c# - How to best implement Equals for custom types?

Question

Say for a Point2 class, and the following Equals:

public override bool Equals ( object obj )

public bool Equals ( Point2 obj )

This is the one that is shown in the Effective C# 3:

public override bool Equals ( object obj )
{
    // STEP 1: Check for null
    if ( obj == null )
    {
        return false;
    }

    // STEP 3: equivalent data types
    if ( this.GetType ( ) != obj.GetType ( ) )
    {
        return false;
    }
    return Equals ( ( Point2 ) obj );
}

public bool Equals ( Point2 obj )
{
    // STEP 1: Check for null if nullable (e.g., a reference type)
    if ( obj == null )
    {
        return false;
    }
    // STEP 2: Check for ReferenceEquals if this is a reference type
    if ( ReferenceEquals ( this, obj ) )
    {
        return true;
    }
    // STEP 4: Possibly check for equivalent hash codes
    if ( this.GetHashCode ( ) != obj.GetHashCode ( ) )
    {
        return false;
    }
    // STEP 5: Check base.Equals if base overrides Equals()
    System.Diagnostics.Debug.Assert (
        base.GetType ( ) != typeof ( object ) );

    if ( !base.Equals ( obj ) )
    {
        return false;
    }

    // STEP 6: Compare identifying fields for equality.
    return ( ( this.X.Equals ( obj.X ) ) && ( this.Y.Equals ( obj.Y ) ) );
}

question from:https://stackoverflow.com/questions/567642/how-to-best-implement-equals-for-custom-types

Answer 1

There is a whole set of guidelines on MSDN as well. You should read them well, it is both tricky and important.

A few points I found most helpful:

• Value Types don't have Identity, so in a struct Point you will usually do a member by member compare.

• Reference Types usually do have identity, and therefore the Equals test usually stops at ReferenceEquals (the default, no need to override). But there are exceptions, like string and your class Point2, where an object has no useful identity and then you override the Equality members to provide your own semantics. In that situation, follow the guidelines to get through the null and other-type cases first.

• And there are good reasons to keep GethashCode() and operator== in sync as well.