<connectionStrings>
        <add name="PGConnectionString" providerName="Npgsql" connectionString="Server=localhost;User Id=postgres;Password=postgres;Port=5432;Database=postgres;"  />
    </connectionStrings>
 

            <asp:GridView ID="GridView1" runat="server" AllowPaging="True" 
            AllowSorting="True" AutoGenerateColumns="False" DataKeyNames="CustomerID" 
            DataSourceID="SqlDataSource1">
            <Columns>
                <asp:BoundField DataField="param_id" HeaderText="param_id" ReadOnly="True"  SortExpression="param_id" />
                <asp:BoundField DataField="source_system" HeaderText="source_system" SortExpression="source_system" />
                <asp:BoundField DataField="param_name" HeaderText="param_name" SortExpression="param_name" />
            </Columns>
        </asp:GridView>
 
        <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
            ConnectionString="<%$ ConnectionStrings:PGConnectionString %>" 
            SelectCommand="SELECT param_id, param_name FROM public.parameters"></asp:SqlDataSource>
 

protected void Page_Load(object sender, EventArgs e)
        {
 
            var connString = "Server=localhost;User Id=postgres;Password=postgres;Port=5432;Database=postgres;";
            using (var conn = new NpgsqlConnection(connString))
            {

.....

 <system.data>
        <DbProviderFactories>
            <add name="Npgsql Data Provider"
                 invariant="Npgsql"
                 description ="Data Provider for PostgreSQL"
                 type="Npgsql.NpgsqlFactory, Npgsql" />
        </DbProviderFactories>
    </system.data>