This question及其答案很好地解释了隐式捕获闭包的概念。但是,我偶尔会看到代码似乎应该生成有问题的警告,实际上并没有。例如:
public static void F()
{
var rnd1 = new Random();
var rnd2 = new Random();
Action a1 = () => G(rnd1);
Action a2 = () => G(rnd2);
}
private static void G(Random r)
{
}
我的期望是,我被警告a1
隐式捕获rnd2
,a2
隐式捕获rnd1
。但是,我根本没有得到任何警告(链接问题中的代码确实为我生成了它)。这是ReSharper的一部分(v9.2)的错误,还是由于某种原因不会在这里发生隐式捕获?
答案 0 :(得分:3)
我认为Resharper由于某种原因无法在这种情况下发现隐式捕获的变量。您可以使用一些反汇编程序验证自己,编译器使用rnd1和rnd2生成单个类。在你的例子中,它不是很清楚,但让我们从Eric Lippert博客文章(https://blogs.msdn.microsoft.com/ericlippert/2007/06/06/fyi-c-and-vb-closures-are-per-scope/)中拿这个例子来描述一个危险的隐式捕获的例子:
Func<Cheap> M() {
var c = new Cheap();
var e = new Expensive();
Func<Expensive> shortlived = () => e;
Func<Cheap> longlived = () => c;
shortlived();
// use shortlived
// use longlived
return longlived;
}
class Cheap {
}
class Expensive {
}
很明显,longlived委托捕获了Expensive变量,并且在它死亡之前不会被收集。但是(至少对我来说),Resharper不会警告你这件事。但不能把它命名为“bug”,但肯定有一个改进的地方。
答案 1 :(得分:0)
当编译器捕获闭包中匿名方法使用的局部变量时,它通过生成特定于包含委托定义的方法范围的辅助类来实现。即使在该范围内有多个委托,每个范围也存在一个这样的方法。请参阅Eric Lippert的解释{{3}}。
借用您的示例,请考虑以下程序:
using System;
namespace ConsoleApplication
{
internal class Program
{
private static void Main(string[] args)
{
F();
}
public static void F()
{
var rnd1 = new Random();
var rnd2 = new Random();
Action a1 = () => G(rnd1);
Action a2 = () => G(rnd2);
}
private static void G(Random r)
{
}
}
}
查看编译器生成的IL,我们看到以下F()
的实现:
.method public hidebysig static
void F () cil managed
{
// Method begins at RVA 0x205c
// Code size 56 (0x38)
.maxstack 2
.locals init (
[0] class ConsoleApplication.Program/'<>c__DisplayClass1_0' 'CS$<>8__locals0',
[1] class [mscorlib]System.Action a1,
[2] class [mscorlib]System.Action a2
)
IL_0000: newobj instance void ConsoleApplication.Program/'<>c__DisplayClass1_0'::.ctor()
IL_0005: stloc.0
IL_0006: nop
IL_0007: ldloc.0
IL_0008: newobj instance void [mscorlib]System.Random::.ctor()
IL_000d: stfld class [mscorlib]System.Random ConsoleApplication.Program/'<>c__DisplayClass1_0'::rnd1
IL_0012: ldloc.0
IL_0013: newobj instance void [mscorlib]System.Random::.ctor()
IL_0018: stfld class [mscorlib]System.Random ConsoleApplication.Program/'<>c__DisplayClass1_0'::rnd2
IL_001d: ldloc.0
IL_001e: ldftn instance void ConsoleApplication.Program/'<>c__DisplayClass1_0'::'<F>b__0'()
IL_0024: newobj instance void [mscorlib]System.Action::.ctor(object, native int)
IL_0029: stloc.1
IL_002a: ldloc.0
IL_002b: ldftn instance void ConsoleApplication.Program/'<>c__DisplayClass1_0'::'<F>b__1'()
IL_0031: newobj instance void [mscorlib]System.Action::.ctor(object, native int)
IL_0036: stloc.2
IL_0037: ret
} // end of method Program::F
注意第一条IL指令: IL_0000: newobj instance void ConsoleApplication.Program/'<>c__DisplayClass1_0'::.ctor()
,它调用编译器生成的辅助类的默认构造函数 - 负责捕获闭包中的局部变量的类。
以下是编译器生成的帮助程序类的IL:
.class nested private auto ansi sealed beforefieldinit '<>c__DisplayClass1_0'
extends [mscorlib]System.Object
{
.custom instance void [mscorlib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
01 00 00 00
)
// Fields
.field public class [mscorlib]System.Random rnd1
.field public class [mscorlib]System.Random rnd2
// Methods
.method public hidebysig specialname rtspecialname
instance void .ctor () cil managed
{
// Method begins at RVA 0x20a3
// Code size 8 (0x8)
.maxstack 8
IL_0000: ldarg.0
IL_0001: call instance void [mscorlib]System.Object::.ctor()
IL_0006: nop
IL_0007: ret
} // end of method '<>c__DisplayClass1_0'::.ctor
.method assembly hidebysig
instance void '<F>b__0' () cil managed
{
// Method begins at RVA 0x20ac
// Code size 13 (0xd)
.maxstack 8
IL_0000: ldarg.0
IL_0001: ldfld class [mscorlib]System.Random ConsoleApplication.Program/'<>c__DisplayClass1_0'::rnd1
IL_0006: call void ConsoleApplication.Program::G(class [mscorlib]System.Random)
IL_000b: nop
IL_000c: ret
} // end of method '<>c__DisplayClass1_0'::'<F>b__0'
.method assembly hidebysig
instance void '<F>b__1' () cil managed
{
// Method begins at RVA 0x20ba
// Code size 13 (0xd)
.maxstack 8
IL_0000: ldarg.0
IL_0001: ldfld class [mscorlib]System.Random ConsoleApplication.Program/'<>c__DisplayClass1_0'::rnd2
IL_0006: call void ConsoleApplication.Program::G(class [mscorlib]System.Random)
IL_000b: nop
IL_000c: ret
} // end of method '<>c__DisplayClass1_0'::'<F>b__1'
} // end of class <>c__DisplayClass1_0
请注意,此帮助程序类包含 rnd1
和rnd2
的字段。
IL级别的F()
的“最终”实现类似于以下内容:
public static void F()
{
var closureHelper = new ClosureHelper();
closureHelper.rnd1 = new Random();
closureHelper.rnd2 = new Random();
Action a1 = closureHelper.MethodOne;
Action a2 = closureHelper.MethodTwo;
}
实施ClosureHelper
的地方类似于:
internal class Program
{
public class ClosureHelper
{
public Random rnd1;
public Random rnd2;
void MethodOne()
{
Program.G(rnd1);
}
void MethodTwo()
{
Program.G(rnd2);
}
}
}
至于ReSharper为什么不警告你在这种情况下发生隐式捕获,我不知道。