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Rounding to specfic digits fails with this double-precision value

I'm attempting to truncate a series of double-precision values in C#. The following value fails no matter what rounding method I use. What is wrong with this value that causes both of these methods to fail? Why does even Math.Round fail to correctly truncate the number? What method can be used instead to correctly truncate such values?
The value :
double value = 0.61740451388888251;
Method 1:
return Math.Round(value, digits);
Method 2:
double multiplier = Math.Pow(10, decimals)
return Math.Round(value * multiplier) / multiplier;
Fails even in VS watch window!

Double is a floating binary point type. They are represented in binary system (like 11010.00110). When double is presented in decimal system it is only an approximation as not all binary numbers have exact representation in decimal system. Try for example this operation:
double d = 3.65d + 0.05d;
It will not result in 3.7 but in 3.6999999999999997. It is because the variable contains a closest available double.
The same happens in your case. Your variable contains closest available double.
For precise operations double/float is not the most fortunate choice.
Use double/float when you need fast performance or you want to operate on larger range of numbers, but where high precision is not required. For instance, it is perfect type for calculations in physics.
For precise decimal operations use, well, decimal.
Here is an article about float/decimal: http://csharpindepth.com/Articles/General/FloatingPoint.aspx

If you need a more exact representation of the number you might have to use the decimal type, which has more precision but smaller range (it's usually used financial calculations).
More info on when to use each here: https://stackoverflow.com/a/618596/1373170

According to this online tool which gives the binary representation of doubles, the two closest double values to 0.62 are:
6.19999999999999995559107901499E-1 or 0x3FE3D70A3D70A3D7
link
6.20000000000000106581410364015E-1 or 0x3FE3D70A3D70A3D8
link
I'm not sure why neither of these agree with your value exactly, but like the others said, it is likely a floating point representation issue.

I think you are running up against the binary limit of a double-precision float (64 bits). From http://en.wikipedia.org/wiki/Double-precision_floating-point_format, a double only gives between 15-17 significant digits.

C# Convert.ToDouble() loses decimal points when converting string to double

Let's say we have the following simple code
string number = "93389.429999999993";
double numberAsDouble = Convert.ToDouble(number);
Console.WriteLine(numberAsDouble);
after that conversion numberAsDouble variable has the value 93389.43. What can i do to make this variable keep the full number as is without rounding it? I have found that Convert.ToDecimal does not behave the same way but i need to have the value as double.
-------------------small update---------------------
putting a breakpoint in line 2 of the above code shows that the numberAsDouble variable has the rounded value 93389.43 before displayed in the console.

93389.429999999993 cannot be represented exactly as a 64-bit floating point number. A double can only hold 15 or 16 digits, while you have 17 digits. If you need that level of precision use a decimal instead.
(I know you say you need it as a double, but if you could explain why, there may be alternate solutions)

This is expected behavior.
A double can't represent every number exactly. This has nothing to do with the string conversion.
You can check it yourself:
Console.WriteLine(93389.429999999993);
This will print 93389.43.
The following also shows this:
Console.WriteLine(93389.429999999993 == 93389.43);
This prints True.

Keep in mind that there are two conversions going on here. First you're converting the string to a double, and then you're converting that double back into a string to display it.
You also need to consider that a double doesn't have infinite precision; depending on the string, some data may be lost due to the fact that a double doesn't have the capacity to store it.
When converting to a double it's not going to "round" any more than it has to. It will create the double that is closest to the number provided, given the capabilities of a double. When converting that double to a string it's much more likely that some information isn't kept.

See the following (in particular the first part of Michael Borgwardt's answer):
decimal vs double! - Which one should I use and when?
A double will not always keep the precision depending on the number you are trying to convert
If you need to be precise you will need to use decimal

This is a limit on the precision that a double can store. You can see this yourself by trying to convert 3389.429999999993 instead.

The double type has a finite precision of 64 bits, so a rounding error occurs when the real number is stored in the numberAsDouble variable.
A solution that would work for your example is to use the decimal type instead, which has 128 bit precision. However, the same problem arises with a smaller difference.
For arbitrary large numbers, the System.Numerics.BigInteger object from the .NET Framework 4.0 supports arbitrary precision for integers. However you will need a 3rd party library to use arbitrary large real numbers.

You could truncate the decimal places to the amount of digits you need, not exceeding double precision.
For instance, this will truncate to 5 decimal places, getting 93389.42999. Just replace 100000 for the needed value
string number = "93389.429999999993";
decimal numberAsDecimal = Convert.ToDecimal(number);
var numberAsDouble = ((double)((long)(numberAsDecimal * 100000.0m))) / 100000.0;

Double precision when number converted from string

I’m getting numbers from a database. Some are stored as double in the database and some are stored as string.
What I want to do is count the decimal number of caracters so : 345.34938 would give me a result of 5.
As I said, some of my double come from the database as double and some as string. I’m wondering if there could be any kind of problem when casting the string numbers to double, hence giving me wrong result when trying to count the decimals.
I think I should be ok but I’m afraid that in some situations I’ll end up receiving wrong double numbers when they’re casted from the string (thinking about having 1.9999999 instead of 2.0, things like that)...
Is there any kind of risk that casting my number from string to double would give me strange result when stored as double in my application ? Am I being to frisky around that ?

Consider converting the string representations to System.Decimal with the decimal.Parse method. Because for a decimal there's a much better correspondence between the value of the number and its string representation. Also, it can handle more digits.
A System.Decimal will preserve trailing zeros present in the string (like "2.7500"), which a System.Double will not.
But if your strings never have more than 15 digits in total (including digits before the decimal point .), your approach with double will probably work. But the exact number represented almost always differs from "what you see" with a double, so the number of decimal figures is to be understood as what double.ToString() shows...
Maybe it's easier to just use the string directly, like
int numberOfDecimals = myString.Length - (myString.IndexOf('.') + 1);

Converting String To Float in C#

I am converting a string like "41.00027357629127", and I am using;
Convert.ToSingle("41.00027357629127");
or
float.Parse("41.00027357629127");
These methods return 4.10002732E+15.
When I convert to float I want "41.00027357629127". This string should be the same...

Your thread's locale is set to one in which the decimal mark is "," instead of ".".
Try using this:
float.Parse("41.00027357629127", CultureInfo.InvariantCulture.NumberFormat);
Note, however, that a float cannot hold that many digits of precision. You would have to use double or Decimal to do so.

You can use the following:
float asd = (float) Convert.ToDouble("41.00027357629127");

First, it is just a presentation of the float number you see in the debugger. The real value is approximately exact (as much as it's possible).
Note: Use always CultureInfo information when dealing with floating point numbers versus strings.
float.Parse("41.00027357629127",
System.Globalization.CultureInfo.InvariantCulture);
This is just an example; choose an appropriate culture for your case.

Use Convert.ToDouble("41.00027357629127");
Convert.ToDouble documentation

The precision of float is 7 digits. If you want to keep the whole lot, you need to use the double type that keeps 15-16 digits. Regarding formatting, look at a post about formatting doubles. And you need to worry about decimal separators in C#.

A 2022 way of converting an string that represents a float value:
(float)Convert.ToDecimal(value, CultureInfo.GetCultureInfo("en-US"));
where you also can choose what kind of float are you expecting to convert, because some CultureInfo instances represents decimal values with a , and others with ..
If you need more decimals to obtain more precision, just not use float
Convert.ToDecimal(value, CultureInfo.GetCultureInfo("en-US"));

You can double.Parse("41.00027357629127");

First you need to using System.Globalization to dealing convertions from string to float/double/decimal without problem.
Then you can call Parse on float(or double/decimal depending at the accuracy you need), and as argument in Parse you need your string (you can store it in a variable if you want) and CultureInfo.InvariantCulture.NumberFormat
So, as previous users already explained:
float.Parse("41.00027357629127", CultureInfo.InvariantCulture.NumberFormat);

You can use parsing with double instead of float to get more precision value.

Multiplying a double value by 100.0 introduces rounding errors?

This is what I am doing, which works 99.999% of the time:
((int)(customerBatch.Amount * 100.0)).ToString()
The Amount value is a double. I am trying to write the value out in pennies to a text file for transport to a server for processing. The Amount is never more than 2 digits of precision.
If you use 580.55 for the Amount, this line of code returns 58054 as the string value.
This code runs on a web server in 64-bit.
Any ideas?

You should really use decimal for money calculations.
((int)(580.55m * 100.0m)).ToString().Dump();

You could use decimal values for accurate calculations. Double is floating point number which is not guaranteed to be precise during calculations.

I'm guessing that 580.55 is getting converted to 58054.99999999999999999999999999..., in which case int will round it down to 58054. You may want to write your own function that converts your amount to a int with some sort of rounding or threshold to make this not happen.

Try
((int)(Math.Round(customerBatch.Amount * 100.0))).ToString()

You really should not be using a double value to represent currency, due to rounding errors such as this.
Instead you might consider using integral values to represent monetary amounts, so that they are represented exactly. To represent decimals you can use a similar trick of storing 580.55 as the value 58055.

no, multiplying does not introduce rounding errors
but not all values can by represented by floating point numbers.
x.55 is one of them )

Decimal has more precision than a double. Give decimal a try.
http://msdn.microsoft.com/en-us/library/364x0z75%28VS.80%29.aspx

My suggestion would be to store the value as the integer number of pennies and take dollars_part = pennies / 100 and cents_part = pennies % 100. This will completely avoid rounding errors.
Edit: when I wrote this post, I did not see that you could not change the number format. The best answer is probably using the round method as others have suggested.
EDIT 2: As others have pointed out, it would be best to use some sort of fixed point decimal variable. This is better than my original solution because it would store the information about the location of the decimal point in the value where it belongs instead of in the code.