title: Fill/Bind DropDownList values based upon keys
body: <p>I am trying to select/show only single group of values in <code>DropDownList</code> based upon another <code>DropDownList</code> value changed.</p>
  
  <p>I have a Sync Type and <strong>Operation Type DDL</strong> and I want to bind/show only those key-values in Operation Type DDL whose key selected in First DDL i.e. <strong>Sync Type DDL</strong>.
  <em>(Note - Values of Sync Type DDL is similar to the key of Operation Type DDL)</em></p>
  
  <p>Here is an Sync Type HTML markup:</p>
  
  <pre><code>&lt;div&gt;
      &lt;label&gt;
          Sync Type
      &lt;/label&gt;
      &lt;asp:DropDownList runat="server" ID="dllSyncType" AutoPostBack="true" OnSelectedIndexChanged="ddlChannel_SelectedIndexChanged"&gt;
          &lt;asp:ListItem Text="Property" Value="1"&gt;&lt;/asp:ListItem&gt;
          &lt;asp:ListItem Text="Room" Value="2"&gt;&lt;/asp:ListItem&gt;
          &lt;asp:ListItem Text="Rate" Value="3"&gt;&lt;/asp:ListItem&gt;
          &lt;asp:ListItem Text="Product" Value="4"&gt;&lt;/asp:ListItem&gt;
          &lt;asp:ListItem Text="Open/Close Property" Value="5"&gt;&lt;/asp:ListItem&gt;
          &lt;asp:ListItem Text="Sync All" Value="6"&gt;&lt;/asp:ListItem&gt;
      &lt;/asp:DropDownList&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>and the Operation Type DDL:</p>
  
  <pre><code>&lt;div&gt;
      &lt;label&gt;
          &lt;%=Resources.Common.OperationType%&gt;
      &lt;/label&gt;
      &lt;asp:DropDownList runat="server" ID="ddlOperationType"&gt;
      &lt;/asp:DropDownList&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>I am using <code>Dictionary</code> containing hard coded key-values record set to bind the values in Operation Type DDL, Like below..</p>
  
  <pre><code>Dictionary&lt;string, string&gt; dicOperationTypes = new Dictionary&lt;string, string&gt;();
  dicOperationTypes.Add("-1", "Select");
  dicOperationTypes.Add("Property", "Check");
  dicOperationTypes.Add("Property", "Open");
  dicOperationTypes.Add("Property", "Close");
  
  dicOperationTypes.Add("Room", "Activate");
  dicOperationTypes.Add("Room", "Deactivate");
  
  dicOperationTypes.Add("Rate", "Update");
  dicOperationTypes.Add("Rate", "Activate");
  dicOperationTypes.Add("Rate", "Deactivate");
  ddlOperationType.DataSource = dicOperationTypes;
  ddlOperationType.DataValueField = "Key";
  ddlOperationType.DataTextField = "Value";
  ddlOperationType.DataBind();
  </code></pre>
  
  <p>How can I do that?</p>