A network-related or instance-specific error occurred when applicatoin connect to a DB

The error as following:

symptom:

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified) 

Cause:

Application log on DB with a instance which is denied or unaccessable.

Solution:

To resolve the issue - I had to change the VSTS Options; From Tools -> Options -> Data Tools -> Data Connection, then set SQL Serevr Instance Name as Blank or an instance name you want.

For example, this paragraph is from a config file, the Server is "./", that means a local server, default instance.

if application need to connect to SQLExpress instance, it should be ...Server=./SQLExpress. Accordingly, you should update the "SQL Serevr Instance Name " in VS also.

 
             connectionString="Server=./;Database=ShotgunAccept;Integrated Security=SSPI;"
         providerName="System.Data.SqlClient" />