CA2218: Override GetHashCode on overriding Equals

 

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The latest version of this topic can be found at CA2218: Override GetHashCode on overriding Equals.

TypeName|OverrideGetHashCodeOnOverridingEquals|
|CheckId|CA2218|
|Category|Microsoft.Usage|
|Breaking Change|Non Breaking|

A public type overrides Object.Equals but does not override Object.GetHashCode.

GetHashCode returns a value, based on the current instance, that is suited for hashing algorithms and data structures such as a hash table. Two objects that are the same type and are equal must return the same hash code to ensure that instances of the following types work correctly:

To fix a violation of this rule, provide an implementation of GetHashCode. For a pair of objects of the same type, you must ensure that the implementation returns the same value if your implementation of Equals returns true for the pair.

Do not suppress a warning from this rule.

Description

The following example shows a class (reference type) that violates this rule.

Code

using System; 

namespace Samples
{    
    // Violates this rule    
    public class Point    
    {        
        private readonly int _X;        
        private readonly int _Y;         
        
        public Point(int x, int y)        
        {            
            _X = x;            
            _Y = y;        
        }         
        
        public int X        
        {            
            get { return _X; }        
        }         
        
        public int Y        
        {            
            get { return _Y; }        
        }               
        
        public override bool Equals(object obj)        
        {            
            if (obj == null)                
                return false;             
            
            if (GetType() != obj.GetType())                
                return false;             
                
            Point point = (Point)obj;             
            
            if (_X != point.X)                
                return false;             
                
            return _Y == point.Y;        
        }    
    }
}

Comments

The following example fixes the violation by overriding T:System.Object.GetHashCode.

Code

using System; 

namespace Samples
{    
    public struct Point : IEquatable<Point>    
    {        
        private readonly int _X;        
        private readonly int _Y;         
        
        public Point(int x, int y)        
        {            
            _X = x;            
            _Y = y;        
        }         
        
        public int X        
        {            
            get { return _X; }        
        }         
        
        public int Y        
        {            
            get { return _Y; }        
        }         
        
        public override int GetHashCode()        
        {            
            return _X ^ _Y;        
        }         
        
        public override bool Equals(object obj)        
        {           
            if (!(obj is Point))                
                return false;             
                
            return Equals((Point)obj);        
        }         
        
        public bool Equals(Point other)        
        {            
            if (_X != other._X)                
                return false;             
                
            return _Y == other._Y;        
        }         
        
        public static bool operator ==(Point point1, Point point2)        
        {            
            return point1.Equals(point2);        
        }         
        
        public static bool operator !=(Point point1, Point point2)        
        {            
            return !point1.Equals(point2);        
        }    
    }
}

Description

The following example shows a structure (value type) that violates this rule.

Code

using System; 

namespace Samples
{    
    // Violates this rule    
    public struct Point : IEquatable<Point>    
    {        
        private readonly int _X;        
        private readonly int _Y;         
        
        public Point(int x, int y)        
        {            
            _X = x;            
            _Y = y;        
        }         
        
        public int X        
        {            
            get { return _X; }        
        }         
        
        public int Y        
        {            
            get { return _Y; }        
        }         
        
        public override bool Equals(object obj)        
        {           
            if (!(obj is Point))                
                return false;             
                
            return Equals((Point)obj);        
        }         
        
        public bool Equals(Point other)        
        {            
            if (_X != other._X)                
                return false;             
                
            return _Y == other._Y;        
        }         
        
        public static bool operator ==(Point point1, Point point2)        
        {            
            return point1.Equals(point2);        
        }         
        
        public static bool operator !=(Point point1, Point point2)        
        {            
            return !point1.Equals(point2);        
        }    
    }
}

Comments

The following example fixes the violation by overriding T:System.Object.GetHashCode.

Code

using System; 

namespace Samples
{    
    public struct Point : IEquatable<Point>    
    {        
        private readonly int _X;        
        private readonly int _Y;         
        
        public Point(int x, int y)        
        {            
            _X = x;            
            _Y = y;        
        }         
        
        public int X        
        {            
            get { return _X; }        
        }         
        
        public int Y        
        {            
            get { return _Y; }        
        }         
        
        public override int GetHashCode()        
        {            
            return _X ^ _Y;        
        }         
        
        public override bool Equals(object obj)        
        {           
            if (!(obj is Point))                
                return false;             
            
            return Equals((Point)obj);        
        }         
        
        public bool Equals(Point other)        
        {            
            if (_X != other._X)                
                return false;             
                
            return _Y == other._Y;        
        }         
        
        public static bool operator ==(Point point1, Point point2)        
        {            
            return point1.Equals(point2);        
        }         
        
        public static bool operator !=(Point point1, Point point2)        
        {            
            return !point1.Equals(point2);        
        }    
    }
}

CA1046: Do not overload operator equals on reference types

CA2225: Operator overloads have named alternates

CA2226: Operators should have symmetrical overloads

CA2224: Override equals on overloading operator equals

CA2231: Overload operator equals on overriding ValueType.Equals

Object.Equals
Object.GetHashCode
T:System.Collections.HashTable
Equality Operators

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