C＃编译器能否优化此转换？

Question

I'm doing the following cast to double and back to long , intentionally:

var adjustedValue = (long)(double)dateTime.Ticks;

(where dateTime is a System.DateTime and Ticks is a long ). Resharper tells me however that the cast to double is "redundant" and this has me wondering if it can be optimized away by the compiler.

Answer 1

I tried out this code in LINQPad :

var dateTime = DateTime.Now;
var adjustedValue = (long)(double)dateTime.Ticks;
adjustedValue.Dump();

Both with and without optimizations, it generated identical IL (the only difference being in the labels):

IL_0000:  call        System.DateTime.get_Now
IL_0005:  stloc.0     // dateTime
IL_0006:  ldloca.s    00 // dateTime
IL_0008:  call        System.DateTime.get_Ticks
IL_000D:  conv.r8     
IL_000E:  conv.i8     
IL_000F:  stloc.1     // adjustedValue
IL_0010:  ldloc.1     // adjustedValue
IL_0011:  call        LINQPad.Extensions.Dump

So the answer is no , the C# compiler does not optimize this double cast. The conversions are there, even when optimizations are turned on.

However, you should also notice that this double-cast does significantly change the original data. Here's a quick demo, also created using LINQPad:

var dateTime = DateTime.Now;
var originalValue = dateTime.Ticks;
var adjustedValue = (long)(double)originalValue;
originalValue.Dump(); // 635385353415288655
adjustedValue.Dump(); // 635385353415288704

This is because a double only has 15-16 digits of precision, but a long may be up to 19 digits long. You're bound to get some amount of rounding error. If this is a problem, you could use a decimal instead. This will avoid any rounding errors, but even in this case, the double-cast will not be optimized away by the C# compiler.

Of course, how the JIT compiler or NGen.exe convert this to native code is an entirely different matter, but I would expect the behavior to be the same.

Answer 2

I just checked it out. With "dotPeek 1.1" I "took a look" into the source code of this.

With configuration Debug it keeps as is, because the /optimize flag is not set. With configuration Release there is indeed something changed, but not the casts. In Release, as default, /optimize switch is turned on.

var adjustedValue = (long)(double)DateTime.Now.Ticks;

Console.WriteLine(adjustedValue);

went to

Console.WriteLine((long) (double) DateTime.Now.Ticks);

Résumé:

It will not optimize this way of cast.