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<h1 id="aspnet-expressions">ASP.NET Expressions</h1>
<p>Posted on Thu 10 May 2012 in <a href="http://www.emadmokhtar.com/category/aspnet.html">ASP.NET</a> <b>Read in 2 min.</b></p>
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<p>ASP.NET Expression is written inline surrounded with <% %> in other words you will write them with your markup in .ASPX files for specific task, for example if want to access a connection string there an expression for that</p>
<div class="highlight"><pre><span></span><span class="cp"><%</span><span class="err">$</span> <span class="ss">ConnectionStrings</span><span class="p">:</span><span class="no">DefaultConnection</span> <span class="cp">%></span><span class="x"></span>
</pre></div>
<p>Let’s lists all ASP.NET Expression and it’s task:</p>
<ol>
<li>
<p><strong>Directive Expression</strong> <%@ .. %> </p>
<ul>
<li>It’s used in Web Forms .ASPX files or User Control .ASCX files
to set settings, for example the Page directive where you can
declare Title, Master Page, Language, etc.
<code>aspx
<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.Master"
AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="ASPNETExpressions._Default" %></code></li>
</ul>
</li>
<li>
<p><strong>Data-Binding Expression</strong> <%# .. %></p>
<ul>
<li>
<p>Create binding between server control with data source when calling DataBind() method, most of the time you’ll see this expression inside Data Bound Controls like GridView and DetailView.</p>
<p><code>aspx
<asp:TextBox runat="server" ID="txtFirstName" Text="<%# Eval("FirstName") %>"/></code></p>
</li>
</ul>
</li>
<li>
<p><strong>Expression Builder</strong> <%\$ .. %></p>
<ul>
<li>This expression is used to set controls properties that located in configuration files <code>Web.Config</code> such as <code>AppSettings</code>, <code>ConnectionStrings</code>, or <code>Resources</code></li>
<li>Its syntax is <code><$ Expression Prefix: Expression Value %></code>
<code>aspx
<%$ ConnectionStrings:DefaultConnection %></code></li>
</ul>
</li>
<li>
<p><strong>Server-side Comment Expression</strong> <%-- .. --%></p>
<ul>
<li>It’s to comment a block of code so that it’ll not rendered or executed inside the page.
<code>aspx
<%— This is a comment —%></code></li>
</ul>
</li>
<li>
<p><strong>Displaying Expression</strong> <%= .. %><strong> </strong></p>
<ul>
<li>It’s the simplest way to display one piece of information such as Integer or String inside the page, it’ll be converted to <code>Response.Write()</code>.
<code>aspx
<%= DateTime.UtcNow.ToString("dd/MM/yyyy") %></code></li>
</ul>
</li>
</ol>
<h1>Tip</h1>
<p>Every time you’ll see <code><%</code> look at the next character:</p>
<ul>
<li>If it @ then it’s <strong>Directive Expression</strong>.</li>
<li>If it = then it’s <strong>Displaying Expression</strong>.</li>
<li>If it # then it’s <strong>Data-Binding Expression</strong>.</li>
<li>If it -- then it’s <strong>Comment</strong>.</li>
<li>If it \$ then it’s <strong>Expression Builder</strong>.</li>
</ul>
<h1>Note:</h1>
<p>If you make incorrect syntax inside these expression, Exception will be thrown.</p>
<p>Microsoft Support: <a href="http://support.microsoft.com/kb/976112">Introduction to ASP.NET inline expressions in the .NET Framework</a></p>
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