How is a decompiler able to recognize a compiled constant?

Question

I am using ILSpy to decompile the assemblies of .Net and look into the code. As I was browsing the code of System.Windows.Vector.AngleBetween(Vector, Vector) in WindowsBase.dll, I stumbled across something bizarre.

This is the full code of the function:

public static double AngleBetween(Vector vector1, Vector vector2)
{
    double y = vector1._x * vector2._y - vector2._x * vector1._y;
    double x = vector1._x * vector2._x + vector1._y * vector2._y;
    return Math.Atan2(y, x) * (180.0 / Math.PI);
}

Apparently ILSpy could recognize Math.PI, which is a constant.

This is what Microsoft Docs says about constants in C#:

In fact, when the compiler encounters a constant identifier in C# source code, it substitutes the literal value directly into the intermediate language (IL) code that it produces.

Based on this, what ILSpy did seems impossible.

Note: this behavior is present even if the "Use variable names from debug symbols, if available" and "Show info from debug symbols, if available" options are unchecked in the settings.

Possibly ILSpy just knows the value of common constants like `Math.PI` when it sees them. — Blorgbeard, Apr 10 '19 at 18:31

score 6 · Accepted Answer · answered Apr 10 '19 at 18:34

6

As you can see in this ILSpy issue and the corresponding pull request, this was specifically implemented (hardcoded) for well-known values such as Math.PI.

From the GitHub issue:

I suppose to calculate pi coefficient by following way: c = Math.PI / constant. If we getting "good" value (equal exactly to 1.0, 2.0, 0.5, 1/180 and so on), we simply replacing it with symbolic expression (Math.PI, Math.PI * 2, Math.PI / 2, Math.PI / 180 and so on).

answered Apr 10 '19 at 18:34

Ghost4Man

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How then the ILSpy is able to recognize not well known constants ? – Dmytro Mukalov Apr 10 '19 at 18:46
Do you have an example of a non-well-known constant that it recognizes? – Bradley Uffner Apr 10 '19 at 18:50
@DmytroMukalov It can probably recognize non-well-known constants from the debug symbols (PDB files), but I haven't tested this. – Ghost4Man Apr 10 '19 at 18:51
@DmytroMukalov I just tested, and it didn't recognize `Math.PI * 254.543`, even with 'use debug symbols' on – AstroRP Apr 10 '19 at 18:54

Bradley Uffner · Answer 2 · 2019-04-10T18:56:44.117

0

A quick test in LINQPad shows some differences in the IL code generated (PI constant value copied from here).

Source:

void Main()
{
    double a = Math.PI;
    double b = 3.14159265358979;
}

IL:

IL_0000:  nop         
IL_0001:  ldc.r8      18 2D 44 54 FB 21 09 40 
IL_000A:  stloc.0     // a
IL_000B:  ldc.r8      11 2D 44 54 FB 21 09 40 
IL_0014:  stloc.1     // b
IL_0015:  ret

There does seem to be a subtle difference in the IL code generated between the constant and the literal values, but I'm not sure exactly what it means yet.

The above values from the MSDN documentation seem to contradict information from the .NET Reference Source (See comments). With adjusted code from the source, the IL is identical.

Source:

void Main()
{
    var a = Math.PI;
    var b = 3.14159265358979323846;
}

IL:

IL_0000:  nop         
IL_0001:  ldc.r8      18 2D 44 54 FB 21 09 40 
IL_000A:  stloc.0     // a
IL_000B:  ldc.r8      18 2D 44 54 FB 21 09 40 
IL_0014:  stloc.1     // b
IL_0015:  ret

As an aside, it looks like there is an open issue to address the documentation / source code inconsistency.

edited Apr 10 '19 at 18:56

answered Apr 10 '19 at 18:27

Bradley Uffner

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`Math.PI` is definitely a const, it's declaration: `public const double PI = 3.1415926535897931;` – AstroRP Apr 10 '19 at 18:32
I was just going by the MSDN documentation from https://learn.microsoft.com/en-us/dotnet/api/system.math.pi?view=netframework-4.7.2, which lists it as a field. – Bradley Uffner Apr 10 '19 at 18:33
The value you give is also different from the value in the linked documentation. – Bradley Uffner Apr 10 '19 at 18:33
According to the [C# Reference Source](https://referencesource.microsoft.com/#mscorlib/system/math.cs,fa5fd1f4e5f6f911,references), you are correct. it is declared as a constant. I will adjust my answer. – Bradley Uffner Apr 10 '19 at 18:36

How is a decompiler able to recognize a compiled constant?

2 Answers2