Passing array of strings to webmethod with variable number of arguments using jQuery AJAX

Revised server-side code:

[WebMethod]
public string Concat(List<string> arr)
{
    string result = "";
    for (int i = 0; i < arr.Count; i++)
    {
        result += arr[i];
    }
    return result;
}

Also, add this above your WebService class declaration:

[System.Web.Script.Services.ScriptService]

Revised client-side code:

    $(document).ready(function () {

        var myCars = new Array();
        myCars[0] = "Saab";
        myCars[1] = "Volvo";
        myCars[2] = "BMW";

        $.ajax({
            type: "POST",
            url: "WebService.asmx/Concat",
            data: JSON.stringify({ arr: myCars }),
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: onSuccess,
            failure: onError
        });
    });

    function onSuccess(response) {
        alert(response.d);
    }

    function onError() {
        alert("fail");
    }

Also, add above that script block a reference to JSON2, such as:

<script src="http://ajax.cdnjs.com/ajax/libs/json2/20110223/json2.js"></script>

Notes:

• I have tested this under .NET 4 and using jQuery 1.6.4.
• Make sure you keep the client and server variable names in sync:
  public string Concat(List<string> arr)
data: JSON.stringify({ arr: myCars })