Request.Form ["name"]什么时候为空，什么时候为空字符串? [英] When is Request.Form["name"] null and when an empty string?

问题描述

为什么即使文本框为空并且即使在回发中也没有被触摸，以下结果仍会导致true子句出现?:

Why do the following result in a true if clause even though the textbox is empty and not even touched on a postback? :

<form action="Default.aspx" runat="server" method="post" id="newform">
<input type="text" id="name" runat="server"/>
</form>

<%
if (Request.Form["name"] != null) // Prints out "Name OK" on postback.
{
    Response.Write("<br/>");
    Response.Write("Name OK");
}
%>

文本框在回发中是否实际上包含空字符串(")?

Does the textbox actually contain an empty string ("") on a postback?

为什么以下结果在第一页加载时导致true if子句，而在回发时却没有true?:

Why do the following result in a true if clause on the first page load but not on a postback? :

<form action="Default.aspx" runat="server" method="post" id="newform">
<input type="text" id="name" runat="server"/>
</form>

<%
if (Request.Form["name"] != "") // Prints out "Name OK" on first page load, but not on postback.
{
    Response.Write("<br/>");
    Response.Write("Name OK");
}
%>

要获得成功和预期的结果，我必须使用以下内容:

To get a successful and expected result I have to use the following:

<form action="Default.aspx" runat="server" method="post" id="newform">
<input type="text" id="name" runat="server"/>
</form>

<%
if (Request.Form["name"] != null && Request.Form["name"] != "")
{
    Response.Write("<br/>");
    Response.Write("Name OK");
}
%>

推荐答案

首先，让我回答您的问题:

首页加载是GET，回发是POST(因此名为 post back).如果页面是通过POST表单加载的，则 Request.Form 仅填充 .

The first page load is a GET, postbacks are a POST (hence the name postback). Request.Form is populated only if the page is loaded though a form POST.

• 在首页加载时， Request.Form 是一个空集合.由于 Request.Form 是 NameValueCollection ，因此在首页加载时返回 null .

• On the first page load, Request.Form is an empty collection. Since Request.Form is a NameValueCollection, accessing a non-existent entry returns null. Thus, Request.Form["whatever"] returns null on the first page load.

回发后， Request.Form 会填充值.由于HTTP POST不了解 null 的值，因此 Request.Form ["whatever"] 会为存在但为空的字段返回空字符串.

After a postback, Request.Form is filled with values. Since HTTP POST does not know about null values, Request.Form["whatever"] returns an empty string for fields which are present but empty.

如果要避免 x！= null&&x！=" 模式，请使用 String.IsNullOrEmpty或空合并运算符:(x??")！=" .

If you want to avoid the x != null && x != "" pattern, use String.IsNullOrEmpty or the null coalescing operator: (x ?? "") != "".

另一方面，您可以通过仅使用内置的WebForms功能来简化生活，而不用自己解析 Request.Form :

On the other hand, you could make your life a lot easier by just using the built-in WebForms features instead of parsing Request.Form yourself:

<form runat="server">
    <asp:TextBox ID="nameBox" runat="server" />
    <asp:Button Text="Do Postback" runat="server" />
</form>

<%
    if (nameBox.Text != "")
    {
        %><br />Name OK<%
    }
%>

自 TextBox.Text 默认为" ，这里不需要检查 null .

Since TextBox.Text defaults to "", there's no need to check for null here.

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