Elegant Invariant Checking with C# 3
Of all the great advice in The Pragmatic Programmer, one of the ideas I’ve found most useful is that of Design by Contract. Most commonly, I check method preconditions using something like:
private void MyMethod(object arg1, string arg2) { Invariant.ArgumentNotNull(arg1, "arg1"); // throws ArgumentNullException Invariant.ArgumentNotEmpty(arg2, "arg2"); // throws ArgumentOutOfRangeException // do stuff }
I quite often check invariants at other points in a method too:
private void MyMethod(object arg1, string arg2) { var myVar = AnotherMethod(); Invariant.IsNotNull(myVar); // throws InvalidOperationException // do stuff with myVar }
This approach works well but, in the case of preconditions, having to duplicate the argument name as a string is obviously not ideal. Perhaps using some of the new features of C# 3 we can do better?
Here’s one API I’ve been playing with:
private void MyMethod(object arg1, string arg2) { Check.Argument(() => arg1.IsNotNull); // throws ArgumentNullException Check.Argument(() => arg2.IsNotEmpty); // throws ArgumentOutOfRangeException // do stuff }
And for simple invariants:
private void MyMethod(object arg1, string arg2) { var myVar = AnotherMethod(); Check.Invariant(() => myVar.IsNotNull); // throws InvalidOperationException // do stuff with myVar }
This approach leverages expression trees and extension methods to create a more fluent API and allows us to do away with the string argument when checking arguments.
Although I’m not 100% happy with it, I think I prefer this API. What do you think?
Download the code here.
