Get Connection String from Web.config in asp.net

Using the ConfigurationManager.ConnectionStrings is about the only proper way, to use it properly with sanity check you can have such code:

public DBGateway()
{
    ConnectionStringSettings mySetting = ConfigurationManager.ConnectionStrings["test"];
    if (mySetting == null || string.IsNullOrEmpty(mySetting.ConnectionString))
        throw new Exception("Fatal error: missing connecting string in web.config file");
    conString = mySetting.ConnectionString;
}

This will throw useful error in case the connection string is missing, instead of cryptic "null object" error.

Worth to mention that the ConnectionStringSettings class is overriding the ToString() method:

public override string ToString()
{
    return this.ConnectionString;
}

So it means that using ConfigurationManager.ConnectionStrings["test"].ToString() is the same like ConfigurationManager.ConnectionStrings["test"].ConnectionString however you still better perform sanity check and personally it looks cleaner to use the actual property and not depend on the class to give it.

Here is the whole solution:-

string constring = System.Configuration.ConfigurationManager.ConnectionStrings["ConnectionStringName"].ConnectionString;
SqlConnection con = new SqlConnection(constring);
DataSet ds = new DataSet();
try
 {
   SqlDataAdapter dataAdapter = new SqlDataAdapter(query, con);
   SqlCommandBuilder commandBuilder = new SqlCommandBuilder(dataAdapter);
   con.Open();
   dataAdapter.Fill(ds, "table");
   return ds;
 }
 catch (Exception ex)
  {
  }
    finally
    {
        if (con.State == System.Data.ConnectionState.Open)
            con.Close();
    }

This is how you can fetch records from database into datatable.

Hope this is what you were looking for.