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    <p>I would advocate Kyra's first solution to modify the dropdown lists so that the same value cannot be selected, it is always better to prevent problems than tell the user what they did was wrong. However, if you do want to use a <a href="http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.customvalidator.aspx" rel="nofollow noreferrer">CustomValidator</a> the following code would work:</p> <pre><code>&lt;asp:CustomValidator ID="dropDownValidation" runat="server" OnServerValidate="dropDownValidation_ServerValidate" ErrorMessage="The same value cannot be selected in more than one drop down." /&gt; </code></pre> <p>And then in the code behind, or a script tag.</p> <pre><code>protected void dropDownValidation_ServerValidate(object sender, ServerValidateEventArgs e) { e.IsValid = !haveSameValue(DropDownList1.SelectedValue, DropDownList2.SelectedValue) &amp;&amp; !haveSameValue(DropDownList1.SelectedValue, DropDownList3.SelectedValue) &amp;&amp; !haveSameValue(DropDownList1.SelectedValue, DropDownList4.SelectedValue) &amp;&amp; !haveSameValue(DropDownList2.SelectedValue, DropDownList3.SelectedValue) &amp;&amp; !haveSameValue(DropDownList2.SelectedValue, DropDownList4.SelectedValue) &amp;&amp; !haveSameValue(DropDownList3.SelectedValue, DropDownList4.SelectedValue); } protected bool haveSameValue(string first, string second) { if (first != null &amp;&amp; second != null) { return first.Equals(second); } return first == null &amp;&amp; second == null; } </code></pre> <p>This can obviously be further refined, and a javascript function can be used to provide client side validation if desired using the <a href="http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.customvalidator.clientvalidationfunction.aspx" rel="nofollow noreferrer">ClientValidationFunction</a> property.</p>
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    1. COHi, many thanks for your help. The custom validator works fine. I will also take your advice and code a client validation function too. Could you let me know how your server validation would accept null values as well? Thanks!
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    2. COI updated my answer to handle null values. Please note that I haven't actually tried this, so it should be used as a reference.
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