The OnPost() handler method is invoked when a POST request is sent to a page. This usually happens when a user submits a form (an HTML <form> element).

Just like with OnGet(), the default behavior of an empty OnPost() method is to send the corresponding page. Without a return statement, this method also returns void.

public void OnPost()
{ }

Usually POST requests come with some information in the form of a query string. For example, say that a form at www.library.com/favorite asks for a book and an author:

<form method="post">
  <div class="form-group">
    <label for="Title">Title</label>
    <input type="text" class="form-control" id="Title" name="Title">
  </div>
  <div class="form-group">
    <label for="Author">Author</label>
    <input type="text" class="form-control" id="Author" name="Author">
  </div>
  <button type="submit" class="btn btn-primary">Submit</button>
</form>

Notice that each <input> has a name attribute — Title and Author, respectively.

In a browser it would look like:

If a user provided the input Where The Wild Things Are and Maurice Sendak, the URL would look like this (+ represents a space):

www.library.com/favorite?Title=Where+The+Wild+Things+Are&Author=Maurice+Sendak

OnPost() can capture the values in the query string via matching method parameters. To capture the above values, the method would look like this:

public void OnPost(string title, string author)
{
  Title = title;
  Author = author;
}

Those method parameters are matched case-insensitively; author or Author or aUTHOR would have worked.