对于集合的遍历，使用foreach是非常方便的，但是Emit动态生成foreach的代码就要复杂很多。它涉及到以下几个方面：
（1）IEnumerable<> 是所有可枚举类型的基础接口。
（2）IEnumerator<>，通过IEnumerable<> 接口的GetEnumerator方法可以获取枚举器IEnumerator<>，而对集合元素的遍历正是由IEnumerator<>的MoveNext方法完成的。
（3）遍历完成以后，需要调用IEnumerator<>的Dispose方法释放它。
（4）为了IEnumerator<>被正常释放，还需要使用try....finally块包含相应的代码。
下面我们来举个例子，比如对于如下的C#代码：
public interface ICompute
{
void Add(int a, int b);
}
public class Compute : ICompute
{
private ICollection<ICompute> computers;
{
foreach (ICompute com in computers)
{
com.Add(a, b);
}
}
}
{
void Add(int a, int b);
}
public class Compute : ICompute
{
private ICollection<ICompute> computers;
public void Add(int a, int b)
{
foreach (ICompute com in computers)
{
com.Add(a, b);
}
}
}
Compute类的Add方法使用了foreach进行遍历操作，我们可以将Add方法的等价形式写出来：
public virtual void Add(int num1, int num2)
{
IEnumerator<ICompute> enumerator = this.computers.GetEnumerator();
try
{
while (enumerator.MoveNext())
{
enumerator.Current.Add(num1, num2);
}
}
finally
{
if (enumerator != null)
{
enumerator.Dispose();
}
}
}
{
IEnumerator<ICompute> enumerator = this.computers.GetEnumerator();
try
{
while (enumerator.MoveNext())
{
enumerator.Current.Add(num1, num2);
}
}
finally
{
if (enumerator != null)
{
enumerator.Dispose();
}
}
}
那么为了Emit类似的代码，需要生成如下的IL：
.method public hidebysig newslot virtual final instance void Add(int32 a, int32 b) cil managed
{
.maxstack 3
.locals init (
[0] class TheTest.ICompute com,
[1] class [mscorlib]System.Collections.Generic.IEnumerator`1<class TheTest.ICompute> CS$5$0000,
[2] bool CS$4$0001)
L_0000: nop
L_0001: nop
L_0002: ldarg.0
L_0003: ldfld class class [mscorlib]System.Collections.Generic.ICollection`1<class TheTest.ICompute> TheTest.Compute::computers
L_000d: callvirt instance class [mscorlib]System.Collections.Generic.IEnumerator`1<!0> [mscorlib]System.Collections.Generic.IEnumerable`1<class TheTest.ICompute>::GetEnumerator()
L_0012: stloc.1
L_0013: br.s L_0027
L_0015: ldloc.1
L_0016: callvirt instance !0 [mscorlib]System.Collections.Generic.IEnumerator`1<class TheTest.ICompute>::get_Current()
L_001b: stloc.0
L_001c: nop
L_001d: ldloc.0
L_001e: ldarg.1
L_001f: ldarg.2
L_0020: callvirt instance void TheTest.ICompute::Add(int32, int32)
L_0025: nop
L_0026: nop
L_0027: ldloc.1
L_0028: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
L_002d: stloc.2
L_002e: ldloc.2
L_002f: brtrue.s L_0015
L_0031: leave.s L_0043
L_0033: ldloc.1
L_0034: ldnull
L_0035: ceq
L_0037: stloc.2
L_0038: ldloc.2
L_0039: brtrue.s L_0042
L_003b: ldloc.1
L_003c: callvirt instance void [mscorlib]System.IDisposable::Dispose()
L_0041: nop
L_0042: endfinally
L_0043: nop
L_0044: ret
.try L_0013 to L_0033 finally handler L_0033 to L_0043
}
{
.maxstack 3
.locals init (
[0] class TheTest.ICompute com,
[1] class [mscorlib]System.Collections.Generic.IEnumerator`1<class TheTest.ICompute> CS$5$0000,
[2] bool CS$4$0001)
L_0000: nop
L_0001: nop
L_0002: ldarg.0
L_0003: ldfld class class [mscorlib]System.Collections.Generic.ICollection`1<class TheTest.ICompute> TheTest.Compute::computers
L_000d: callvirt instance class [mscorlib]System.Collections.Generic.IEnumerator`1<!0> [mscorlib]System.Collections.Generic.IEnumerable`1<class TheTest.ICompute>::GetEnumerator()
L_0012: stloc.1
L_0013: br.s L_0027
L_0015: ldloc.1
L_0016: callvirt instance !0 [mscorlib]System.Collections.Generic.IEnumerator`1<class TheTest.ICompute>::get_Current()
L_001b: stloc.0
L_001c: nop
L_001d: ldloc.0
L_001e: ldarg.1
L_001f: ldarg.2
L_0020: callvirt instance void TheTest.ICompute::Add(int32, int32)
L_0025: nop
L_0026: nop
L_0027: ldloc.1
L_0028: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
L_002d: stloc.2
L_002e: ldloc.2
L_002f: brtrue.s L_0015
L_0031: leave.s L_0043
L_0033: ldloc.1
L_0034: ldnull
L_0035: ceq
L_0037: stloc.2
L_0038: ldloc.2
L_0039: brtrue.s L_0042
L_003b: ldloc.1
L_003c: callvirt instance void [mscorlib]System.IDisposable::Dispose()
L_0041: nop
L_0042: endfinally
L_0043: nop
L_0044: ret
.try L_0013 to L_0033 finally handler L_0033 to L_0043
}
请注意红色代码部分，这与我们上面的描述是一致的。其它的IL代码的Emit相对简单，这里我们只提一下如何Emit最后这句try...finally块：
.try L_0013 to L_0033 finally handler L_0033 to L_0043
对于try...catch...finnally块的标准Emit过程时这样的：
ILGenerator methodGen = ..... ;
//开始try块
methodGen.BeginExceptionBlock();
//......
//开始catch块
methodGen.BeginCatchBlock(typeof(Exception));
//......
//开始finally块
methodGen.BeginFinallyBlock();
//......
//结束try/Catch/finally块
methodGen.EndExceptionBlock();
methodGen.BeginExceptionBlock();
//......
//开始catch块
methodGen.BeginCatchBlock(typeof(Exception));
//......
//开始finally块
methodGen.BeginFinallyBlock();
//......
//结束try/Catch/finally块
methodGen.EndExceptionBlock();
如果只有try...catch...，则
ILGenerator methodGen = ..... ;
methodGen.BeginExceptionBlock();
//......
//开始catch块
methodGen.BeginCatchBlock(typeof(Exception));
//......
//结束try/Catch块
methodGen.EndExceptionBlock();
//开始try块
methodGen.BeginExceptionBlock();
//......
//开始catch块
methodGen.BeginCatchBlock(typeof(Exception));
//......
//结束try/Catch块
methodGen.EndExceptionBlock();
如果只有try...finnally...，则
ILGenerator methodGen = ..... ;
//开始try块
methodGen.BeginExceptionBlock();
//......
//开始finally块
methodGen.BeginFinallyBlock();
//......
//结束try/finally块
methodGen.EndExceptionBlock();
//开始try块
methodGen.BeginExceptionBlock();
//......
//开始finally块
methodGen.BeginFinallyBlock();
//......
//结束try/finally块
methodGen.EndExceptionBlock();