【Emit基础】在IL中进行异常处理

本文通过一个简单的示例来说明在IL中进行异常处理时要注意的关键点。

我们来看一个包含try...catch...finally的示例：

<!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--> public void TestEF()
{
TransactionScopeFactoryfactory = new TransactionScopeFactory( null );
TransactionScopescope = factory.NewTransactionScope( false );

try
{
scope.Commit();
}
catch (Exceptionee)
{
string msg = ee.Message;
}
finally
{
scope.Dispose();
}
}

这段代码实际上与使用using是等价的：

<!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--> public void TestEF()
{
TransactionScopeFactoryfactory = new TransactionScopeFactory( null );
using (TransactionScopescope = factory.NewTransactionScope( false ))
{
try
{
scope.Commit();
}
catch (Exceptionee)
{
string msg = ee.Message;
}
}
}

它们对应的IL代码如下所示：

<!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--> .method public hidebysiginstance void TestEF()cilmanaged
{
.maxstack 2
.localsinit(
[ 0 ] class [DataRabbit.Application]DataRabbit.Application.TransactionScopeFactoryfactory,
[ 1 ] class [DataRabbit.Application]DataRabbit.Application.TransactionScopescope,
[ 2 ] class [mscorlib]System.Exceptionee,
[ 3 ] string msg)
L_0000:nop
L_0001:ldnull
L_0002:newobjinstance void [DataRabbit.Application]DataRabbit.Application.TransactionScopeFactory::.ctor( class [DataRabbit]DataRabbit.DataConfiguration)
L_0007:stloc. 0
L_0008:ldloc. 0
L_0009:ldc.i4. 0
L_000a:callvirtinstance class [DataRabbit.Application]DataRabbit.Application.TransactionScope[DataRabbit.Application]DataRabbit.Application.TransactionScopeFactory::NewTransactionScope( bool )
L_000f:stloc. 1
L_0010:nop
L_0011:ldloc. 1
L_0012:callvirtinstance void [DataRabbit.Application]DataRabbit.Application.TransactionScope::Commit()
L_0017:nop
L_0018:nop
L_0019:leave.sL_0027
L_001b:stloc. 2
L_001c:nop
L_001d:ldloc. 2
L_001e:callvirtinstance string [mscorlib]System.Exception::get_Message()
L_0023:stloc. 3
L_0024:nop
L_0025:leave.sL_0027
L_0027:nop
L_0028:leave.sL_0034
L_002a:nop
L_002b:ldloc. 1
L_002c:callvirtinstance void [DataRabbit.Application]DataRabbit.Application.TransactionScope::Dispose()
L_0031:nop
L_0032:nop
L_0033:endfinally
L_0034:nop
L_0035:ret
. tryL_0010toL_001bcatch [mscorlib]System.ExceptionhandlerL_001btoL_0027
. tryL_0010toL_002afinally handlerL_002atoL_0034
}

我们来剖析这段IL中的异常处理流程：

1.有最后的两句代码，我们看到：

（1）try...catch...finally 是由try...catch 和 try...finally两部分构成。

（2）try...finally 中的try块内含了catch块。

即类似这样：

<!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--> .try
{
. try
{
}
catch
{
}
}
finally
{
}

2.try块、catch块（catch handler）只能通过leave（或leave.s）退出。

3.finally块（finally handler）必须通过endfinally退出。

4.由于try...catch 和 try...finally两部分都需要退出try块，所以我们看到在L_0019 和 L_0028 处都有对应的leave.s指令。

5.程序中如果没有finally块，则IL中只需要处理try...catch 部分；同理，如果程序中没有catch块，则IL只需要处理try...finally部分。

6.总结起来，IL的异常处理类似这个样子：

<!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />-->.try
{
.try
{

leave L1
}
catch
{

leave L1
}

leaveL2

}
finally
{

endfinally
}