IEnumerable扩展支持Add，Remove等操作

public static class CollectionExtend
{
/// <summary>
/// 向集合中添加元素
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="collection"></param>
/// <param name="value"></param>
public static void Add<T>(this IEnumerable<T> collection, T value)
{
var list = collection as List<T>;
if (list != null) list.Add(value);
}

/// <summary>
/// 从集合中删除元素
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="collection"></param>
/// <param name="value"></param>
public static void Remove<T>(this IEnumerable<T> collection, T value)
{
var list = collection as List<T>;
if (list != null) list.Remove(value);
}
/// <summary>
/// 检索集合中是否包含某个元素
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="collection"></param>
/// <param name="value"></param>
/// <returns></returns>
public static bool Contains<T>(this IEnumerable<T> collection, T value)
{
var list = collection as List<T>;
return list != null && list.Contains(value);
}
}