参考http://www.cnblogs.com/shenggege/p/5165616.html
最近从unity5.1.3升级到5.3.4的时候，发现有个问题：
system.io.file' does not contain a definition for readalltext on webplayer in Unity, 上网查资料，有个老外说This is due to the sandboxing security of the webplayer, which has certain limitations imposed to prevent malicious code being executed by a unity application via the browser.
Unity's docs include a page explaining this:
http://docs.unity3d.com/Manual/SecuritySandbox.html
Which includes the following brief description of the limitations:
- Restrictions on accessing data on a domain other than the one hosting your .unity3d file.
- Some limitation on the usage of the Sockets.
- Disallowing invocation of any method we deemed off limits. (things
like File.Delete, etc). - Disallowing the usage of System.Reflection.* to call private/internal methods in classes you did not write yourself.
In short, you cannot access the filesystem directly using the System.IO namespace. This has been disabled intentionally by Unity.
The best practice is to use the PlayerPrefs, or to load/save data via a remote web service. There's a discussion on the topic here:
传送门：http://*.com/questions/29233253/system-io-file-does-not-contain-a-definition-for-readalltext-on-webplayer-in-un
既需要转换platform，但是发现按钮是灰色的，点击右边的Module Manager按钮也不管用，如图：
只好重新打开安装助手，勾选Android Build Support安装即可。ios也一样。