To connect the attachment .mdf file through .net

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C#

SQL-Server

SQL-Server-2008

I have install the SQL server 2008 R2 & configure the server Mixed mode authentication with username = 'sa' & password = 'pw'
and my attachment .mdf file is located in d:\Test folder and service is running.

Now i connect the server2008 attachment .mdf file through coding but not connect. Error msg shown('An error has occurred while establishing a connection to the server. When connecting to SQL Server 2005, this failure may be caused by the fact that under the default settings SQL Server does not allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)')

My code is below :->
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SqlConnection con = new SqlConnection();
con.ConnectionString = @"Data Source=.\MSSQLEXPRESS;AttachDbFilename=D:\Test\db.mdf;
Integrated Security=True;Connect Timeout=30;User Instance=True";
con.Open();

How will i solve above problem???
Please help me any instruction......