Question

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decimal vs double! – Which one should I use and when?

I’m using double type for price in my trading software.
I’ve noticed that sometimes there are a odd errors.
They occur if price contains 4 digits after “dot”, like 2.1234.

When I sent from my program “2.1234” on the market order appears at the price of “2.1235”.

I don’t use decimal because I don’t need “extreme” precision. I don’t need to distinguish for examle “2.00000000003” from “2.00000000002”. I need maximum 6 digits after a dot.

The question is – where is the line? When to use decimal?

Should I use decimal for any finansical operations? Even if I need just one digit after the dot? (1.1 1.2 etc.)

I know decimal is pretty slow so I would prefer to use double unless decimal is absolutely required.

Answer 1

Use decimal whenever you’re dealing with quantities that you want to (and can) be represented exactly in base-10. That includes monetary values, because you want 2.1234 to be represented exactly as 2.1234.

Use double when you don’t need an exact representation in base-10. This is usually good for handling measurements, because those are already approximations, not exact quantities.

Of course, if having or not an exact representation in base-10 is not important to you, other factors come into consideration, which may or may not matter depending on the specific situation:

• double has a larger range (it can handle very large and very small magnitudes);
• decimal has more precision (has more significant digits);
• you may need to use double to interact with some older APIs that are not aware of decimal;
• double is faster than decimal;
• decimal has a larger memory footprint;