扩展IQueryable查询的属性

public static IQueryable<T> WhereIf<T>(this IQueryable<T> query, bool condition, Expression<Func<T, bool>> predicate)
{
    return condition ? query.Where(predicate) : query;
}

public static IQueryable<T> OrderByDescendingIf<T, TKey>(this IQueryable<T> query, bool condition, Expression<Func<T, TKey>> keySelector)
{
    return condition ? query.OrderByDescending(keySelector) : query;
}

var parameter = Expression.Parameter(typeof(T), "x");
var property = Expression.Property(parameter, propertyName);
var expression = Expression.Lambda<Func<T, object>>(Expression.Convert(property, typeof(object)), parameter);
return query.Select(expression);