Create temporary file using C # and ASP.NET

I believe that to follow this path you will face some problems, in any case I have some suggestions.

To return the HTML file to the user, you can convert string to byte[] and return FileResult to mine-type text/html :

public FileResult Index()
{
    var model = new
    {
        guid = Guid.NewGuid(),
        nome = "Toby Mosque"
    };
    var html = $@"
        <div>
            <div>
                <label>
                    GUID:
                    <input id='guid' type='text' value='{model.guid}' />
                </label>
            </div>
            <div>
                <label>
                    Nome:
                    <input id='nome' type='text' value='{model.nome}' />
                </label>
            </div>
        </div>
    ";

    var binary = Encoding.UTF8.GetBytes(html);
    return File(binary, "text/html");
}

Note that in our example above the model has a unique identifier, so we can store the file's binary in the cache and use GUID as a key.

MemoryCache.Default.Add(model.guid.ToString(), binary, new CacheItemPolicy
{
    Priority = CacheItemPriority.NotRemovable,
    AbsoluteExpiration = MemoryCache.InfiniteAbsoluteExpiration,
    SlidingExpiration = MemoryCache.NoSlidingExpiration
});

Of course, in a production environment it is not desirable to have a Cache with an extreme priority that never expires, so configure it with care.

Then when calling this controller, you can check if the above binary exists in the cache and you will not need to create it again.

var binary = default(byte[]);
if (MemoryCache.Default.Contains(model.guid.ToString()))
{
    binary = MemoryCache.Default.Get(model.guid.ToString()) as byte[];
}
else
{
    var html = string.Empty;
    /* Logica de montagem do HTML aqui  */
    binary = Encoding.UTF8.GetBytes(html);
}
return File(binary, "text/html");

Remembering to use MemoryCache you must add the Assembly System.Runtime.Caching to your project.

But if your templates are a bit more complex, you can use Handlebars.NET , in this if its html will give rise to a Function<string, object> .

using (var reader = new StringReader(html))
{
    var hbTemplate = Handlebars.Compile(reader);
    var stream = hbTemplate.template(model);
    return File.Stream(stream, "text/html");
}

In this case, it is desirable that you keep your templates compiled in the Cache.

To learn more about the syntax of Handlebars , you can look at his version site for JavaScript :

HandlebarsJS