FormBlock.cs

public static HtmlTag FormBlock<T>(this HtmlHelper<T> helper,
    Expression<Func<T, object>> expression,
    Action<HtmlTag> labelModifier = null,
    Action<HtmlTag> inputBlockModifier = null,
    Action<HtmlTag> inputModifier = null,
    Action<HtmlTag> validatorModifier = null
    ) where T : class
{
    labelModifier = labelModifier ?? (_ => { });
    inputBlockModifier = inputBlockModifier ?? (_ => { });

    var divTag = new HtmlTag("div");
    divTag.AddClass("form-group");

    var labelTag = helper.Label(expression);
    labelModifier(labelTag);

    var inputBlockTag = helper.InputBlock(
        expression, 
        inputModifier, 
        validatorModifier);
    inputBlockModifier(inputBlockTag);

    divTag.Append(labelTag);
    divTag.Append(inputBlockTag);

    return divTag;
}