Question

Implementing Equals() for reference types is harder than it seems. My current canonical implementation goes like this:

public bool Equals( MyClass obj ) {   // If both refer to the same reference they are equal.   if( ReferenceEquals( obj, this ) )     return true;    // If the other object is null they are not equal because in C# this cannot be null.   if( ReferenceEquals( obj, null ) )    return false;     // Compare data to evaluate equality        return _data.Equals( obj._data ); }  public override bool Equals( object obj ) {   // If both refer to the same reference they are equal.   if( ReferenceEquals( obj, this ) )     return true;    // If the other object is null or is of a different types the objects are not equal.    if( ReferenceEquals( obj, null ) || obj.GetType() != GetType() )     return false;    // Use type-safe equality comparison   return Equals( (MyClass)obj ); }  public override int GetHashCode() {   // Use data's hash code as our hashcode     return _data.GetHashCode(); } 

I think that this covers all corner (inheritance and such) cases but I may be wrong. What do you guys think?

Answer 1

I wrote a fairly comprehensive guide to this a while back. For a start your equals implementations should be shared (i.e. the overload taking an object should pass through to the one taking a strongly typed object). Additionally you need to consider things such as your object should be immutable because of the need to override GetHashCode. More info here:

http://gregbeech.com/blog/implementing-object-equality-in-dotnet