double c1 = 182273d;
double c2 = 0.888d;
Expression c1e = Expression.Constant(c1,typeof(double));
Expression c2e = Expression.Constant(c2,typeof(double));
Expression<Func<double,double>> sinee = a => Math.Sin(a);
Expression sine = ((MethodCallExpression)sinee.Body).Update(null,new[] { c1e });
Expression sum = Expression.Add(sine,c2e);
Func<double> f = Expression.Lambda<Func<double>>(sum).Compile();
double r = f();
double rr = Math.Sin(c1) + c2;
Console.WriteLine(r.ToString("R"));
Console.WriteLine(rr.ToString("R"));
将输出：
0.082907514933846488 0.082907514933846516
为什么r和rr不同？
更新：
发现如果选择“x86”平台目标或使用“任何cpu”检查“首选32位”,则重现此项.在64x模式下正常工作.
解决方法
首先,只有在使用调试标志进行编译时才会出现问题(在发布模式下它不会出现),并且实际上只有在运行为x86时才出现.
如果我们反编译表达式编译的方法,我们将看到这个(在调试和发布中)：
IL_0000: ldc.r8 182273 // push first value IL_0009: call float64 [mscorlib]System.Math::Sin(float64) // call Math.Sin() IL_000e: ldc.r8 0.888 // push second value IL_0017: add // add IL_0018: ret
但是,如果我们查看在调试模式下编译的类似方法的IL代码,我们将看到：
.locals init ( [0] float64 V_0 ) IL_0001: ldc.r8 182273 IL_000a: call float64 [mscorlib]System.Math::Sin(float64) IL_000f: ldc.r8 0.888 IL_0018: add IL_0019: stloc.0 // save to local IL_001a: br.s IL_001c // basically nop IL_001c: ldloc.0 // V_0 // pop from local to stack IL_001d: ret // return
您会看到编译器将(不必要的)结果保存并加载到本地变量(可能用于调试目的).现在我不确定,但据我所知,在x86架构上,双值可能存储在80位cpu寄存器中(引自here)：
By default,in code for x86 architectures the compiler uses the
coprocessor’s 80-bit registers to hold the intermediate results of
floating-point calculations. This increases program speed and
decreases program size. However,because the calculation involves
floating-point data types that are represented in memory by less than
80 bits,carrying the extra bits of precision—80 bits minus the number
of bits in a smaller floating-point type—through a lengthy calculation
can produce inconsistent results.
所以我的猜测是这个本地存储和从本地加载导致从64位转换到80位(因为寄存器)和返回,这会导致你观察到的行为.
另一种解释可能是JIT在调试和释放模式之间表现不同(可能仍然与将中间计算结果存储在80位寄存器中有关).
希望有些知道更多的人可以确认我是否正确.
更新以回应评论.反编译表达式的一种方法是创建动态程序集,将表达式编译到那里的方法,保存到磁盘,然后查看任何反编译器(我使用JetBrains DotPeek).例：
var asm = AppDomain.CurrentDomain.DefineDynamicAssembly(
new AssemblyName("dynamic_asm"),AssemblyBuilderAccess.Save);
var module = asm.DefineDynamicModule("dynamic_mod","dynamic_asm.dll");
var type = module.DefineType("DynamicType");
var method = type.DefineMethod(
"DynamicMethod",MethodAttributes.Public | MethodAttributes.Static);
Expression.Lambda<Func<double>>(sum).CompileToMethod(method);
type.CreateType();
asm.Save("dynamic_asm.dll");
