在登录系统，通过linq查询时发生错误，the underlying provider failed on open，如何解决，请看：
Step 1:
Open Internet Information Service Manager
Step 2:
Click on Application Pools in left navigation tree.
Step 3:
Select your version Pool. In my case, I am using ASP .Net v4.0. If you dont have this version, select DefaultAppPool.
Step 4:
Right click on step 3, and select advanced settings.
Step 5:
Select Identity in properties window and click the button to change the value.
Step 6:
Select Local System in Built-in accounts combo box and click ok.
That's it. Now run your application. Everything works well.