12/28/2010 Simplifying Value Comparison Semantics

A common chore in developing real-world C# applications is implementing value semantics for equality.  This involves implementing IEquatable<T>, overriding Equals() and GetHashCode(), and overloading the == and != operators.

Implementing these methods is a time-consuming and repetitive task, and is easy to get wrong, especially GetHashCode().  In particular, the best way implement GetHashCode() is much more complicated than return x.GetHashCode() ^ y.GetHashCode().

To simplify this task, I created a ValueComparer class:

///<summary>
/// Contains all of the properties of a class that 
/// are used to provide value semantics.
///</summary>
///<remarks>
/// You can create a static readonly ValueComparer for your class,
/// then call into it from Equals, GetHashCode, and CompareTo.
///</remarks>
class ValueComparer<T> : IComparer<T>, IEqualityComparer<T> {
    public ValueComparer(params Func<T, object>[] props) {
        Properties = new ReadOnlyCollection<Func<T, object>>(props);
    }

    public ReadOnlyCollection<Func<T, object>> Properties
            { get; private set; }

    public bool Equals(T x, T y) {
        if (ReferenceEquals(x, y)) return true;
        if (x == null || y == null) return false;
        //Object.Equals handles strings and nulls correctly
        return Properties.All(f => Equals(f(x), f(y)));    
    }

    //http://stackoverflow.com/questions/263400/263416#263416
    public int GetHashCode(T obj) {
        if (obj == null) return -42;
        unchecked {
            int hash = 17;
            foreach (var prop in Properties) {
                object value = prop(obj);
                if (value == null)
                    hash = hash * 23 - 1;
                else
                    hash = hash * 23 + value.GetHashCode();
            }
            return hash;
        }
    }

    public int Compare(T x, T y) {
        foreach (var prop in Properties) {
            //The properties can be any type including null.
            var comp = Comparer.DefaultInvariant
                .Compare(prop(x), prop(y));    
            if (comp != 0)
                return comp;
        }
        return 0;
    }
}

This class implements an external comparer that compares two instances by an ordered list of properties.

ValueComparer can be used as a standalone IComparer<T> or IEqualityComparer<T> implementation.

It can also be used to implement value semantics within a type.
For example:

class Person : IComparable<Person>, IEquatable<Person>, IComparable {
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Address { get; set; }
    public string Phone { get; set; }
    public string Email { get; set; }

    public override int GetHashCode() { return Comparer.GetHashCode(this); }
    public int CompareTo(Person obj) { return Comparer.Compare(this, obj); }
    int IComparable.CompareTo(object obj) { return CompareTo(obj as Person); }
    public bool Equals(Person obj) { return Comparer.Equals(this, obj); }
    public override bool Equals(object obj) { return Equals(obj as Person); }
    static readonly ValueComparer<Person> Comparer = new ValueComparer<Person>(
        o => o.LastName,
        o => o.FirstName,
        o => o.Address,
        o => o.Phone,
        o => o.Email
    );
}

To simplify this task, I created a code snippet:

<?xml version="1.0" encoding="utf-8" ?>
<CodeSnippets  xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
    <CodeSnippet Format="1.0.0">
        <Header>
            <Title>ValueComparer</Title>
            <Shortcut>vc</Shortcut>
            <Description>Code snippet for equality methods using ValueComparer</Description>
            <Author>SLaks</Author>
            <SnippetTypes>
                <SnippetType>Expansion</SnippetType>
            </SnippetTypes>
        </Header>
        <Snippet>
            <Declarations>
                <Literal Editable="false">
                    <ID>classname</ID>
                    <ToolTip>Class name</ToolTip>
                    <Default>ClassNamePlaceholder</Default>
                    <Function>ClassName()</Function>
                </Literal>
            </Declarations>
            <Code Language="csharp">
                <![CDATA[public override int GetHashCode() { return Comparer.GetHashCode(this); }
        public int CompareTo($classname$ obj) { return Comparer.Compare(this, obj); }
        int IComparable.CompareTo(object obj) { return CompareTo(obj as $classname$); }
        public bool Equals($classname$ obj) { return Comparer.Equals(this, obj); }
        public override bool Equals(object obj) { return Equals(obj as $classname$); }
        static readonly ValueComparer<$classname$> Comparer = new ValueComparer<$classname$>(
            o => o.$end$
        );]]>
            </Code>
        </Snippet>
    </CodeSnippet>
</CodeSnippets>

It can also be downloaded here; save it to My Documents\Visual Studio 2010\Code Snippets\Visual C#\My Code Snippets\