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  "contentMarkdown": "# IEnumerable\n\n> 创建时间：2020/5/18 13:20\n\nenumerable 可数的，可枚举的\n在使用Linq查询数据时经常以IEnumerable来作为数据查询返回对象\n使用foreach进行遍历时需要该对象实现IEnumerable接口\n定义了一个GetEnumerator()方法\n依靠MoveNext和Current来达到Foreach的遍历\n\n定义一个集合FruitShop类，它是一个关于Fruit对象的集合\n流程:\n第一次遇到foreach里的fruitShop对象时就会去执行FruitShop中的GetEnumerator方法\n接着每次执行in关键字就会去执行MoveNext方法\n每次取数据则是调用Current属性\n\n```csharp\nclass MyClass<T, U>\n    where T : class\n    where U : struct\n{ }\n\n```\n\nIEnumerable还有4个扩展方法\n\n  * Cast()\n\n  * OfType()\n非泛型集合比如ArrayList，它只实现了IEnumerable接口而没有实现IEnumerable接口，因此无法使用标准查询运算。\n但是标准查询运算如此的方便，.NET肯定不会允许这样的不完美发生，于是为IEnumerable提供了2个扩展方法进行强制转换。\nOfType比Cast更加强大，它除了进行强制转换外还可以实现类型的过滤，遇到无法强制转换的数据将不会转换\n进行转换时OfType是首选\n\n```csharp\nclass MyClass<T, U>\n    where T : class\n    where U : struct\n{ }\n\n```\n"
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