How to add a second css class with a conditional value in razor MVC 4

I believe that there can still be and valid logic on views. But for this kind of things I agree with @BigMike, it is better placed on the model. Having said that the problem can be solved in three ways:

Your answer (assuming this works, I haven't tried this):

<div class="details @(@Model.Details.Count > 0 ? "show" : "hide")">

Second option:

@if (Model.Details.Count > 0) {
    <div class="details show">
}
else {
    <div class="details hide">
}

Third option:

<div class="@("details " + (Model.Details.Count>0 ? "show" : "hide"))">

This:

    <div class="details @(Model.Details.Count > 0 ? "show" : "hide")">

will render this:

    <div class="details hide">

and is the mark-up I want.

You can add property to your model as follows:

    public string DetailsClass { get { return Details.Count > 0 ? "show" : "hide" } }

and then your view will be simpler and will contain no logic at all:

    <div class="details @Model.DetailsClass"/>

This will work even with many classes and will not render class if it is null:

    <div class="@Model.Class1 @Model.Class2"/>

with 2 not null properties will render:

    <div class="class1 class2"/>

if class1 is null

    <div class=" class2"/>

You can use String.Format function to add second class based on condition:

<div class="@String.Format("details {0}", Details.Count > 0 ? "show" : "hide")">