(sort of) First Class Classes in C#

2010-08-10 c / functional-programming

It seems, at first, that C# doesn't have first-class classes. But ...

I find myself writing some C# code while still thinking in Python. One thing in particular caught me out … it seems, at first, that C# doesn’t have first class classes. This is annoying, because I’d started writing some device driver classes where each class is a type of device, and instances represent the individual devices themselves. And I wanted to construct a list of these classes, and call a “probe” classmethod on each of them to ask the class to go search out any devices which were available. In Python, this would look something like:

device_classes = (FooDevice, BarDevice, BazDevice)

for device_class in device_classes:
    device_class.probe()

See? The classes are being treated just like any other variable, because they are, they’re just instances of type classobj. But the equivalent doesn’t work in C# — doing this:

Type[] DeviceClasses = {
    FooDevice,
    BarDevice,
    BazDevice
};

… complains that ‘FooDevice’ is a ‘type’ but is used like a ‘variable’. At first it seemed that C# didn’t have first class classes, and indeed a few web searches came up empty handed.

Thankfully after a bit more exploration it turns out that all that is needed is some syntactic nastiness … namely, typeof(), GetMethod() and Invoke() (Passing null to Invoke() works for static methods):

Type[] DeviceClasses = {
    typeof(FooDevice),
    typeof(BarDevice),
    typeof(BazDevice)
};

foreach (Type dct in DeviceClasses) {
    dct.GetMethod("Probe").Invoke(null, new object[] {} );
}

Now, quite why a shiny new programming language has to get saddled with such godawful syntax is a bit beyond me, but so it goes.

As always, this is lovingly documented in MSDN, in such a way that the answer is clear so long as you already know what you’re looking for.

(As a bonus, yes, you can use reflection to find the list of Devices in the first place. It just wasn’t all that relevant to this example)

... and another thing ...

(sort of) First Class Classes in C#