The underlying provider failed on Open.

I tried to connect the database on my local machine. API throws exception "The underlying provider failed on Open". I checked the inner exception to find out more my fault.

The solution key is in your Web.config file. I use this configuration and it works.

  <connectionStrings>  
  <add name="MyDBEntities" connectionString="metadata=res://*/DBAAA.csdl|res://*/DBAAA.ssdl|res://*/DBAAA.msl;provider=System.Data.SqlClient;
        provider connection string=&quot;
        data source=MyMachineDomainHQGWD2S\SQLEXPRESS;
        initial catalog=My_DB_TEST;
        integrated security=True;
        MultipleActiveResultSets=True;
        App=EntityFramework&quot;" 
        providerName="System.Data.EntityClient" />
  </connectionStrings>

Hope it can help :D

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UPDATE
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I've got the problem again! I want to add the strongly note that YOU CANNOT SET data source AS YOU LOCAL IP!! PLEASE SET IT TO $DOMAIN\$MACHINE_NAME

Then you can access the database with locahost:port/service or _your_ip:port/service
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This site can’t be reached localhost refused to connect

I found this problem when I started debugging web service on VS2015. Refer to the answer http://stackoverflow.com/a/35418904/6879961, I added a step before following them.

I deleted .vs\config\applicationhost.config and then I create the virtual directory. This action will generate the new applicationhost.config for you with the correct URL.


* this image is from stackoverflow