Undefault.NET

Undefault.NET is a small utility library that provides an interface for the configuration of certain .NET framework default implementations.

Injection points are provided for assigning a custom implementation to EqualityComparer<T>.Default and Comparer<T>.Default. These properties play a major role in determining the default behavior of: .NET collections, tuple classes, anonymous objects and LINQ extension methods.

Inspiration

I wrote the code for the initial commit in response to a stackoverflow question with concerns about inconsistencies in how .NET's default IEqualityComparer<Type> handles equality. Specifically, the default implementation is asymmetrical, which violates the definition of equality. My stackoverflow answer provides some explanation of this code and its usage, but I have made a few changes to the API prior to publishing on GitHub.

Example

Below is an example usage of this library that demonstrates how to configure the equality comparer for System.Double to round each value prior to comparing the values. One non-obvious feature that I'd like to point out is that assigning a custom default EqualityComparer<T> for a given type automatically applies a modification to the default Comparer<T> to ensure compatibility. The rule is: for any two T values, x and y, if EqualityComparer<T>.Default.Equals(x,y) == true then Comparer<T>.Default.Compare(x,y) == 0.

WARNING: Please understand that this example is contrived solely for the purpose of demonstration, and that altering the way that primitives work is almost certainly a terrible idea. This library applies modifications at the level of the entire AppDomain and could potentially affect the functionality of any and all referenced assemblies.

void Main()
{	
	var array = new []{ 0.8d, 1.55d, 2.2d };
	
	Console.WriteLine(array.Contains(2d));  // False
	Console.WriteLine(Array.IndexOf(array,2d));  // -1 
	Console.WriteLine(Comparer<double>.Default.Compare(array[1],array[2]));  // -1
	
	Undefault
		.ComparisonConfigurator
		.Configure<double>(new RoundingEqualityComparer());
	
	Console.WriteLine(array.Contains(2d));  // True
	Console.WriteLine(Array.IndexOf(array,2d));  // 1 
	Console.WriteLine(Comparer<double>.Default.Compare(array[1],array[2]));  // 0
}

public class RoundingEqualityComparer : EqualityComparer<double>
{
	public RoundingEqualityComparer(){} 
	public override bool Equals(double x, double y){ return Math.Round(x).Equals(Math.Round(y)); }
	public override int GetHashCode(double x){ return Math.Round(x).GetHashCode(); }
}

The library also provides a method for reverting configurations back to the .NET defaults:

	Undefault.ComparisonConfigurator.Revert<double>();