我将模型切回了一个领域：

//模型

public class LetterViewModel
{
    public string LetterText;
}

//控制器

public ActionResult Index()
{
    var model = new LetterViewModel();
    model.LetterText = "Anything";

    return View(model);
}

[HttpPost]
public ActionResult Index(LetterViewModel model)
{ 
    //model.LetterText == null
    return View(model);
}

//视图

@model Test.Models.LetterViewModel
@{
    Layout = "~/Views/Shared/_Layout.cshtml";
    ViewBag.Title = "Create a Letter";
}
@using (Html.BeginForm())
{
    <div id="Bottom">
        @Html.TextAreaFor(m => m.LetterText)
        <input type="submit" value="Ok" class="btn btn-default" />
    </div>
}

当我在开发工具中检查“网络”选项卡时，它显示输入的值已包含在请求中。但是，当HttpPost控制器被激发时，该字段为空。