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 an IComparer<T>
from a projection.
So your example would be:
int index = Array.BinarySearch(widgets, x,
ProjectionComparer<Widget>.Create(x => x.foo));
Or you could implement your own extension methods on T[]
to do the same sort of thing:
public static int BinarySearchBy<TSource, TKey>(
this TSource[] array,
TSource value,
Func<TSource, TKey> keySelector)
{
return Array.BinarySearch(array, value,
ProjectionComparer.Create(array, keySelector));
}
You can use my ValueComparer<T>
class:
int index = Array.BinarySearch(
widgets, x,
new ValueComparer<Widget>(x => x.Foo)
);
You can compare by multiple properties by passing multiple lambda expressions.
Try this:
public static class ComparisonEx
{
public static IComparer<T> AsComparer<T>(this Comparison<T> @this)
{
if (@this == null)
throw new System.ArgumentNullException("Comparison<T> @this");
return new ComparisonComparer<T>(@this);
}
public static IComparer<T> AsComparer<T>(this Func<T, T, int> @this)
{
if (@this == null)
throw new System.ArgumentNullException("Func<T, T, int> @this");
return new ComparisonComparer<T>((x, y) => @this(x, y));
}
private class ComparisonComparer<T> : IComparer<T>
{
public ComparisonComparer(Comparison<T> comparison)
{
if (comparison == null)
throw new System.ArgumentNullException("comparison");
this.Comparison = comparison;
}
public int Compare(T x, T y)
{
return this.Comparison(x, y);
}
public Comparison<T> Comparison { get; private set; }
}
}
It lets you use this code:
Comparison<int> c = (x, y) => x == y ? 0 : (x <= y ? -1 : 1);
IComparer<int> icc = c.AsComparer();
Func<int, int, int> f = (x, y) => x == y ? 0 : (x <= y ? -1 : 1);
IComparer<int> icf = f.AsComparer();