I want my entity classes to rely on their ID when checking equality, the ID is populated from the repository when I retrieve an object and automatically generated when I save a new entity.

This poses the problem of how I compare two entities before they have been saved while working with them in my domain, or, when I have ‘newed’ them up myself for unit testing. The answer is to use a transient ID.

Normally an Entity Base class might look like this:

public abstract class EntityBase : IEquatable<EntityBase> {

    public virtual int Id { get; protected set; }

    public override bool Equals(object other) {
        return Equals(other as EntityBase);
    }

    public override int GetHashCode() {
        return Id;
    }

    public virtual bool Equals(EntityBase other) {
        if (other == null) return false;
        return other.Id == Id && other.GetType() == GetType();
    }

}

If two new objects are created from a derived class then object1.Equals(object2) will be true as Id will be 0 for both objects. To avoid this I need to check if an object has not yet been saved:

private bool IsTransient {
    get { return Id == 0; }
}

And just check for referential equality if this is the case:

public virtual bool Equals(EntityBase other) {
    if (other == null) return false;
    if (IsTransient) return ReferenceEquals(this, other);
    return other.Id == Id && other.GetType() == GetType();
}

GetHashCode() needs to be overridden to reflect this:

public override int GetHashCode() {
    if (IsTransient) return base.GetHashCode();
    return Id;
}

The full code for EntityBase is:

public abstract class EntityBase : IEquatable<EntityBase> {

    public virtual int Id { get; protected set; }

    private bool IsTransient {
        get { return Id == 0; }
    }

    public override bool Equals(object other) {
        return Equals(other as EntityBase);
    }

    public override int GetHashCode() {
        if (IsTransient) return base.GetHashCode();
        return Id;
    }

    public virtual bool Equals(EntityBase other) {
        if (other == null) return false;
        if (IsTransient) return ReferenceEquals(this, other);
        return other.Id == Id && other.GetType() == GetType();
    }

}