How to create a connection string in asp.net c#

Demo :

<connectionStrings>
<add name="myConnectionString" connectionString="server=localhost;database=myDb;uid=myUser;password=myPass;" />
</connectionStrings>

Based on your question:

<connectionStrings>
    <add name="itmall" connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=D:\19-02\ABCC\App_Data\abcc.mdf;Integrated Security=True;User Instance=True" />
    </connectionStrings>

Refer links:

http://www.connectionstrings.com/store-connection-string-in-webconfig/

Retrive connection string from web.config file:

write the below code in your file where you want;

string connstring=ConfigurationManager.ConnectionStrings["itmall"].ConnectionString;

SqlConnection con = new SqlConnection(connstring);

or you can go in your way like

SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["itmall"].ConnectionString);

Note:

The "name" which you gave in web.config file and name which you used in connection string must be same(like "itmall" in this solution.)

add this in web.config file

           <configuration>
              <appSettings>
                 <add key="ConnectionString" value="Your connection string which contains database id and password"/>
            </appSettings>
            </configuration>

.cs file

 public ConnectionObjects()
 {           
    string connectionstring= ConfigurationManager.AppSettings["ConnectionString"].ToString();
 }

Hope this helps.

Add this in your web.config file

<connectionStrings>
<add name="itmall" 
connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=D:\19-
02\ABCC\App_Data\abcc.mdf;Integrated Security=True;User Instance=True" />
</connectionStrings>

Add this connection string tag in web.config file:

<connectionStrings>
  <add name="itmall" 
    connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=D:\19-02\ABCC\App_Data\abcc.mdf;Integrated Security=True;User Instance=True"/>
</connectionStrings>

And use it like you mentioned. :)

It occurs when IIS is not being connected to SQL SERVER. For a solution, see this screenshot:

string connectionstring="DataSource=severname;InitialCatlog=databasename;Uid=; password=;"
SqlConnection con=new SqlConnection(connectionstring)

        // Web config file into connection string //
        <connectionStrings>
                <add name="constr" connectionString="Data Source=sourcename;Initial Catalog=database name ; Integrated Security=True;" providerName="System.Data.SqlClient"/>
            </connectionStrings>
   //end web config file //

// below this code use in web form page //
    
     protected void Page_Load(object sender, EventArgs e)
            {
                string cs = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
          
                SqlConnection conn = new SqlConnection(cs);
                conn.Open();
                if (conn.State == ConnectionState.Open)
                {
                    Response.Write("<script>alert('open')</script>");
                }
                else
                {
                    Response.Write("<script>alert('not open')</script>");
                }
    
            }