
if (a != null)
{
len += a.Length;
}
}
sw.Stop();
Console.WriteLine("As and null check: {0} : {1}", len,
(long)sw.ElapsedMilliseconds);
}
static void FindLengthWithCast(object[] values)
{
Stopwatch sw = Stopwatch.StartNew();
int len = 0;
foreach (object o in values)
{
string a = (string)o;
len += a.Length;
}
sw.Stop();
Console.WriteLine("Cast: {0} : {1}", len,
(long)sw.ElapsedMilliseconds);
}
static void FindLengthWithAs(object[] values)
{
Stopwatch sw = Stopwatch.StartNew();
int len = 0;
foreach (object o in values)
{
string a = o as string;
len += a.Length;
}
sw.Stop();
Console.WriteLine("As: {0} : {1}", len,
(long)sw.ElapsedMilliseconds);
}
}
结果:
Is and Cast: 30000000 : 88
Is and As: 30000000 : 93
As and null check: 30000000 : 56
Cast: 30000000 : 66
As: 30000000 : 46
不要像我一样专注于速度，因为所有这些都是非常非常快的。
除了这里已经暴露的所有内容，我刚刚发现了一个我认为值得注意的实际差异，在显式选角之间
var x = (T) ...
相对于使用as操作符。
下面是例子:
class Program
{
static void Main(string[] args)
{
Console.WriteLine(GenericCaster<string>(12345));
Console.WriteLine(GenericCaster<object>(new { a = 100, b = "string" }) ?? "null");
Console.WriteLine(GenericCaster<double>(20.4));
//prints:
//12345
//null
//20.4
Console.WriteLine(GenericCaster2<string>(12345));
Console.WriteLine(GenericCaster2<object>(new { a = 100, b = "string" }) ?? "null");
//will not compile -> 20.4 does not comply due to the type constraint "T : class"
//Console.WriteLine(GenericCaster2<double>(20.4));
}
static T GenericCaster<T>(object value, T defaultValue = default(T))
{
T castedValue;
try
{
castedValue = (T) Convert.ChangeType(value, typeof(T));
}
catch (Exception)
{
castedValue = defaultValue;
}
return castedValue;
}
static T GenericCaster2<T>(object value, T defaultValue = default(T)) where T : class
{
T castedValue;
try
{
castedValue = Convert.ChangeType(value, typeof(T)) as T;
}
catch (Exception)
{
castedValue = defaultValue;
}
return castedValue;
}
}
底线:GenericCaster2不能用于结构类型。GenericCaster意志。
这是另一个答案，带有一些IL比较。考虑这个类:
public class MyClass
{
public static void Main()
{
// Call the 2 methods
}
public void DirectCast(Object obj)
{
if ( obj is MyClass)
{
MyClass myclass = (MyClass) obj;
Console.WriteLine(obj);
}
}
public void UsesAs(object obj)
{
MyClass myclass = obj as MyClass;
if (myclass != null)
{
Console.WriteLine(obj);
}
}
}
现在看看每种方法产生的IL。即使操作码对您没有任何意义，您也可以看到一个主要的区别-在DirectCast方法中，isinst被调用后是castclass。所以基本上是两次调用而不是一次。
.method public hidebysig instance void DirectCast(object obj) cil managed
{
// Code size 22 (0x16)
.maxstack 8
IL_0000: ldarg.1
IL_0001: isinst MyClass
IL_0006: brfalse.s IL_0015
IL_0008: ldarg.1
IL_0009: castclass MyClass
IL_000e: pop
IL_000f: ldarg.1
IL_0010: call void [mscorlib]System.Console::WriteLine(object)
IL_0015: ret
} // end of method MyClass::DirectCast
.method public hidebysig instance void UsesAs(object obj) cil managed
{
// Code size 17 (0x11)
.maxstack 1
.locals init (class MyClass V_0)
IL_0000: ldarg.1
IL_0001: isinst MyClass
IL_0006: stloc.0
IL_0007: ldloc.0
IL_0008: brfalse.s IL_0010
IL_000a: ldarg.1
IL_000b: call void [mscorlib]System.Console::WriteLine(object)
IL_0010: ret
} // end of method MyClass::UsesAs
isinst关键字与castclass的对比
这篇博文对这两种方法进行了比较。他的总结是:
在直接比较中，isinst比castclass快(尽管只有一点点)
当必须执行检查以确保转换成功时，isinst明显比castclass快
不应该使用isinst和castclass的组合，因为这比最快的“安全”转换慢得多(慢12%以上)
我个人总是使用a，因为它易于阅读，并且是。net开发团队(或者Jeffrey Richter)推荐的。
