swap-netfx4.cs

public static void Swap<T>(Expression<Func<T>> left, Expression<Func<T>> right)
{
    var lvalue = left.Compile()();
    Expression.Lambda<Func<T>>(Expression.Assign(left.Body, Expression.Constant(right.Compile()()))).Compile()();
    Expression.Lambda<Func<T>>(Expression.Assign(right.Body, Expression.Constant(lvalue))).Compile()();
}

public class Test
{
    public int X { get; set; }
    public int Y { get; set; }
}

static void Main()
{
    var obj = new Test() { X = 123, Y = 456, };
    Swap(() => obj.X, () => obj.Y);
    Console.WriteLine("obj.X = {0}, obj.Y = {1}", obj.X, obj.Y);
}