CSHARP.IDEF.EQUALSNOHC : Defines equals but not hashCode (C#)

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CSHARP.IDEF.EQUALSNOHC : Defines equals but not hashCode (C#)

Summary

Redefinitions of the Equals()/GetHashCode() methods from System.Object must be consistent in the sense that, for instance, if two objects are Equals() then GetHashCode() must yield the same value on both. The validity of such a property is in general undecidable, but most incorrect definitions amount to simple cases, where one of the two methods is missing. However, it is often correct to redefine only one of those methods and the analyzer is aware of many such situations.

Inconsistent definitions of Equals()/GetHashCode() induce unexpected behaviors when objects are put inside most collection classes of the standard .NET library.

Properties

Example


namespace DocumentationExamples
{


    public class EqualsHashCode
    {
        public static void Main(string[] args)
        { } 

        private readonly string name;
        public EqualsHashCode(string name)
        {
            this.name = name;
        }
        public override bool Equals(object other) // "Defines equals but not hashCode (C#)" warning issued here
        {
            return other is EqualsHashCode && ((EqualsHashCode)other).name.Equals(name);
        }
    }

    public class EqualsHashCodeB
    {
        private readonly string name;
        public EqualsHashCodeB(string name)
        {
            this.name = name;
        }
        public override int GetHashCode()
        {
            return name.GetHashCode();
        }
        public override bool Equals(object other)                // ok: class also overrides GetHashCode()
        {
            return other is EqualsHashCodeB && ((EqualsHashCodeB)other).name.Equals(name);
        }
    }
    public class EqualsHashCodeC : EqualsHashCodeB
    {
        private readonly int age;
        public EqualsHashCodeC(string name, int age) : base(name)
        {
            this.age = age;
        }
        public override bool Equals(object other)                 // ok: superclass overrides GetHashCode() and this redefinition of Equals() requires equality with respect to the superclass.
        {
            return base.Equals(other) && other is EqualsHashCodeC && ((EqualsHashCodeC)other).age == age;
        }
    }
}

In this example, the programmer should add the following definition (or similar) to class EqualsHashCode.

public override int GetHashCode() {
    return name.GetHashCode();
}

Resolution

Make Equals() consistent with GetHashCode(). In many cases, this just amounts to provide the missing definition for one of them.

Relevant Configuration File Parameters

The following configuration file parameters affect checks for this warning class.