Type or namespace name 'Data' does not exist in the namespace 'System'
<p>I'm trying to build my first ASP.NET Web Forms project but I'm facing some serious problem.
I created two project files in my project named: BLL and DAL.
I created classes named <code>class.cs</code> and <code>class1.cs</code> in both the above files respectively. When I add <code>using System.Data;</code> in any of the <code>.cs</code> files, it displays the following errors:<br></p>

<p>1: <code>Error   CS0234  The type or namespace name 'Data' does not exist in the namespace 'System' (are you missing an assembly reference?) DAL..NET Platform 5.4</code><br>
2. <code>Error   CS0234  The type or namespace name 'Data' does not exist in the namespace 'System' (are you missing an assembly reference?) BLL..NET Platform 5.4</code><br></p>

<p>I tried adding assembly references by right click on references -> Add reference -> Checking <code>System.Data</code> and rebuilding it again but it didn't help me.<br></p>

<p>When I hover mouse over <code>using System.Data;</code> it displays the following thing:<br></p>

<p><a href="https://i.stack.imgur.com/FA0Zv.png" rel="noreferrer"><img src="https://i.stack.imgur.com/FA0Zv.png" alt="enter image description here"></a></p>