Cast a Double Variable to Decimal

Question

How does one cast a double to decimal which is used when doing currency development. Where does the M go?

decimal dtot = (decimal)(doubleTotal);

Solution 1

You only use the M for a numeric literal, when you cast it's just:

decimal dtot = (decimal)doubleTotal;

Note that a floating point number is not suited to keep an exact value, so if you first add numbers together and then convert to Decimal you may get rounding errors. You may want to convert the numbers to Decimal before adding them together, or make sure that the numbers aren't floating point numbers in the first place.

Solution 2

You can cast a double to a decimal like this, without needing the M literal suffix:

double dbl = 1.2345D;
decimal dec = (decimal) dbl;

You should use the M when declaring a new literal decimal value:

decimal dec = 123.45M;

(Without the M, 123.45 is treated as a double and will not compile.)

Solution 3

use default convertation class: Convert.ToDecimal(Double)

Solution 4

Convert.ToDecimal(the double you are trying to convert);

Solution 5

Well this is an old question and I indeed made use of some of the answers shown here. Nevertheless, in my particular scenario it was possible that the double value that I wanted to convert to decimal was often bigger than decimal.MaxValue. So, instead of handling exceptions I wrote this extension method:

    public static decimal ToDecimal(this double @double) => 
        @double > (double) decimal.MaxValue ? decimal.MaxValue : (decimal) @double;

The above approach works if you do not want to bother handling overflow exceptions and if such a thing happen you want just to keep the max possible value(my case), but I am aware that for many other scenarios this would not be the expected behavior and may be the exception handling will be needed.

View more solutions

Comments

[Peter Ritchie]

Really, you're suggesting a call to a Convert method instead of a cast operator??

[Tom]

I've learnt that the Convert class is much more flexible and safe than a cast in C#.

[Peter Ritchie]

"Safe"? as in when it can't cast it throws an exception at run-time instead of a compiler error? I've been bitten by that so many times that I actively avoid Convert...

[Tom]

That's true, I usually know what I'm converting so in that case I use the Convert class. A simple try catch could get rid of the error but I agree it is more work.

[Mike Johnson]

@PeterRitchie thread is a bit old but this should be said: Calling the Convert method directly would be the more appropriate approach. Maybe I'm just an optimization freak but one less instruction to resolve is a bonus (since using the explicit (Type) cast syntax is just an operator overload that calls Convert).

[supercat]

@PeterRitchie: From a language-design perspective, it would have been better to require a programmer to use one of two conversion methods rather than allowing a typecast from double to decimal, given that for a double value like (1000000.0/3.0) one would in some cases want to clip "excess" precision yielding 333333.333333333D, but in others cases one would want to retain it, yielding 333333.333333333313931D. Rather than simply saying "convert to decimal", code should specify how that conversion should be performed.

[Peter Ritchie]

@supercat I don't see how "two conversion methods" would be from a language-design perspective. The would have to do with interface design using the language in question. Given that the OP only details that a given double needs to be assigned to a decimal, I don't see how anyone could infer how the conversion should be performed and thus which of these two non-existent methods could be chosen for an answer to the question. I agree, it would be great if the question included that detail; but it didn't

[supercat]

@PeterRitchie: From a language design perspective, I would favor allowing casts only in cases where any value in the source type would have one clearly and unambiguously "best" value in the destination type. For conversion from Decimal to Double, that would apply, but for conversion from Double to Decimal it would not.

[Peter Ritchie]

@supercat which really seems unrelated to my first comment because using Convert.ToDecimal(double) is the same as (decimal)doubleTotal, except if doubleTotal changed to a different type you'd probably avoid a compile-time error and introduce a harder-to-find run-time error because a different ToDecimal override might get called. Cast operator is much more explicit...

[Admin]

as a follow up question, why is the explicit conversion needed? I've tried it out and I get an error that a double cannot be explicitly cast to a decimal, but doesn't a decimal carry more precision? (i.e. much like casting from an int to a double can be implicit.)

[Guffa]

@Cortana: The precision of a decimal is higher, but the range is smaller. A double value may be out of range for a decimal. See: stackoverflow.com/questions/7817866/…

[ToXinE]

No because it will throw an OverflowException double vol_y = (double)Decimal.MaxValue + 10E+28D; Console.WriteLine("Convert.ToDecimal(vol_y) = " + Convert.ToDecimal(vol_y));

[Martin Eyles]

This would fail in the following case double _double = (double)decimal.MaxValue; I would suggest using >= in the comparison public static decimal ToDecimal(this double _double) => _double >= (double) decimal.MaxValue ? decimal.MaxValue : (decimal) _double;

[this.myself]

@ToXinE IMHO in most cases an OverflowException is better than a silently creating wrong data