What is the difference between FormUrlEncodedContent and query string?

Question

I'm doing some web scraping using the following methods

This is successful for most websites.

var content = new FormUrlEncodedContent(new[]
{
    new KeyValuePair<string, string>("v1", "value1"),
    new KeyValuePair<string, string>("v2", "value2"),
    new KeyValuePair<string, string>("v3", "value3"),
});
var response = await client.PostAsync("http://url.com", content);
html = await response.Content.ReadAsStringAsync();

Sometimes certain website needs this way to get response.

var url = "http://url.com?v1=value1&v2=value2&v3=value3";
var response = await client.PostAsync(url, null);
html = await response.Content.ReadAsStringAsync();

And sometimes needs this to get response.

var query = "v1=value1&v2=value2&v3=value3";
var content = new ByteArrayContent(Encoding.UTF8.GetBytes(query));
var response = await client.PostAsync("http://url.com", content);
html = await response.Content.ReadAsStringAsync();

I really have no idea what is the difference.

Answer 1

If you use FormUrlEncodedContent your parameters will be sent in request body and formatted as query string.

POST http://url.com/ HTTP/1.1
Host: url.com
Content-Length: 29
Expect: 100-continue
Connection: Keep-Alive

v1=value1&v2=value2&v3=value3 

In the second case parameters will be sent as query part of URL.

POST http://url.com/?v1=value1&v2=value2&v3=value3 HTTP/1.1
Host: url.com
Content-Length: 0 

In the third case you sent a content of query in the request body.

In your sample it has the same effect as in the first case, but you made formatting by hand.

Answer 2

The URL (query string) has a (browser dependant) size limit. The body of a POST request doesn't have this limit. So you would use the body to send a file.

On the other hand, a URL can be used in a link.