[Proposal] Add new overload to Math.Truncate

[VBAndCs]

Background and Motivation

A common task is to truncate doubles to a specific places after the decimal point. There is no easy, fast and precise way to do this in Math class. Either we can round the number, or use this formula:

var n2 = Math.Truncate(n1 * Math.power(10, precision)) / 100;

more discussion about this is here:
https://stackoverflow.com/questions/3143657/truncate-two-decimal-places-without-rounding

Proposed API

So, I suggest to add a new overloads to the Truncate method:
Math.Truncate(double number, int precision)
Math.Truncate(decimal number, int precision)
so it can achieve this goal with a fast and accurate way to give us the best performance.

Usage Examples

var n2 = Math.Truncate(n1, precision); 

[SingleAccretion]

I think you can now do this with Math.Round:

Math.Round(3.4611, 2, MidpointRounding.ToZero); //3.46
Math.Round(-3.4611, 2, MidpointRounding.ToZero); //-3.46
Math.Round(3.4679, 2, MidpointRounding.ToZero); //3.46
Math.Round(-3.4679, 2, MidpointRounding.ToZero); //-3.46

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[VBAndCs]

Thanks.
I am not aware of that option, and I think the reason is that the name MidpointRounding is misleading. These options are about the mid point (when the fraction digit is 5), and ir is not convenient to affect other non-midpoints.
So, either you change the name MidpointRounding, or add a third optional param to the round to specify the round (default), truncate or ceiling options.

[SingleAccretion]

@VBAndCs That is true and the only reason why I know about MidpointRounding.ToZero myself :).

[Gnbrkm41]

To me, Truncate means only one thing, and it's dropping the value past the decimal bit entirely instead of selectively rounding towards zero to a certain decimal points. This is true in libraries of other languages as well, where it does the same, for example, C/C++ <math.h>'s trunc function and Rust's f32::trunc. For that reason I don't think it's a great idea to expose these as an overload of Truncate.

I do believe that those behaviours you've described already exists in Round (by specifying MidpointRounding.ToZero). I do agree that the name "MidpointRounding" is very misleading. I think we should work towards fixing that instead - as @​SingleAccretion mentioned, ​#38160 tracks improving the existing Round to be more clearer.

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