Though TPL is widely accepted as a feature of C# 4.0 , but heard that it is not supported on SL4, but SL5.
Could not find any solid evidence of it.
So why the do MicroSoft implemented a feature in WPF but not Silverlight, though both sl4 and wpf4 released around same time.
If you need TCL in Silverlight 4 you can add the "Microsoft.Bcl" and "Microsoft.Bcl.Async" nuGet packages to your project.
They implement part of what TCL delivers in .net. More info can be found here
Don't forget you have to use the TaskEx
class instead.
Hoped it helped.
来源:https://stackoverflow.com/questions/18396139/which-version-of-silverlight-supports-task-parallel-library