为什么在尝试与SQL Server建立连接时出现此错误？

Question

I'm trying to insert values in the table but when I click on the SignUp button it gives me error

An attempt to attach an auto-named database for file Database.mdf failed. A database with the same name exists, or specified file cannot be opened, or it is located on UNC share.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.SqlClient.SqlException: An attempt to attach an auto-named database for file Database.mdf failed. A database with the same name exists, or specified file cannot be opened, or it is located on UNC share.

Source Error:

Line 22: string con =ConfigurationManager.ConnectionStrings["connection"].ConnectionString;
Line 23: SqlConnection conn = new SqlConnection(con);
Line 24: conn.Open(); //error line
Line 25: if (selectques.SelectedItem.Text == "Write your own question?")
Line 26: {

My button event

protected void signup_Click(object sender, EventArgs e)
{
   string con = ConfigurationManager.ConnectionStrings["connection"].ConnectionString;
   SqlConnection conn = new SqlConnection(con);

   conn.Open();

   if (selectques.SelectedItem.Text == "Write your own question?")
   {
      SqlCommand cmd = new SqlCommand("insert into registration values('" + username + "','" + passwrd + "','" + emailadd + "','" + alterquestion + "','" + securityanswer + "'", conn);
      cmd.ExecuteNonQuery();
      conn.Close();
   }
   else
   {
      SqlCommand cmd = new SqlCommand("insert into registration values('" + username + "','" + passwrd + "','" + emailadd + "','" + selectques + "','" + securityanswer + "'", conn);
      cmd.ExecuteNonQuery();
      conn.Close();
   }

And my web.config connectionstring

<configuration>
    <appSettings/>
  <connectionStrings>
    <add name="connection" 
         connectionString="server=.\sqlexpress; AttachdbFilename=Database.mdf; integrated security=true; user instance=true"/>
  </connectionStrings>

Answer 1

This part of your connection string is wrong:

AttachdbFilename=Database.mdf

AttachdbFilename needs a full path to the MDF file, not just its name.

Source:
Connecting to SQL Server Express User Instances (ADO.NET)

Quote from the link:

The AttachDbFileName connection string keyword is used to attach the primary database file (.mdf), which must include the full path name.

Answer 2

change:

AttachdbFilename=Database.mdf

to this:

AttachdbFilename=|DataDirectory|Database.mdf