I'm making a form in Umbraco 7.7.7 and I'll translate the labels. For this I use the Display-attribute and the UmbracoHelper class to get access to the dictionary.

public class ContactViewModel 
{
    private UmbracoHelper _helper = new UmbracoHelper();

    [Required]
    [Display(Name = _helper.Field("#First name"))] // ← error on this line.
    public string FirstName { get; set; }

    [Required]
    [Display(Name = _helper.Field("#Last name"))]  // ← error on this line.
    public string LastName { get; set; }

    [Required]
    [EmailAddress]
    [Display(Name = _helper.Field("#Email"))]      // ← error on this line.
    public string Email { get; set; }

    [Required]
    [Display(Name = _helper.Field("#Message"))]    // ← error on this line.
    public string Message { get; set; }

    // Some other properties...

    public ContactViewModel()
    {
        _helper = new UmbracoHelper(UmbracoContext.Current);
    }
}

Here is my code in my view:

@using (Html.BeginUmbracoForm<FormController>("contact", FormMethod.Post))
{
    @(Html.EditorFor<ContactViewModel>())

    <div class="form-control">
        <input type="submit" value="verzenden" class="submit-button" />
    </div>
}

But I've got this error on the marked lines:

An object reference is required for the non-static field, method, or property ContactViewModel._helper.

I've also tried with this code:

private static UmbracoHelper _helper = new UmbracoHelper(UmbracoContext.Current);

and convert the result of _helper.Field() to a string instead of IHtmlString.

[Required]
[Display(Name = _helper.Field("#First name").ToString())] // ← error on this line.
public string FirstName { get; set; }

But code now this error also on the marked lines:

An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type.

From Stack Overflow I know this:

Attributes are metadata and must be know at compile time. _helper.Field() is a method and its results cannot be determined until runtime. You need to use a constant, for example [Display(Name = "First name")].

P.S.: I'm following this documentation from our.umbraco.org.

Update: Is there a other way to do more or less the same thing?