方法一： 安裝官方出的Microsoft.Bcl.Async包
最新發布日期為 2014/4/12，版本1.0.168
（不支持VS2010）
1、解決方案-右鍵-管理解決方案的NuGet程序包
2、搜索Microsoft.Bcl.Async，點擊安裝
3、等待下載完成
4、點擊我接受
5、發現引用下多出7個引用
方法二、
2011年微軟出了async的預覽版，當時使用async的方式如下
1、准備好Visual Studio 2010 + SP1，無論什么版本。
VS2010 SP1升級補丁： Microsoft Visual Studio 2010 Service Pack 1 官方離線下載版（ISO）1.48G地址： http://download.microsoft.com/download/E/B/A/EBA0A152-F426-47E6-9E3F-EFB686E3CA20/VS2010SP1dvd1.iso
2、把
Visual Studio Async CTP下載下來，並安裝。
3、建個項目，現在你已經可以使用
async和
await關鍵字了，而諸如
WebClient下的
DownloadStringTaskAsync方法，則是在
%MyDocument%\Microsoft Visual Studio Async CTP\Samples\AsyncCtpLibrary.dll下定義的擴展方法。
安裝VS2010 SP1 升級程序
如果雙擊安裝程序，出現的界面不是下面，而是標題出現XXX維護
則需要在注冊表下，hkey_current_user\software\microsoft\installer\products\
在Products下找到一個ProductName叫 Microsoft Visual Studio 2010 Service Pack 1的項，並刪除掉項9AC087DE786021C34B9333962F4249F1
再次打開安裝程序就正常了
安裝完SP1后，打開彈框報錯：未能正確加載 “Microsoft.Data.Entity.Design.BootstrapPackage.BootstrapPackage,”
reg query HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\AutoLoadPackages\{adfc4e66-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\AutoLoadPackages\{93694fa0-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\Packages\{7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /s reg query HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\10.0\AutoLoadPackages\{adfc4e66-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\10.0\AutoLoadPackages\{93694fa0-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\10.0\Packages\{7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /s reg query HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\10.0_Config\AutoLoadPackages\{adfc4e66-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query delete HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\10.0_Config\AutoLoadPackages\{93694fa0-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\10.0_Config\Packages\{7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /s reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\AutoLoadPackages\{adfc4e66-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /f reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\AutoLoadPackages\{93694fa0-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /f reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\Packages\{7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /va /f reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\10.0\AutoLoadPackages\{adfc4e66-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /f reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\10.0\AutoLoadPackages\{93694fa0-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /f reg delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\10.0\Packages\{7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /va /f reg delete HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\10.0_Config\AutoLoadPackages\{adfc4e66-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /f reg delete HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\10.0_Config\AutoLoadPackages\{93694fa0-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /f reg delete HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\10.0_Config\Packages\{7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /va /f reg query HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\AutoLoadPackages\{adfc4e66-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\AutoLoadPackages\{93694fa0-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\Packages\{7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /s reg query HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\10.0\AutoLoadPackages\{adfc4e66-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\10.0\AutoLoadPackages\{93694fa0-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\10.0\Packages\{7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /s reg query HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\10.0_Config\AutoLoadPackages\{adfc4e66-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query delete HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\10.0_Config\AutoLoadPackages\{93694fa0-0397-11d1-9f4e-00a0c911004f} /v {7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} reg query HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\10.0_Config\Packages\{7A4E8D96-5D5B-4415-9FAB-D6DCC56F47FB} /s
將上述代碼復制到cmd下即可~如果一般不行來兩遍。