R – .NET RIA Services – DataService problem

float and double are floating binary point types. In other words, they represent a number like this:

10001.10010110011

The binary number and the location of the binary point are both encoded within the value.

decimal is a floating decimal point type. In other words, they represent a number like this:

12345.65789

Again, the number and the location of the decimal point are both encoded within the value – that's what makes decimal still a floating point type instead of a fixed point type.

The important thing to note is that humans are used to representing non-integers in a decimal form, and expect exact results in decimal representations; not all decimal numbers are exactly representable in binary floating point – 0.1, for example – so if you use a binary floating point value you'll actually get an approximation to 0.1. You'll still get approximations when using a floating decimal point as well – the result of dividing 1 by 3 can't be exactly represented, for example.

As for what to use when:

• For values which are "naturally exact decimals" it's good to use decimal. This is usually suitable for any concepts invented by humans: financial values are the most obvious example, but there are others too. Consider the score given to divers or ice skaters, for example.

• For values which are more artefacts of nature which can't really be measured exactly anyway, float/double are more appropriate. For example, scientific data would usually be represented in this form. Here, the original values won't be "decimally accurate" to start with, so it's not important for the expected results to maintain the "decimal accuracy". Floating binary point types are much faster to work with than decimals.

Since you have a DomainService with a parameter in its ctor, and more generally needs to be constructed through some sort of IoC container or dependency injection system, you'll need to provide an app-level domain service factory. Your factory is then responsible for instantiating the domain service (and disposing it), and it can do so by calling into another API, such as Unity in your case.

Here's a basic example:

In Global.asax.cs of your app, add the following:

public class Global : HttpApplication {

    static Global() {
        DomainService.Factory = new MyAppDomainServiceFactory();
    }
}

internal sealed class MyAppDomainServiceFactory : IDomainServiceFactory {

    public DomainService CreateDomainService(Type domainServiceType,
                                             DomainServiceContext context) {
        DomainService ds = ... // code to create a service, or look it up
                               // from a container

        if (ds != null) {
            ds.Initialize(context);
        }
        return ds;
    }

    public void ReleaseDomainService(DomainService domainService) {
        // any custom logic that must be run to dispose a domain service
        domainService.Dispose();
    }
}

Hope that helps!