Unable to cast object of type 'ASP.site_master' to type 'System.Web.UI.WebControls.Button'. in asp.net
<p>I just created ten buttons and last two buttons named btn_green and btn_red,.</p>

<p>I wanna to change all button color to the green and red when click those buttons green and red.</p>

<p>My C# code is</p>

<pre><code>protected void btn_green_Click(object sender, EventArgs e)
        {
            foreach (Button btn in this.Controls)
            {
                btn.BackColor = Color.DarkGreen;
            }
        }     

        protected void btn_red_Click(object sender, EventArgs e)
        {
            foreach (Button btn in this.Controls)
            {
                btn.BackColor = Color.Red;
            }
        }
</code></pre>

<p>but my bed luck i got only error </p>

<blockquote>
  <p>Description: An unhandled exception occurred during the execution of
  the current web request. Please review the stack trace for more
  information about the error and where it originated in the code. </p>
  
  <p>Exception Details: System.InvalidCastException: Unable to cast object
  of type 'ASP.site_master' to type 'System.Web.UI.WebControls.Button'.</p>
</blockquote>

<pre><code>Line 20:             foreach (Button btn in this.Controls)
</code></pre>

<p>how i escape this problem?.</p>
