上一篇博客 《代码覆盖从简到繁（二）》介绍了Visual Studio所采用的Block覆盖中Block是如何定义的，并且展示了代码行与Block之间其实并不是严格对应的。本篇博客将通过.NET中间语言（IL）进一步分析Visual Studio是如何划分Block的，从而更准确回答代码行与Block不能严格对应的原因。
使用Visual Studio获取code coverage数据是非常简单的，只需要在配置中选择“Code Coverage”选项，然后执行测试用例就可以了，覆盖数据会直接在“Code Coverage Results”窗口中呈现出来，这些在《代码覆盖从简到繁（一）》中都有介绍。其实要获取覆盖数据，首先要对被测试的.exe或者.dll进行instrument，所谓instrument实际上就是向文件注入特定的用于收集覆盖数据的代码；然后，启动覆盖数据的监听服务，刚才注入代码会在被指定到时项监听程序发出报告；接下来就是要执行你的测试用例（可以是自动或者手动测试用例）；停止监听服务，生成代码覆盖报告。为了易于使用，Visual Studio自动为我们上述很多工作。除了Visual Studio IDE， 通过命令行工具 VsInstr.exe，VsPerfmon和VsPerfCmd来完成获取覆盖数据的操作，Code Coverage Basics with Visual Studio Team System有详细的介绍，这里就不再赘述！这里需要注意：这些命令不只是用于代码覆盖，而是性能Profiling的 工具。
这里我们用到了VsInstr.exe -coverage命令，用它来instrument我们前面编写的代码，然后通过.NET的Ildasm.exe在IL层观察一下上一篇博客中使用的GetInteger(）函数是如何被划分block的，下面就是Instrument之后的GetInteger()函数的IL代码（这里使用的Visual Studio 2010带的C#编译器，编译器不同产生的代码也会不同）：
.method public hidebysig instance int32 GetInteger(int32 arg1, int32 arg2) cil managed
{
// Code size 204 (0xcc)
.maxstack 3
.locals init ([0] int32 CS$1$0000, [1] bool CS$4$0001)
IL_0000: call void Microsoft.VisualStudio.Coverage.Init_bbf9568946f2545aaa9b589093700f85::Register()
IL_0005: ldsfld uint64[] Microsoft.VisualStudio.Coverage.Init_bbf9568946f2545aaa9b589093700f85::m_vscov
IL_000a: ldc.i4 0x5
IL_000f: ldelem.i8
IL_0010: ldc.i8 0x1
IL_0019: add
IL_001a: conv.i
IL_001b: ldc.i4.1
IL_001c: stind.i1
IL_001d: nop 判断 arg1 > 0
IL_001e: ldarg.1
IL_001f: ldc.i4.0
IL_0020: ble.s IL_0043 如果 arg1 <= 0，跳转到0043处。
IL_0022: ldsfld uint64[] Microsoft.VisualStudio.Coverage.Init_bbf9568946f2545aaa9b589093700f85::m_vscov
IL_0027: ldc.i4 0x5
IL_002c: ldelem.i8
IL_002d: ldc.i8 0x2
IL_0036: add
IL_0037: conv.i
IL_0038: ldc.i4.1
IL_0039: stind.i1
IL_003a: ldarg.2 判断 arg2 < 0
IL_003b: ldc.i4.0
IL_003c: clt
IL_003e: ldc.i4.0
IL_003f: ceq 如果 arg2 < 0, 向求值栈（evaluation stack）加载 0；否则为1；
IL_0041: br.s IL_005c
IL_0043: ldsfld uint64[] Microsoft.VisualStudio.Coverage.Init_bbf9568946f2545aaa9b589093700f85::m_vscov
IL_0048: ldc.i4 0x5
IL_004d: ldelem.i8
IL_004e: ldc.i8 0x3
IL_0057: add
IL_0058: conv.i
IL_0059: ldc.i4.1
IL_005a: stind.i1
IL_005b: ldc.i4.1 （arg1 <= 0时）向求值栈（evaluation stack）加载 1
IL_005c: ldsfld uint64[] Microsoft.VisualStudio.Coverage.Init_bbf9568946f2545aaa9b589093700f85::m_vscov
IL_0061: ldc.i4 0x5
IL_0066: ldelem.i8
IL_0067: ldc.i8 0x4
IL_0070: add
IL_0071: conv.i
IL_0072: ldc.i4.1
IL_0073: stind.i1
IL_0074: stloc.1 判断 arg1 > 0 && arg2 < 0 最终结果
IL_0075: ldloc.1
IL_0076: brtrue.s IL_0095
IL_0078: ldsfld uint64[] Microsoft.VisualStudio.Coverage.Init_bbf9568946f2545aaa9b589093700f85::m_vscov
IL_007d: ldc.i4 0x5
IL_0082: ldelem.i8
IL_0083: ldc.i8 0x5
IL_008c: add
IL_008d: conv.i
IL_008e: ldc.i4.1
IL_008f: stind.i1
IL_0090: nop 准备return 0
IL_0091: ldc.i4.0
IL_0092: stloc.0
IL_0093: br.s IL_00b2
IL_0095: ldsfld uint64[] Microsoft.VisualStudio.Coverage.Init_bbf9568946f2545aaa9b589093700f85::m_vscov
IL_009a: ldc.i4 0x5
IL_009f: ldelem.i8
IL_00a0: ldc.i8 0x6
IL_00a9: add
IL_00aa: conv.i
IL_00ab: ldc.i4.1
IL_00ac: stind.i1
IL_00ad: nop 准备return 1
IL_00ae: ldc.i4.1
IL_00af: stloc.0
IL_00b0: br.s IL_00b2
IL_00b2: ldsfld uint64[] Microsoft.VisualStudio.Coverage.Init_bbf9568946f2545aaa9b589093700f85::m_vscov
IL_00b7: ldc.i4 0x5
IL_00bc: ldelem.i8
IL_00bd: ldc.i8 0x7
IL_00c6: add
IL_00c7: conv.i
IL_00c8: ldc.i4.1
IL_00c9: stind.i1
IL_00ca: ldloc.0
IL_00cb: ret
} // end of method Program::GetInteger |
