When is Request.Form["name"] null and when an empty string? - asp.net

Question

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");
}
%>
Does the textbox actually contain an empty string ("") on a postback?
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");
}
%>
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");
}
%>

Answer 1

First, let me answer your question:
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.
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.
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.
If you want to avoid the x != null && x != "" pattern, use String.IsNullOrEmpty or the null coalescing operator: (x ?? "") != "".
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<%
}
%>
Since TextBox.Text defaults to "", there's no need to check for null here.

Answer 2

Request.Form is NameValueCollection, which returns null if specified key is not found, returns value (which is an empty string) otherwise.
You may use string.IsNullOrEmpty() method.
if (!string.IsNullOrEmpty(Request.Form["name"]))
{
Response.Write("<br/>");
Response.Write("Name OK");
}

Answer 3

Request.Form["ControlName"] returns null if Control is not present on form.
If Control is present, but it contains null or empty value, then Request.Form["ControlName"] will always return String.Empty.
So it's good practice, instead of comparing (Request.Form["ControlName"] != null), use (!String.IsNullOrEmpty(Request.Form["ControlName"]))

Answer 4

Base on this site: https://www.mikesdotnetting.com/article/293/request-form-is-empty-when-posting-to-aspx-page
This is the answer. Remark this line:
settings.AutoRedirectMode = RedirectMode.Permanent;
in routeConfig.cs