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：
public void TestEF()
{
TransactionScopeFactory factory = new TransactionScopeFactory(null);
TransactionScope scope = factory.NewTransactionScope(false);
try
{
scope.Commit();
}
catch (Exception ee)
{
string msg = ee.Message;
}
finally
{
scope.Dispose();
}
}
这段代码实际上与使用using是等价的：
public void TestEF()
{
TransactionScopeFactory factory = new TransactionScopeFactory(null);
using (TransactionScope scope = factory.NewTransactionScope(false))
{
try
{
scope.Commit();
}
catch (Exception ee)
{
string msg = ee.Message;
}
}
}
它们对应的IL代码如下所示：
.method public hidebysig instance void TestEF() cil managed
{
.maxstack 2
.locals init (
[0] class [DataRabbit.Application]DataRabbit.Application.TransactionScopeFactory factory,
[1] class [DataRabbit.Application]DataRabbit.Application.TransactionScope scope,
[2] class [mscorlib]System.Exception ee,
[3] string msg)
L_0000: nop
L_0001: ldnull
L_0002: newobj instance 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: callvirt instance 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: callvirt instance void [DataRabbit.Application]DataRabbit.Application.TransactionScope::Commit()
L_0017: nop
L_0018: nop
L_0019: leave.s L_0027
L_001b: stloc.2
L_001c: nop
L_001d: ldloc.2
L_001e: callvirt instance string [mscorlib]System.Exception::get_Message()
L_0023: stloc.3
L_0024: nop
L_0025: leave.s L_0027
L_0027: nop
L_0028: leave.s L_0034
L_002a: nop
L_002b: ldloc.1
L_002c: callvirt instance void [DataRabbit.Application]DataRabbit.Application.TransactionScope::Dispose()
L_0031: nop
L_0032: nop
L_0033: endfinally
L_0034: nop
L_0035: ret
.try L_0010 to L_001b catch [mscorlib]System.Exception handler L_001b to L_0027
.try L_0010 to L_002a finally handler L_002a to L_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：
.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：
.try
{
.try
{
leave L1
}
catch
{
leave L1
}
leave L2
}
finally
{
endfinally
}