ASP.NET MVC Html.ValidationSummary(true) does not display model errors

I believe the way the ValidationSummary flag works is it will only display ModelErrors for string.empty as the key. Otherwise it is assumed it is a property error. The custom error you're adding has the key 'error' so it will not display in when you call ValidationSummary(true). You need to add your custom error message with an empty key like this:

ModelState.AddModelError(string.Empty, ex.Message);

This works better, as you can show validationMessage for a specified key:

    ModelState.AddModelError("keyName","Message");

and display it like this:

    @Html.ValidationMessage("keyName")

I know this is kind of old and has been marked as answers with 147 up votes, but there is something else to consider.

You can have all the model errors, the property named and string.Empty keys alike, be shown in the ValidationSummary if you need to. There is an overload in the ValidationSummary that will do this.

    //   excludePropertyErrors:
    //   true to have the summary display model-level errors only, or false to have
    //   the summary display all errors.
    public static MvcHtmlString ValidationSummary(this HtmlHelper htmlHelper, bool excludePropertyErrors);