How to pass json string to webmethod c# ASP.NET

First, you need to use:

var jSon = JSON.stringify({obj:javascriptObject});

instead of:

var jSon = JSON.stringify(javascriptObject);

Then your WebMethod would be like:

[WebMethod]
public static string Updatera(aData obj)
{
    // logic code 
}

Now here aData is your class something like below :

public class aData { 
    public string Foretagsnamn  { get; set; }
    public string BGFarg  { get; set; }
    public string TextColor  { get; set; }
    public string FooterFarg  { get; set; }
    public string Email  { get; set; }
}

---

So your final code look like jQuery:

var jSon = JSON.stringify({ obj:javascriptObject });
$.ajax({
    type: "POST",
    url: "Post/Installningar.aspx/Updatera",
    data: jsonData,
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    success: OnSuccess,
    error: OnErrorCall
});

function OnSuccess(response){
    // Do something
}
function OnErrorCall(){
    // Do something
}

Code Behind:

public class aData { 
    public string Foretagsnamn { get; set; }
    public string BGFarg { get; set; }
    public string TextColor { get; set; }
    public string FooterFarg { get; set; }
    public string Email { get; set; }
}


[WebMethod]
public static string Updatera(aData obj)
{
    // Logic code
}

Do check jQuery Ajax JSON Example in Asp.net