列表不规则地优化
如果我执行以下代码并且不按控制台上的任何键
class Program
{
static void Main(string[] args)
{
List<Test> list = new List<Test>(1) {new Test()};
Console.ReadKey();
GC.KeepAlive(list);
var x = list[0];
Console.WriteLine((x.ToString()));
}
}
class Test
{
public override string ToString()
{
return "Empty object";
}
}
然后在windbg中分析数组时,我看到列表中没有包含我添加的测试对象.
第0位的元素是我不确定的其他东西
但是,如果我像我这样添加一个字符串属性到我的Test类
class Test
{
public string Name = "Rohit";
public override string ToString()
{
return "Empty object";
}
}
然后这次windbg揭示了这个对象
有人可以帮忙解释一下发生了什么吗？我在x64 windows 7上使用Visual Studio 2015(.net 4兼容模式)测试了上述内容
旁白:即使我要求列表大小为一(对于我的测试),我看到它的默认大小为128.所以基本上初始容量是基于一些启发式等？
从List到对象
从评论我看到一个混乱是否List<T>将其项目存储在Object[]或T[].
我在VS 2013中为.NET 4.0编译了程序,我在WinDbg 6.2.9600中调试:
0:007> !dumpheap -stat
Statistics:
MT Count TotalSize Class Name
000007feed598130 1 24 System.Security.HostSecurityManager
000007feed597158 1 24 System.Collections.Generic.ObjectEqualityComparer`1[[System.Type, mscorlib]]
000007fe91bd40c0 1 24 ConsoleWriteLine.Test
000007feed592090 1 28 System.Char[]
000007feed5980b8 1 32 System.Security.Policy.Evidence+EvidenceLockHolder
000007feed5975e8 1 32 System.Security.Policy.AssemblyEvidenceFactory
000007feed5974a0 1 32 Microsoft.Win32.SafeHandles.SafePEFileHandle
000007feed594810 1 32 System.Text.DecoderReplacementFallback
000007feed594780 1 32 System.Text.EncoderReplacementFallback
000007feed536fd8 1 40 Microsoft.Win32.Win32Native+InputRecord
000007fe91bd4150 1 40 System.Collections.Generic.List`1[[ConsoleWriteLine.Test, ConsoleWriteLine]]
000007feed591480 1 48 System.SharedStatics
000007feed5945a8 1 56 System.Text.UnicodeEncoding
000007feed5943e0 1 56 System.Reflection.RuntimeAssembly
000007feed598038 1 64 System.Threading.ReaderWriterLock
000007feed597548 1 64 System.Security.Policy.PEFileEvidenceFactory
000007feed592610 1 64 System.Security.PermissionSet
000007feed593af0 1 72 System.RuntimeFieldInfoStub
000007feed592478 1 72 System.Security.Policy.Evidence
000007feed5913e8 3 72 System.Object
000007feeced7d10 1 80 System.Collections.Generic.Dictionary`2[[System.Type, mscorlib],[System.Security.Policy.EvidenceTypeDescriptor, mscorlib]]
000007feed591e00 1 128 System.AppDomainSetup
000007feed591310 1 160 System.ExecutionEngineException
000007feed591298 1 160 System.StackOverflowException
000007feed591220 1 160 System.OutOfMemoryException
000007feed591038 1 160 System.Exception
000007feed591540 1 216 System.AppDomain
00000000002e9e60 8 216 Free
000007feed591388 2 320 System.Threading.ThreadAbortException
000007feed593920 4 492 System.Int32[]
000007feed597fd8 3 720 System.Collections.Generic.Dictionary`2+Entry[[System.Type, mscorlib],[System.Security.Policy.EvidenceTypeDescriptor, mscorlib]][]
000007feed592eb8 21 1176 System.RuntimeType
000007feed590e08 37 2786 System.String
000007feed524918 8 34808 System.Object[]
Total 112 objects
与你的输出相比,我没有String[],没有Type[],没有Test[].相反,我有8 Object[].和你类似,我只有1个List<T>.让我们找出它使用的阵列.您的计算机上的步骤应该相同,但您可能会得到Test[]结果.
第1步:全部转储List<T>:
0:007> !dumpheap -mt 000007fe91bd4150
Address MT Size
00000000021a2de8 000007fe91bd4150 40
Statistics:
MT Count TotalSize Class Name
000007fe91bd4150 1 40 System.Collections.Generic.List`1[[ConsoleWriteLine.Test, ConsoleWriteLine]]
Total 1 objects
第2步:转储List<T>那里唯一的:
0:007> !do 00000000021a2de8
Name: System.Collections.Generic.List`1[[ConsoleWriteLine.Test, ConsoleWriteLine]]
MethodTable: 000007fe91bd4150
EEClass: 000007feecf7ea08
Size: 40(0x28) bytes
File: C:\Windows\Microsoft.Net\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089\mscorlib.dll
Fields:
MT Field Offset Type VT Attr Value Name
000007feed524918 4000cd1 8 System.Object[] 0 instance 00000000021a2e10 _items
000007feed593980 4000cd2 18 System.Int32 1 instance 1 _size
000007feed593980 4000cd3 1c System.Int32 1 instance 1 _version
000007feed5913e8 4000cd4 10 System.Object 0 instance 0000000000000000 _syncRoot
000007feed524918 4000cd5 8 System.Object[] 0 static <no information>
在这里查看_items类型的属性Object[].
第3步:转储支持Object[]:
0:007> !da 00000000021a2e10
Name: ConsoleWriteLine.Test[]
MethodTable: 000007feed524918
EEClass: 000007feecf77f58
Size: 40(0x28) bytes
Array: Rank 1, Number of elements 1, Type CLASS
Element Methodtable: 000007fe91bd40c0
[0] 00000000021a2e38
有一个有趣的发现:当转储数组时,它发现它实际上是一个Test[].可能是您的SOS版本比我的版本更智能并正确显示类型!dumpheap -stat.我的版本是4.0.30319.34209.
第4步:转储数组的唯一对象
0:007> !do 00000000021a2e38
Name: ConsoleWriteLine.Test
MethodTable: 000007fe91bd40c0
EEClass: 000007fe91ce23d8
Size: 24(0x18) bytes
File: E:\...\bin\Debug\ConsoleWriteLine.exe
Fields:
None
这是一个没有字段的测试对象,正如预期的那样.
还有什么可以帮助？
你选择了Object[]列出的第二个!dumpheap -mt 7fee816f150.我不知道你为什么选择那个,但可能是因为你检测到了那个数组的差异.然后,您将对象转储到该数组中,该数组是一个空字符串.第二次,这个列表包含一个字符串.我可以重现这一点.
要了解有关此内容的更多信息,请使用!gcroot以查看对象的使用位置.由于.NET对象类似于指针,因此两个数组可以指向同一个对象(字符串),因此可以-all用作参数.
0:007> !gcroot -all 00000000021a1420
Thread 109c:
000000000013e8c0 000007feedbca151 System.Console.ReadKey(Boolean)
rdi: (interior)
-> 00000000121a1038 System.Object[]
-> 00000000021a1420 System.String
HandleTable:
00000000008517e8 (pinned handle)
-> 00000000121a32e8 System.Object[]
-> 00000000021a1420 System.String
00000000008517f8 (pinned handle)
-> 00000000121a1038 System.Object[]
-> 00000000021a1420 System.String
Found 3 roots.
另一个有用的命令可能是!dso显示堆栈引用的对象(例如局部变量):
0:007> ~0s
0:000> !dso
OS Thread Id: 0x109c (0)
RSP/REG Object Name
000000000013E950 00000000021a2f10 System.Object
000000000013E9F0 00000000021a2e38 ConsoleWriteLine.Test
000000000013EA00 00000000021a2de8 System.Collections.Generic.List`1[[ConsoleWriteLine.Test, ConsoleWriteLine]]
000000000013EA10 00000000021a2de8 System.Collections.Generic.List`1[[ConsoleWriteLine.Test, ConsoleWriteLine]]
000000000013EA28 00000000021a2de8 System.Collections.Generic.List`1[[ConsoleWriteLine.Test, ConsoleWriteLine]]
000000000013EA30 00000000021a2de8 System.Collections.Generic.List`1[[ConsoleWriteLine.Test, ConsoleWriteLine]]
000000000013EA38 00000000021a2de8 System.Collections.Generic.List`1[[ConsoleWriteLine.Test, ConsoleWriteLine]]
000000000013EA40 00000000021a2e38 ConsoleWriteLine.Test
000000000013EA48 00000000021a2e38 ConsoleWriteLine.Test
000000000013EA80 00000000021a2dc8 System.Object[] (System.String[])
000000000013EBB8 00000000021a2dc8 System.Object[] (System.String[])
000000000013ECD8 00000000021a2dc8 System.Object[] (System.String[])
000000000013EEA8 00000000021a2dc8 System.Object[] (System.String[])
000000000013F478 00000000021a1440 System.SharedStatics