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IEqualityComparer that uses ReferenceEquals

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野趣味
野趣味 2020-11-28 09:33

Is there a default IEqualityComparer implementation that uses ReferenceEquals?

EqualityComparer.Default uses

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  • 我在风中等你
    我在风中等你 (楼主)
    2020-11-28 10:17

    I thought it was time to update the previous answers implementation to .Net4.0+ where it simplifies by becoming non-generic thanks to contravariance on the IEqualityComparer interface:

    using System.Collections;
using System.Collections.Generic;
using System.Runtime.CompilerServices;

public sealed class ReferenceEqualityComparer
    : IEqualityComparer, IEqualityComparer
{
    public static readonly ReferenceEqualityComparer Default
        = new ReferenceEqualityComparer(); // JIT-lazy is sufficiently lazy imo.

    private ReferenceEqualityComparer() { } // <-- A matter of opinion / style.

    public bool Equals(object x, object y)
    {
        return x == y; // This is reference equality! (See explanation below.)
    }

    public int GetHashCode(object obj)
    {
        return RuntimeHelpers.GetHashCode(obj);
    }
}



    Now there only needs to exist one instance for all your reference-equality checking instead of one for each type T as was the case before.
    


    Also you save typing by not having to specify T every time you want to use this!
    


    


    To clarify for those who are not familiar with the concepts of Covariance and Contravariance...
    


    class MyClass
{
    ISet setOfMyClass = new HashSet(ReferenceEqualityComparer.Default);
}

    


    ...will work just fine. This is not limited to e.g. HashSet or similar (in .Net4.0).
    


    


    Also for anyone wondering why x == y is reference equality, it is because the ==operator is a static method, which means it is resolved at compile-time, and at compile-time x and y are of type object so here it resolves to the ==operator of object - which is the real reference equality method. (In fact the Object.ReferenceEquals(object, object) method is simply a redirect to the object equals operator.)
    
    
    
             
    
                                                        
    
            
            
    
              
                
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