在 web 方法中从数据库获取数据返回 vb.net 中的 null

Question

I am trying to get data from a stored procedure in a webmethod inside an asmx service but whatever I try it returns null.

<WebMethod()>
<ScriptMethod(ResponseFormat:=ResponseFormat.Json, UseHttpGet:=False, 
XmlSerializeString:=False)>
Public Function SendIdDocuments(ByVal idReferto As String)
'This id has been sent from ajax as json and now I am converting it to string 
'in order to use it as parameter in stored procedure.

Dim idRef As String = JsonConvert.DeserializeObject(idReferto)
Dim dtRefertoDocIndex As DataTable = operations.G_REPORT_BY_ID(idRef)
Dim version As String = dtRefertoDocIndex.Rows(0)("VERSION")
MsgBox(Cstr(version))
End If

I have tried these:

1) Dim dtRefertoDocIndex As DataTable = operations.G_REPORT_BY_ID(Cstr(idRef)) 
2) Dim dtRefertoDocIndex As DataTable = operations.G_REPORT_BY_ID(idRef.ToString)

These dont work too. I have checked the idRef that has a value and also data from database isn't returning null and version has a value, but in the code dtRefertoDocIndex is null and for this reason version comes as null too. Any help would be appriciated.

Answer 1

A few things. Obviously you can't really use msgbox.

Next up, .net should return JSON without you having to specify it, but we can leave that.

Your web method WILL automatic split out the parameters for you.

So, your web method ALSO needs to be a function that RETURNS a value, say a string

thus it should be:

<WebMethod()>
Public Function SendIdDocuments(ByVal idReferto As String) as string

    Dim dtRefertoDocIndex As DataTable = operations.G_REPORT_BY_ID(idRef)
    Dim strResult as string = dtRefertoDocIndex.Rows(0)("VERSION")
    return strResult

End If

Note this part: Public function bla bla bla) as string <---- your function HAS to return something, right?

Now WHY did I create a string to hold the return value and THEN return it?

Answer:

Because I do NOT want to take a chance that the return type is a column cell, or ANY OTHER data type OTHER then a string. So, to be 100%, 200%, 300% of the correct data type being returned? I create a nice separate string, and stuff the value into that string, and THEN return that SIMPLE string type. This removes ANY doubt that we ARE in fact returning a simple string type. When we execute the return strResult?

It will be converted back into a json result.

Ok, now, lets setup the JavaScript (client side) ajax call.

jQuery:

        function GetDocByID() {

            var MyDocID = 123;

            $.ajax({
                type: "POST",
                url: 'WebService1.asmx/MakeName',
                data: JSON.stringify({idReferto: MyDocID}),
                contentType: "application/json; charset=utf-8",
                datatype: "json",
                success: function (em) {
                    alert('VERSION result = ' + em.d);
                }
            });
        }

If you not using jQuery, then of course you have to re-write above.

So, .net will automatic convert to and from json for you. And ALSO VERY careful note how the parameter name in JavaScript (jQuery) MUST match the parameters in the public function you created.

Here is another simple example. We pass firstname, last name, and return the two as a single string:

<WebMethod()>
Public Function MakeName(FirstName As String, LastName As String) As String

    Dim strResult As String = ""

    strResult = FirstName & " " & LastName

    Return strResult

End Function

and the js/jQuery is:

        function GetNameTest() {

            var MyFirstName = 'Albert';
            var MyLastName = 'Kallal';

            $.ajax({
                type: "POST",
                url: 'WebService1.asmx/MakeName',
                data: JSON.stringify({ FirstName: MyFirstName, LastName: MyLastName }),
                contentType: "application/json; charset=utf-8",
                datatype: "json",
                success: function (em) {
                    alert('name result = ' + em.d);
                }
            });
        }

Note again HOW we used the actual vb.net FirstName and LastName DEFINED in the public function.

Note again: the function has a return value (string).

And if you look at the web service template, note that you WANT to un-comment the FIRST line as outlined like this:

' To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
<System.Web.Script.Services.ScriptService()>