how to serialize a json string to a form post data

Question

I need to POST a JSON string to a page. The page is external and out of my control, and it expects the post data to be in the web-form post format (key1=value1&key2=value2)

How can I convert the JSON string to this format?

Answer 1

This can be done by first deserializing your JSON to a Dictionary<string, string>, then iterating through the key-value pairs in the dictionary and building up a querystring from that.

However, keep in mind that querystring format (application/x-www-form-urlencoded) is not a hierarchical format, while JSON is. So your JSON object can only be a simple object with key-value pairs (no arrays or nested objects). If your JSON is more complicated than that, you will have to do some more work to flatten it before you can convert it to a querystring.

Demo:

class Program
{
    static void Main(string[] args)
    {
        string json = @"
        {
            ""key1"" : ""value1"",
            ""key2"" : ""value2"",
            ""int"" : 5,
            ""bool"" : true,
            ""decimal"" : 3.14,
            ""punct"" : ""x+y=z""
        }";

        var dict = JsonConvert.DeserializeObject<Dictionary<string, string>>(json);

        StringBuilder sb = new StringBuilder();
        foreach (KeyValuePair<string, string> kvp in dict)
        {
            if (!string.IsNullOrEmpty(kvp.Key) && !string.IsNullOrEmpty(kvp.Value))
            {
                if (sb.Length > 0) sb.Append('&');
                sb.Append(HttpUtility.UrlEncode(kvp.Key));
                sb.Append('=');
                sb.Append(HttpUtility.UrlEncode(kvp.Value));
            }
        }
        var postDataString = sb.ToString();

        Console.WriteLine(postDataString);
    }
}

Output:

key1=value1&key2=value2&int=5&bool=True&decimal=3.14&punct=x%2by%3dz

---

As was mentioned in the comments, you can use the FormUrlEncodedContent class to do the same thing. Replace the StringBuilder and foreach loop in the code above with the following (but note this approach requires async/await):

var formUrlEncodedContent = new FormUrlEncodedContent(dict);
var postDataString = await formUrlEncodedContent.ReadAsStringAsync();

Answer 2

You don't post JSON like that. You set the Content-Type header to "application/json" and then you simply fill the content body with the JSON as-is.

There is no built in support in C# or JSON.NET to serialize JSON into form post data, but you can probably use LINQ to JSON to write a translater yourself relatively easy, assuming the JSON format is simple enough.