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How to use a custom comparer for EqualityComparer.Default?

How to use the IEqualityComparerHow do you give a C# Auto-Property a default value?How do I enumerate an enum in C#?Why we need the IEqualityComparer,IEqualityComparer<T> interface?Result of calling IEquatable<T>.Equals(T obj) when this == null and obj == null?Not-hash-based set collection for storing unique objects with custom equality comparer - C#What is the difference between using IEqualityComparer and Equals/GethashCode Override?add Equality Comparer class to base class for custom property classes in c#C# Errors when doing a simple datarow comparer64bit HashCodes, IEqualityComparer & Intersect/ExceptEqualityComparer<T>.Default doesn't return the derived EqualityComparer

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-1

Note: my case is for byte[] but I believe a good answer would work for any type.

Visual Studio's auto-generated implementation of Equals uses EqualityComparer<T>.Default.Equals(T x, T y) for reference types. I have a lot of classes with byte arrays that needs to be included in Equals so I'd like to keep Visual studio's code if possible but Default returns a ObjectEqualityComparer for byte arrays. I've written a simple byte array comparer but I'm not sure how to proceed to have it used instead of ObjectEqualityComparer.

public class Foo

 public int Id get;set;
 public byte[] Data get;set;

 public override bool Equals(object obj)
 
 var foo = obj as Foo;
 return foo != null &&
 Id == foo.Id &&
 EqualityComparer<byte[]>.Default.Equals(Data, foo.Data);
 


static void Main

 Foo f1 = new Foo Id = 1, Data = new byte[1] 0xFF ;
 Foo f2 = new Foo Id = 1, Data = new byte[1] 0xFF ;
 bool result = f1.Equals(f2); // false


public class ByteArrayComparer

 public bool Equals(byte[] x, byte[] y)
 
 return x.SequenceEqual(y);
 

 public int GetHashCode(byte[] obj)
 
 return obj.GetHashCode(); 
 // as Servy said, this is wrong but it's not the point of the question, 
 // assume some working implementation
 

Should ByteArrayComparer implement IEqualityComparer, inherit from EqualityComparer and override the methods, or something else?

c# iequalitycomparer

share|improve this question

edited Mar 26 at 16:10

0xFF

asked Mar 26 at 15:59

0xFF0xFF

58811 gold badge88 silver badges2727 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  There is no way to get EqualityComparer<byte[]>.Default to return anything else no matter what you write. It'd be quite the global problem if you could -- whose "global default equality comparer for byte[]" is the right one? A custom type can implement IEquatable, but for the existing system types you don't get to assign new, "more sensible" defaults. The best you can do is create new comparers, and use those explicitly.
  
  – Jeroen Mostert
  Mar 26 at 16:02
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Your comparer doesn't return the same hash code for "equal" arrays, so it doesn't even work. Be thankful you aren't able to make that the default comparer.
  
  – Servy
  Mar 26 at 16:05
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @JeroenMostert Right, I figured it wasn't possible.
  
  – 0xFF
  Mar 26 at 16:07
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  At least it should implement IEqualityComparer<byte[]> to use it e.g. in linq methods, but you will not be able to let EqualityComparer<byte[]>.Default return an instance of your class.
  
  – René Vogt
  Mar 26 at 16:07
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Servy: thanks, you have eloquently elaborated the purpose of my quotes around "correct".
  
  – Jeroen Mostert
  Mar 26 at 16:22
  

  
  
  
  

 | 
show 6 more comments

-1

Note: my case is for byte[] but I believe a good answer would work for any type.

Visual Studio's auto-generated implementation of Equals uses EqualityComparer<T>.Default.Equals(T x, T y) for reference types. I have a lot of classes with byte arrays that needs to be included in Equals so I'd like to keep Visual studio's code if possible but Default returns a ObjectEqualityComparer for byte arrays. I've written a simple byte array comparer but I'm not sure how to proceed to have it used instead of ObjectEqualityComparer.

public class Foo

 public int Id get;set;
 public byte[] Data get;set;

 public override bool Equals(object obj)
 
 var foo = obj as Foo;
 return foo != null &&
 Id == foo.Id &&
 EqualityComparer<byte[]>.Default.Equals(Data, foo.Data);
 


static void Main

 Foo f1 = new Foo Id = 1, Data = new byte[1] 0xFF ;
 Foo f2 = new Foo Id = 1, Data = new byte[1] 0xFF ;
 bool result = f1.Equals(f2); // false


public class ByteArrayComparer

 public bool Equals(byte[] x, byte[] y)
 
 return x.SequenceEqual(y);
 

 public int GetHashCode(byte[] obj)
 
 return obj.GetHashCode(); 
 // as Servy said, this is wrong but it's not the point of the question, 
 // assume some working implementation
 

Should ByteArrayComparer implement IEqualityComparer, inherit from EqualityComparer and override the methods, or something else?

c# iequalitycomparer

share|improve this question

edited Mar 26 at 16:10

0xFF

asked Mar 26 at 15:59

0xFF0xFF

58811 gold badge88 silver badges2727 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  There is no way to get EqualityComparer<byte[]>.Default to return anything else no matter what you write. It'd be quite the global problem if you could -- whose "global default equality comparer for byte[]" is the right one? A custom type can implement IEquatable, but for the existing system types you don't get to assign new, "more sensible" defaults. The best you can do is create new comparers, and use those explicitly.
  
  – Jeroen Mostert
  Mar 26 at 16:02
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Your comparer doesn't return the same hash code for "equal" arrays, so it doesn't even work. Be thankful you aren't able to make that the default comparer.
  
  – Servy
  Mar 26 at 16:05
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @JeroenMostert Right, I figured it wasn't possible.
  
  – 0xFF
  Mar 26 at 16:07
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  At least it should implement IEqualityComparer<byte[]> to use it e.g. in linq methods, but you will not be able to let EqualityComparer<byte[]>.Default return an instance of your class.
  
  – René Vogt
  Mar 26 at 16:07
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Servy: thanks, you have eloquently elaborated the purpose of my quotes around "correct".
  
  – Jeroen Mostert
  Mar 26 at 16:22
  

  
  
  
  

 | 
show 6 more comments

Note: my case is for byte[] but I believe a good answer would work for any type.

Visual Studio's auto-generated implementation of Equals uses EqualityComparer<T>.Default.Equals(T x, T y) for reference types. I have a lot of classes with byte arrays that needs to be included in Equals so I'd like to keep Visual studio's code if possible but Default returns a ObjectEqualityComparer for byte arrays. I've written a simple byte array comparer but I'm not sure how to proceed to have it used instead of ObjectEqualityComparer.

public class Foo

 public int Id get;set;
 public byte[] Data get;set;

 public override bool Equals(object obj)
 
 var foo = obj as Foo;
 return foo != null &&
 Id == foo.Id &&
 EqualityComparer<byte[]>.Default.Equals(Data, foo.Data);
 


static void Main

 Foo f1 = new Foo Id = 1, Data = new byte[1] 0xFF ;
 Foo f2 = new Foo Id = 1, Data = new byte[1] 0xFF ;
 bool result = f1.Equals(f2); // false


public class ByteArrayComparer

 public bool Equals(byte[] x, byte[] y)
 
 return x.SequenceEqual(y);
 

 public int GetHashCode(byte[] obj)
 
 return obj.GetHashCode(); 
 // as Servy said, this is wrong but it's not the point of the question, 
 // assume some working implementation
 

Should ByteArrayComparer implement IEqualityComparer, inherit from EqualityComparer and override the methods, or something else?

c# iequalitycomparer

share|improve this question

edited Mar 26 at 16:10

0xFF

asked Mar 26 at 15:59

0xFF0xFF

58811 gold badge88 silver badges2727 bronze badges

public class Foo

 public int Id get;set;
 public byte[] Data get;set;

 public override bool Equals(object obj)
 
 var foo = obj as Foo;
 return foo != null &&
 Id == foo.Id &&
 EqualityComparer<byte[]>.Default.Equals(Data, foo.Data);
 


static void Main

 Foo f1 = new Foo Id = 1, Data = new byte[1] 0xFF ;
 Foo f2 = new Foo Id = 1, Data = new byte[1] 0xFF ;
 bool result = f1.Equals(f2); // false


public class ByteArrayComparer

 public bool Equals(byte[] x, byte[] y)
 
 return x.SequenceEqual(y);
 

 public int GetHashCode(byte[] obj)
 
 return obj.GetHashCode(); 
 // as Servy said, this is wrong but it's not the point of the question, 
 // assume some working implementation
 

share|improve this question

edited Mar 26 at 16:10

0xFF

asked Mar 26 at 15:59

0xFF0xFF

58811 gold badge88 silver badges2727 bronze badges

share|improve this question

edited Mar 26 at 16:10

0xFF

asked Mar 26 at 15:59

0xFF0xFF

58811 gold badge88 silver badges2727 bronze badges

asked Mar 26 at 15:59

0xFF0xFF

58811 gold badge88 silver badges2727 bronze badges

asked Mar 26 at 15:59

0xFF0xFF

58811 gold badge88 silver badges2727 bronze badges

1 Answer
1

active

oldest

votes

1

Create and use an instance of your custom comparer instead of using EqualityComparer<byte[]>.Default in your class:

public class Foo

 public int Id get; set; 
 public byte[] Data get; set; 

 private readonly ByteArrayComparer _comparer = new ByteArrayComparer();

 public override bool Equals(object obj)
 
 var foo = obj as Foo;
 return foo != null &&
 Id == foo.Id &&
 _comparer.Equals(Data, foo.Data);
 

You may also want to implement IEqualityComparer<T> and GetHashCode() in your ByteArrayComparer class. EqualityComparer<T>.Default returns an instance of a class that implements this interface, but I assume you don't want to use this one as you have implemented your own custom comparer.

How to use the IEqualityComparer

share|improve this answer

edited Mar 27 at 14:50

answered Mar 26 at 16:01

mm8mm8

97.3k99 gold badges1919 silver badges3434 bronze badges

add a comment | 

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1

Create and use an instance of your custom comparer instead of using EqualityComparer<byte[]>.Default in your class:

public class Foo

 public int Id get; set; 
 public byte[] Data get; set; 

 private readonly ByteArrayComparer _comparer = new ByteArrayComparer();

 public override bool Equals(object obj)
 
 var foo = obj as Foo;
 return foo != null &&
 Id == foo.Id &&
 _comparer.Equals(Data, foo.Data);
 

You may also want to implement IEqualityComparer<T> and GetHashCode() in your ByteArrayComparer class. EqualityComparer<T>.Default returns an instance of a class that implements this interface, but I assume you don't want to use this one as you have implemented your own custom comparer.

How to use the IEqualityComparer

share|improve this answer

edited Mar 27 at 14:50

answered Mar 26 at 16:01

mm8mm8

97.3k99 gold badges1919 silver badges3434 bronze badges

add a comment | 

1

Create and use an instance of your custom comparer instead of using EqualityComparer<byte[]>.Default in your class:

public class Foo

 public int Id get; set; 
 public byte[] Data get; set; 

 private readonly ByteArrayComparer _comparer = new ByteArrayComparer();

 public override bool Equals(object obj)
 
 var foo = obj as Foo;
 return foo != null &&
 Id == foo.Id &&
 _comparer.Equals(Data, foo.Data);
 

You may also want to implement IEqualityComparer<T> and GetHashCode() in your ByteArrayComparer class. EqualityComparer<T>.Default returns an instance of a class that implements this interface, but I assume you don't want to use this one as you have implemented your own custom comparer.

How to use the IEqualityComparer

share|improve this answer

edited Mar 27 at 14:50

answered Mar 26 at 16:01

mm8mm8

97.3k99 gold badges1919 silver badges3434 bronze badges

add a comment | 

Create and use an instance of your custom comparer instead of using EqualityComparer<byte[]>.Default in your class:

public class Foo

 public int Id get; set; 
 public byte[] Data get; set; 

 private readonly ByteArrayComparer _comparer = new ByteArrayComparer();

 public override bool Equals(object obj)
 
 var foo = obj as Foo;
 return foo != null &&
 Id == foo.Id &&
 _comparer.Equals(Data, foo.Data);
 

You may also want to implement IEqualityComparer<T> and GetHashCode() in your ByteArrayComparer class. EqualityComparer<T>.Default returns an instance of a class that implements this interface, but I assume you don't want to use this one as you have implemented your own custom comparer.

How to use the IEqualityComparer

share|improve this answer

edited Mar 27 at 14:50

answered Mar 26 at 16:01

mm8mm8

97.3k99 gold badges1919 silver badges3434 bronze badges

public class Foo

 public int Id get; set; 
 public byte[] Data get; set; 

 private readonly ByteArrayComparer _comparer = new ByteArrayComparer();

 public override bool Equals(object obj)
 
 var foo = obj as Foo;
 return foo != null &&
 Id == foo.Id &&
 _comparer.Equals(Data, foo.Data);
 

share|improve this answer

edited Mar 27 at 14:50

answered Mar 26 at 16:01

mm8mm8

97.3k99 gold badges1919 silver badges3434 bronze badges

answered Mar 26 at 16:01

mm8mm8

97.3k99 gold badges1919 silver badges3434 bronze badges

answered Mar 26 at 16:01

mm8mm8

97.3k99 gold badges1919 silver badges3434 bronze badges