Proposal: NumberBox enhancement for the decimal type

Proposal

NumberBox enhancement for the decimal type

Summary

The NumberBox is based on the double type. It should be as easy to use the decimal type instead.

Rationale

Many applications use the decimal instead of the double. If a property of type decimal is used with compiled binding, a casting exception is thrown. With reflection-based binding, the value is always 0.

• Decimals are often used with properties, and are needed to bind to UI elements, e.g. for money

Scope

Capability	Priority
This proposal will allow developers to bind the NumberBox to a decimal type	Must

Open Questions

Because a decimal is not part of the Windows Runtime base data types there might not be that many options for this functionality. I was successfully in creating and using a converter to convert from decimal to double - however, this workaround shouldn’t be needed. Probably there’s a way for a built-in conversion, or other options?

[robloo]

I asked for the same when we were first discussing the spec for the NumberBox

A double value is NOT ideal for this. The number of decimal places is important and instead the control ideally would be backed by a Decimal type. I would suggest changing the spec for Value from double to decimal to support this type of use case. https://github.com/microsoft/microsoft-ui-xaml/issues/483#issuecomment-497545781

This is important for currency and other monetary information. However, you are correct that this isn’t possible because decimal is not in WinRT. (using type conversion like you suggested between double and decimal would be useless, the precision is already lost)

@robloo, about that final comment, there is no Decimal type in WinRT. https://github.com/microsoft/microsoft-ui-xaml/issues/483#issuecomment-497564896

There are several sacrifices made in WinRT so it is compatible with the most languages/frameworks. I see this as one of them. If WinRT ever does get decimal support I would love to switch NumberBox to it. However, that would now be a breaking change that Microsoft likely wouldn’t support. So I think this should be closed:

1. WinRT doesn’t support decimal
2. Even if decimal type support was added to WinRT, using it in NumberBox would be a breaking change that is likely unjustifiable.

The ship has already sailed I’m afraid.

[robloo]

@christiannagel

using type conversion like you suggested between double and decimal would be useless, the precision is already lost

That’s an important point. In several scenarios I’m using this wouldn’t be an issue - however this again would limit the usability, and there should be a different option.

If precision isn’t an issue, I don’t think you need decimal. Just use Convert to go between double/decimal as you desire (assuming .net). Converting values is quite easy and I don’t think limits usability of NumberBox at all. In fact I could argue the opposite and say double is far more commonplace than decimal and switching to decimal hinders ease of use.

@mdtauk

My suggestion was that the Value type would change, based on the ValueType chosen. So it could be a double by default, but if the control was set to a ValueType of Decimal, then that would be the type used by the Value property.

That isn’t possible without changing to dynamic or object as the backing type for Value. This violates all sorts of API design principles. @christiannagel is correct and we would need another property of type decimal.