C# lambda expressions and IComparer

问题:

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?

回答1:

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)); } 

回答2:

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.

回答3:

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(); 

文章来源: C# lambda expressions and IComparer