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You can achieve this through the master property of the child page and the FindControl() method.<br />
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To demonstrate how, add a button named Button1 to the master page and write the following code in the child page:<br />

<pre><code class="code-1 nolinks" id="code-234307"><span>    Button b = (Button) Master.FindControl(&quot;Button1&quot;);</span>
<span>    b.Text = &quot;Hi&quot;;</span>
</code></pre>
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i have coppied the following text from the link below<br />
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You can use two methods. The first is by using Page.Master.FindControl('c<wbr />ontrolID')<wbr />. Then you can cast it to the type of your user control. The second method is by adding a &lt;%@ MasterType VirtualPath=&quot;&quot; TypeName=&quot;&quot;%&gt; tag to your aspx page. In the VirtualPath add the virtual path to the master page, and the class in the TypeName. You can then access everything with intellisense.<br />
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<a href="http://stackoverflow.com/questions/382358/how-to-access-a-user-control-in-a-masterpage-from-a-content-page" rel="ugc">http://stackoverflow.com/questions/382358/how-to-access-a-user-control-in-a-masterpage-from-a-content-page</a>
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How do I find the type to cast to? &nbsp;Like, instead of (Button) ?
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I have a user control in a master page, and I need to set one of it's custom properties from a child page. &nbsp;How can I do this?
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I have a user control in a master page, and I need to set one of it's custom properties from a child page.  How can I do this?

How do I set a usercontrol property from a child page

C# is an object-oriented programming language created in conjunction with Microsoft’s .NET framework. Compilation is usually done into the Microsoft Intermediate Language (MSIL), which is then JIT-compiled to native code (and cached) during execution in the Common Language Runtime (CLR).