Initialising a System.Decimal v Initialisting a decimal data type

Question

I am use to using the decimal data type like this:

decimal Cost = 30M;

This initialises the decimal with £30 (£ is in the context of my application). I am planning to use a type alias for Decimal in my application like this:

using Cost = System.Decimal

I believe I have to use the object rather than the primitive type when using a type alias. Please let me know if that is not correct?

Also I have noticed that I cannot do this:

Decimal cost = new Decimal(30M);

I have to do this instead:

Decimal cost = new Decimal(30);

Is the initialisation code above suitable for a currency?

new Decimal? Why would you want to use a Decimal constructor? Also, why alias a built-in type? — Camilo Terevinto
– Camilo Terevinto, Commented Feb 3, 2018 at 15:40
@CamiloTerevinto I'm guessing to make explicit in code that the type is used as a monetary value. — InBetween
– InBetween, Commented Feb 3, 2018 at 15:48

InBetween · Accepted Answer · 2018-02-03 15:43:49Z

This initialises the decimal with £30 (£ is in the context of my application). I am planning to use a type alias for Decimal in my application like this: using Cost = System.Decimal

Ok, thats a valid option.

I believe I have to use the object rather than the primitive type when using a type alias. Please let me know if that is not correct?

I'm not sure what you are asking, but if you are using a type alias, the whole idea is to use it:

var cost = new Cost(10);

Rememnber, a type alias is a type alias; wherever you can System.Decimal, Cost is valid too.

Also I have noticed that I cannot do this: Decimal cost = new Decimal(30M);

Yup, and you can't do Cost cost = new Cost(30M); either, because decimal doesn't define such constructor overload. The reason being, if you already have a Decimal / Cost in hand why in the world would you want to new it up again? Cost cost = 30M; would do just fine.

Also notice that Cost cost = 30; is just fine too, the compiler will perform the implicit conversion for you.

Thanks. I also have a DateTime. Can I initialise this without the new operator?
@w0051977 No, C# does not support date constant literals (var d = #01/10/18 or similar), you need the new operator.

DavidG · Accepted Answer · 2018-02-03 15:48:14Z

Yes, you need to use the fully qualified type name to use an alias. From the docs:

Create a using alias to make it easier to qualify an identifier to a namespace or type. The right side of a using alias directive must always be a fully-qualified type regardless of the using directives that come before it.

It's worth noting that using a type alias is very uncommon, particularly when using a built in value type like decimal. I would not recommend that you do that.
You can't do new Decimal(30M) because the type doesn't have a constructor that takes a decimal, after all it would be pointless because you can simply do this (where the M qualifier is optional):
```
Decimal cost = 30M; 
```

The M qualifier is NOT optional. Without it, the number is treated as a double and generates a compiler error.
@DavidG: The following decimal dd = 30.54; produces compiler error CS0664 Literal of type double cannot be implicitly converted to type 'decimal'; use an 'M' suffix to create a literal of this type.
@MarkBenningfield Indeed, but this question doesn't contain that number, it contains an integer.

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