A couple months ago I posted about our query API along with an explanation of how it works. Our query API allows us to express our queries using strongly typed C# syntax:
List<Customer> customers = repository.FindAll(Customer.Columns.Age == 20 & Customer.Columns.Name == “foo”);
One of the things I pointed out in my previous posts was that I couldn’t overload the && and || operators directly since the framework doesn’t allow such craziness…at least not directly.
In particular, it is not possible to overload member access, method invocation, or the =, &&, ||, ?:, checked, unchecked, new, typeof, as, and is operators.
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/csspec/html/vclrfcsharpspec_7_2_2.asp
Over the last month I’ve done a little investigation on the topic to see if and how I can get && and || to behave the way I want. This evening I came across the Conditional logical operators page on MSDN which gave me the answer I was looking for:
- The operation
x && y is evaluated as T.false(x) ? x : T.&(x, y), where T.false(x) is an invocation of the operator false declared in T, and T.&(x, y) is an invocation of the selected operator &. In other words, x is first evaluated and operator false is invoked on the result to determine if x is definitely false. Then, if x is definitely false, the result of the operation is the value previously computed for x. Otherwise, y is evaluated, and the selected operator & is invoked on the value previously computed for x and the value computed for y to produce the result of the operation.
- The operation
x || y is evaluated as T.true(x) ? x : T.|(x, y), where T.true(x) is an invocation of the operator true declared in T, and T.|(x, y) is an invocation of the selected operator |. In other words, x is first evaluated and operator true is invoked on the result to determine if x is definitely true. Then, if x is definitely true, the result of the operation is the value previously computed for x. Otherwise, y is evaluated, and the selected operator | is invoked on the value previously computed for x and the value computed for y to produce the result of the operation.
Since we already have the & and | operators in place it was simply a matter of overloading the true and false operators to both return false. This results in the & and | operators always being called which in turn results in the two criteria objects being turned into an AndCriteria/OrCriteria!
So now we can express our criteria expressions using the && and || syntax we’re accustomed to.
repository.FindAll(Customer.Columns.Age == 20 && Customer.Columns.Name == “foo”);
repository.FindAll(Customer.Columns.FirstName == “Foo” || Customer.Columns.LastName == “Bar”);
The relevant operator overloads are shown below.
public static bool operator true(Criteria<T> criteria) {
return false;
}
public static bool operator false(Criteria<T> criteria) {
return false;
}
public static Criteria<T> operator &(Criteria<T> lhs, Criteria<T> rhs) {
return new AndCriteria<T>(lhs, rhs);
}
public static Criteria<T> operator |(Criteria<T> lhs, Criteria<T> rhs) {
return new OrCriteria<T>(lhs, rhs);
}