I am using lambda expressions to sort and search an array in C#. I don't want to implement the IComparer interface in my class, because I need to sort and search on multiple member fields.
class Widget
{
public int foo;
public void Bar()
{
Widget[] widgets;
Array.Sort(widgets, (a, b) => a.foo.CompareTo(b.foo));
Widget x = new Widget();
x.foo = 5;
int index = Array.BinarySearch(widgets, x,
(a, b) => a.foo.CompareTo(b.foo));
}
}
While the sort works fine, the binary search gives a compilation error Cannot convert lambda expression to type 'System.Collections.IComparer<Widget>' because it is not a delegate type. For some reason, Sort has overloads for both IComparer and Comparison, but BinarySearch only supports IComparer. After some research, I discovered the clunky ComparisonComparer<T>
to convert the Comparison to an IComparer:
public class ComparisonComparer<T> : IComparer<T>
{
private readonly Comparison<T> comparison;
public ComparisonComparer(Comparison<T> comparison)
{
this.comparison = comparison;
}
int IComparer<T>.Compare(T x, T y)
{
return comparison(x, y);
}
}
This allows the binary search to work as follows:
int index = Array.BinarySearch(
widgets,
x,
new ComparisonComparer<Widget>((a, b) => a.foo.CompareTo(b.foo)));
Yuck. Is there a cleaner way?
Well, one option is to create something like
ProjectionComparer
instead. I've got a version of that in MiscUtil - it basically creates anIComparer<T>
from a projection.So your example would be:
Or you could implement your own extension methods on
T[]
to do the same sort of thing: