Microsoft Icon from social bootstrap are working in Asp.Net 5 Mvc 6

I think that the control of the Microsoft.AspNet.Mvc.TagHelpers does not have the @helper in order to GetFontClass to retrieve a windows icon into the Microsoft bootstrap, for example:

<div class="col-md-8"><button type="submit"
                                            class="btn btn-default btn-block btn-social btn-@provider.AuthenticationScheme.ToLower()"
                                            style="font-family:'Khmer OS Bokor'; font-size: 14px;"
                                            name="provider" value="@provider.AuthenticationScheme"
                                            title="ចុះឈ្មោះចូលប្រើជាមួយនឹងគណនី @provider.DisplayName របស់លោកអ្នក។"><i class="fa fa-@SocialStyleHelpers.GetFontClass(provider.AuthenticationScheme)"></i>@provider.AuthenticationScheme</button></div>

Then I create a general helper in SocialStyleHelpers.cshtml to show the owesome windows icon in the Microsoft Social button, but the @helper was not available, so that the GetFontClass can not process to retrieve the windows icon to show in the Microsoft social button, here below is my code:

@helper GetFontClass(string authenticationScheme)
{
	if (authenticationScheme == "Microsoft")
	{
        @("windows")
	}
	else
	{
        @authenticationScheme.ToLower()
	}
}

Can you help me to correct it and show how we can have @helper in view razor working in Asp.Net 5 Mvc 6 or it change the way to retrieve Windows icon on the Microsoft social button?