Auto-filter for Microsoft WPF DataGrid

//******************************
// Written by Peter Golde
// Copyright (c) 2004-2005, Wintellect
//
// Use and restribution of this code is subject to the license agreement 
// contained in the file "License.txt" accompanying this file.
//******************************

using System;
using System.Collections.Generic;

namespace Wintellect.PowerCollections
{
    /// <summary>
    /// Stores a pair of objects within a single struct. This struct is useful to use as the
    /// T of a collection, or as the TKey or TValue of a dictionary.
    /// </summary>
    [Serializable]
    public struct Pair<TFirst, TSecond> : IComparable, IComparable<Pair<TFirst, TSecond>>
    {
        /// <summary>
        /// Comparers for the first and second type that are used to compare
        /// values.
        /// </summary>

        private static IComparer<TFirst> firstComparer = Comparer<TFirst>.Default;
        private static IComparer<TSecond> secondComparer = Comparer<TSecond>.Default;

        private static IEqualityComparer<TFirst> firstEqualityComparer = EqualityComparer<TFirst>.Default;
        private static IEqualityComparer<TSecond> secondEqualityComparer = EqualityComparer<TSecond>.Default;

        /// <summary>
        /// The first element of the pair.
        /// </summary>
        public TFirst First;

        /// <summary>
        /// The second element of the pair.
        /// </summary>
        public TSecond Second;

        /// <summary>
        /// Creates a new pair with given first and second elements.
        /// </summary>
        /// <param name="first">The first element of the pair.</param>
        /// <param name="second">The second element of the pair.</param>
        public Pair(TFirst first, TSecond second)
        {
            this.First = first;
            this.Second = second;
        }

        /// <summary>
        /// Creates a new pair using elements from a KeyValuePair structure. The
        /// First element gets the Key, and the Second elements gets the Value.
        /// </summary>
        /// <param name="keyAndValue">The KeyValuePair to initialize the Pair with .</param>
        public Pair(KeyValuePair<TFirst,TSecond> keyAndValue)
        {
            this.First = keyAndValue.Key;
            this.Second = keyAndValue.Value;
        }

        /// <summary>
        /// Determines if this pair is equal to another object. The pair is equal to another object 
        /// if that object is a Pair, both element types are the same, and the first and second elements
        /// both compare equal using object.Equals.
        /// </summary>
        /// <param name="obj">Object to compare for equality.</param>
        /// <returns>True if the objects are equal. False if the objects are not equal.</returns>
        public override bool Equals(object obj)
        {
            if (obj != null && obj is Pair<TFirst, TSecond>) {
                Pair<TFirst, TSecond> other = (Pair<TFirst, TSecond>)obj;

                return Equals(other);
            }
            else {
                return false;
            }
        }

        /// <summary>
        /// Determines if this pair is equal to another pair. The pair is equal if  the first and second elements
        /// both compare equal using IComparable&lt;T&gt;.Equals or object.Equals.
        /// </summary>
        /// <param name="other">Pair to compare with for equality.</param>
        /// <returns>True if the pairs are equal. False if the pairs are not equal.</returns>
        public bool Equals(Pair<TFirst, TSecond> other)
        {
            return firstEqualityComparer.Equals(First, other.First) && secondEqualityComparer.Equals(Second, other.Second);
        }

        /// <summary>
        /// Returns a hash code for the pair, suitable for use in a hash-table or other hashed collection.
        /// Two pairs that compare equal (using Equals) will have the same hash code. The hash code for
        /// the pair is derived by combining the hash codes for each of the two elements of the pair.
        /// </summary>
        /// <returns>The hash code.</returns>
        public override int GetHashCode()
        {
            // Build the hash code from the hash codes of First and Second. 
            int hashFirst = (First == null) ? 0x61E04917 : First.GetHashCode();
            int hashSecond = (Second == null) ? 0x198ED6A3 : Second.GetHashCode();
            return hashFirst ^ hashSecond;
        }

        /// <summary>
        /// <para> Compares this pair to another pair of the some type. The pairs are compared by using
        /// the IComparable&lt;T&gt; or IComparable interface on TFirst and TSecond. The pairs
        /// are compared by their first elements first, if their first elements are equal, then they
        /// are compared by their second elements.</para>
        /// <para>If either TFirst or TSecond does not implement IComparable&lt;T&gt; or IComparable, then
        /// an NotSupportedException is thrown, because the pairs cannot be compared.</para>
        /// </summary>
        /// <param name="other">The pair to compare to.</param>
        /// <returns>An integer indicating how this pair compares to <paramref name="other"/>. Less
        /// than zero indicates this pair is less than <paramref name="other"/>. Zero indicate this pair is
        /// equals to <paramref name="other"/>. Greater than zero indicates this pair is greater than
        /// <paramref name="other"/>.</returns>
        /// <exception cref="NotSupportedException">Either FirstSecond or TSecond is not comparable
        /// via the IComparable&lt;T&gt; or IComparable interfaces.</exception>
        public int CompareTo(Pair<TFirst, TSecond> other)
        {
            try {
                int firstCompare = firstComparer.Compare(First, other.First);
                if (firstCompare != 0)
                    return firstCompare;
                else
                    return secondComparer.Compare(Second, other.Second);
            }
            catch (ArgumentException) {
                // Determine which type caused the problem for a better error message.
                if (!typeof(IComparable<TFirst>).IsAssignableFrom(typeof(TFirst)) &&
                    !typeof(System.IComparable).IsAssignableFrom(typeof(TFirst))) {
                    throw new NotSupportedException(string.Format(Strings.UncomparableType, typeof(TFirst).FullName));
                }
                else if (!typeof(IComparable<TSecond>).IsAssignableFrom(typeof(TSecond)) &&
                    !typeof(System.IComparable).IsAssignableFrom(typeof(TSecond))) {
                    throw new NotSupportedException(string.Format(Strings.UncomparableType, typeof(TSecond).FullName));
                }
                else
                    throw;              // Hmmm. Unclear why we got the ArgumentException. 
            }
        }

        /// <summary>
        /// <para> Compares this pair to another pair of the some type. The pairs are compared by using
        /// the IComparable&lt;T&gt; or IComparable interface on TFirst and TSecond. The pairs
        /// are compared by their first elements first, if their first elements are equal, then they
        /// are compared by their second elements.</para>
        /// <para>If either TFirst or TSecond does not implement IComparable&lt;T&gt; or IComparable, then
        /// an NotSupportedException is thrown, because the pairs cannot be compared.</para>
        /// </summary>
        /// <param name="obj">The pair to compare to.</param>
        /// <returns>An integer indicating how this pair compares to <paramref name="obj"/>. Less
        /// than zero indicates this pair is less than <paramref name="other"/>. Zero indicate this pair is
        /// equals to <paramref name="obj"/>. Greater than zero indicates this pair is greater than
        /// <paramref name="obj"/>.</returns>
        /// <exception cref="ArgumentException"><paramref name="obj"/> is not of the correct type.</exception>
        /// <exception cref="NotSupportedException">Either FirstSecond or TSecond is not comparable
        /// via the IComparable&lt;T&gt; or IComparable interfaces.</exception>
        int IComparable.CompareTo(object obj)
        {
            if (obj is Pair<TFirst, TSecond>)
                return CompareTo((Pair<TFirst, TSecond>)obj);
            else
                throw new ArgumentException(Strings.BadComparandType, "obj");
        }

        /// <summary>
        /// Returns a string representation of the pair. The string representation of the pair is
        /// of the form:
        /// <c>First: {0}, Second: {1}</c>
        /// where {0} is the result of First.ToString(), and {1} is the result of Second.ToString() (or
        /// "null" if they are null.)
        /// </summary>
        /// <returns> The string representation of the pair.</returns>
        public override string ToString()
        {
            return string.Format("First: {0}, Second: {1}", (First == null) ? "null" : First.ToString(), (Second == null) ? "null" : Second.ToString());
        }

        /// <summary>
        /// Determines if two pairs are equal. Two pairs are equal if  the first and second elements
        /// both compare equal using IComparable&lt;T&gt;.Equals or object.Equals.
        /// </summary>
        /// <param name="pair1">First pair to compare.</param>
        /// <param name="pair2">Second pair to compare.</param>
        /// <returns>True if the pairs are equal. False if the pairs are not equal.</returns>
        public static bool operator ==(Pair<TFirst, TSecond> pair1, Pair<TFirst, TSecond> pair2)
        {
            return firstEqualityComparer.Equals(pair1.First, pair2.First) && secondEqualityComparer.Equals(pair1.Second, pair2.Second);
        }

        /// <summary>
        /// Determines if two pairs are not equal. Two pairs are equal if  the first and second elements
        /// both compare equal using IComparable&lt;T&gt;.Equals or object.Equals.
        /// </summary>
        /// <param name="pair1">First pair to compare.</param>
        /// <param name="pair2">Second pair to compare.</param>
        /// <returns>True if the pairs are not equal. False if the pairs are equal.</returns>
        public static bool operator !=(Pair<TFirst, TSecond> pair1, Pair<TFirst, TSecond> pair2)
        {
            return !(pair1 == pair2);
        }

        /// <summary>
        /// Converts a Pair to a KeyValuePair. The Key part of the KeyValuePair gets
        /// the First element, and the Value part of the KeyValuePair gets the Second 
        /// elements.
        /// </summary>
        /// <param name="pair">Pair to convert.</param>
        /// <returns>The KeyValuePair created from <paramref name="pair"/>.</returns>
        public static explicit operator KeyValuePair<TFirst,TSecond>(Pair<TFirst, TSecond> pair)
        {
            return new KeyValuePair<TFirst, TSecond>(pair.First, pair.Second);
        }

        /// <summary>
        /// Converts this Pair to a KeyValuePair. The Key part of the KeyValuePair gets
        /// the First element, and the Value part of the KeyValuePair gets the Second 
        /// elements.
        /// </summary>
        /// <returns>The KeyValuePair created from this Pair.</returns>
        public KeyValuePair<TFirst, TSecond> ToKeyValuePair()
        {
            return new KeyValuePair<TFirst, TSecond>(this.First, this.Second);
        }

        /// <summary>
        /// Converts a KeyValuePair structure into a Pair. The
        /// First element gets the Key, and the Second element gets the Value.
        /// </summary>
        /// <param name="keyAndValue">The KeyValuePair to convert.</param>
        /// <returns>The Pair created by converted the KeyValuePair into a Pair.</returns>
        public static explicit operator Pair<TFirst, TSecond>(KeyValuePair<TFirst, TSecond> keyAndValue)
        {
            return new Pair<TFirst, TSecond>(keyAndValue);
        }
    }
}