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?
OP的问题仅限于特定的选角情况。标题涵盖了更多的情况。
以下是我目前能想到的所有相关选角情况的概述:
private class CBase
{
}
private class CInherited : CBase
{
}
private enum EnumTest
{
zero,
one,
two
}
private static void Main (string[] args)
{
//########## classes ##########
// object creation, implicit cast to object
object oBase = new CBase ();
object oInherited = new CInherited ();
CBase oBase2 = null;
CInherited oInherited2 = null;
bool bCanCast = false;
// explicit cast using "()"
oBase2 = (CBase)oBase; // works
oBase2 = (CBase)oInherited; // works
//oInherited2 = (CInherited)oBase; System.InvalidCastException
oInherited2 = (CInherited)oInherited; // works
// explicit cast using "as"
oBase2 = oBase as CBase;
oBase2 = oInherited as CBase;
oInherited2 = oBase as CInherited; // returns null, equals C++/CLI "dynamic_cast"
oInherited2 = oInherited as CInherited;
// testing with Type.IsAssignableFrom(), results (of course) equal the results of the cast operations
bCanCast = typeof (CBase).IsAssignableFrom (oBase.GetType ()); // true
bCanCast = typeof (CBase).IsAssignableFrom (oInherited.GetType ()); // true
bCanCast = typeof (CInherited).IsAssignableFrom (oBase.GetType ()); // false
bCanCast = typeof (CInherited).IsAssignableFrom (oInherited.GetType ()); // true
//########## value types ##########
int iValue = 2;
double dValue = 1.1;
EnumTest enValue = EnumTest.two;
// implicit cast, explicit cast using "()"
int iValue2 = iValue; // no cast
double dValue2 = iValue; // implicit conversion
EnumTest enValue2 = (EnumTest)iValue; // conversion by explicit cast. underlying type of EnumTest is int, but explicit cast needed (error CS0266: Cannot implicitly convert type 'int' to 'test01.Program.EnumTest')
iValue2 = (int)dValue; // conversion by explicit cast. implicit cast not possible (error CS0266: Cannot implicitly convert type 'double' to 'int')
dValue2 = dValue;
enValue2 = (EnumTest)dValue; // underlying type is int, so "1.1" beomces "1" and then "one"
iValue2 = (int)enValue;
dValue2 = (double)enValue;
enValue2 = enValue; // no cast
// explicit cast using "as"
// iValue2 = iValue as int; error CS0077: The as operator must be used with a reference type or nullable type
}
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。
