I not seen much (if any) posted code for using [AjaxMethod]. That requires a reference to the ajax.dll, and on page load you have to execute a command to setup and load a few moving parts.

WebMethod should be used in near all cases, since:

It works and plays nice using a jQuery.ajax call.

WebMethod supports soap, ajax, REST, xml, json, and text, and DOES so without ANY changes to your code.

So, assuming you have jQuery installed (don't we all???, and by default even any asp.net project for about 10+ years has included jQuery).

So, then this code can work just fine:

Markup:

<h4>Select fighter</h4>
<asp:DropDownList ID="cboFigher" runat="server" Width="300"
    DataTextField="Fighter"
    DataValueField="ID"
    onchange="mychange(this)">
</asp:DropDownList>
<br />


<div class="mybox" style="float: left; border: solid 1px">
    <div style="text-align: center; padding: 2px 10px 12px 10px">
        <h3 id="Fighter" runat="server"></h3>
        <asp:Image ID="Image2" runat="server"
            Width="180" Height="120" />
        <h4>Engine</h4>
        <asp:Label ID="EngineLabel" runat="server" Text="" />
        <h4>Description</h4>
        <asp:Label ID="DescLabel" runat="server" Width="400px"
            Text="" Style="text-align: left" Font-Size="Large" />
    </div>
</div>

<script>

    function mychange(myitem) {
        // get data record from server, update
        // dom values
        var iPK = myitem.value
        $.ajax({
            type: "POST",
            url: "FighterOneAJ.aspx/GetFighter",
            data: JSON.stringify({ PK: iPK }),
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: function (rData) {
                var F = rData.d
                $('#Image2').attr("src", F.ImagePath)
                $('#Fighter').text(F.Fighter)
                $('#EngineLabel').text(F.Engine)
                $('#DescLabel').text(F.Description)

            },
            failure: function (rData) {
                alert("error " + rData.d);
            }
        });

    }

</script>

And the code behind is this:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    If Not IsPostBack Then

        cboFigher.DataSource =
                MyRst("SELECT ID, Fighter FROM Fighters ORDER BY Fighter")
        cboFigher.DataBind()
        cboFigher.Items.Insert(0, New ListItem("Select Fighter", "0"))

    End If

End Sub


<WebMethod()>
Public Shared Function GetFighter(PK As String) As clsFigher

    Dim OneFigher As New clsFigher

    OneFigher.ID = PK
    Return OneFigher


End Function


Public Class clsFigher
    Public Fighter As String
    Public Engine As String
    Public Thrust As String
    Public Description As String
    Public ImagePath As String
    Private m_id As Integer

    Public Property ID As Integer
        Get
            Return m_id
        End Get
        Set(value As Integer)
            m_id = value

            Dim cmdSQL As New _
                    SqlCommand("SELECT * FROM Fighters WHERE ID = @ID")

            cmdSQL.Parameters.Add("@ID", SqlDbType.Int).Value = m_id

            With MyRstP(cmdSQL).Rows(0)
                Fighter = .Item("Fighter")
                Engine = .Item("Engine")
                Thrust = .Item("Thrust")
                Description = .Item("Description")
                ImagePath = Replace(.Item("ImagePath"), "~/", "../")

            End With
        End Set
    End Property

End Class

And now the result is this:

So, the main difference is we don't see much code or use of [AjaxMethod], and use of that requires a reference to the ajax.dll, and also requires that you on page load event execute some code to load the script part to use AjaxMehods.

I can safe suggest that I see VERY little reason to use [AjaxMethods] when using WebMethod works fine, and does not require extra setup, extra .dll, and extra code on page load event for AjaxMethods to work.

Edit: Using and calling a webmethod in a ascx file

Ok, this takes a "bit" of a extra step.

So, you can NOT directly use nor call a web method in a ascx page (this is because such pages are ASSUMED to be part of a existing page).

So, I turned the justguageJS into a UC. So, to display a gauge on a page.

The simple answer???

You can't really directly call a ascx method. So you MUST place a method and code stub in the current aspx page.

So, say this markup:

        <div style="float:left">
            <uc1:GaugeJS runat="server" 
                ID="GaugeJS1" 
                Donut="true" 
                Label="Tempature\nF"
                Max="80" 
                Min="0" 
                Value=0
                Counter="true"
                FontColor="black"
                Width="100"
                />
        </div>

        <div style="float:left">
            <uc1:GaugeJS runat="server" 
                ID="GaugeJS2" 
                Donut="true" 
                Label="Tempature\nC"
                Max="80" 
                Min="0" 
                Value="0" Counter="true"
                FontColor="black"
                Width="100"
                />
        </div>
        <br />

        <asp:Button ID="cmdGetFuel" runat="server" 
            Text="Convert to C"
            CssClass="btn"
            OnClientClick="webcalltest();return false"
            />

        <script>
            var F = 68

            function mytest() {
                GaugeJS1.refresh(F, 80, 0)
            }

            function webcalltest() {
                $.ajax({
                    type: "POST",
                    url: '<%= ResolveUrl("~/Controls/TestGuage.aspx/GetTemp") %>',
                    dataType: "json",
                    contentType: "application/json; charset=utf-8",
                    data: JSON.stringify({F : F }),
                    success: function (data) {
                        var C = data.d
                        GaugeJS2.refresh(C, 80, 0)
                    },
                    error: function (msg) {
                        alert("Failed: " + msg.status + ": " + msg.statusText);
                    }
                });
            }


            $(window).on('load',function () {
                GaugeJS1.refresh(F, 80, 0)
            });

        </script>

So, on the "aspx" page, I want to call a web method in the ascx file.

but, I have the ajax call to CURRENT page, and the code then looks like this:

<WebMethod()>
Public Shared Function GetTemp(F As Integer) As String

    Return GaugeJS.GetTemp(F).ToString

End Function

So, I THEN in above call the public function in the ascx (code behind).

The web method in the ascx page is thus this:

<WebMethod>
Public Shared Function GetTemp(F As Integer) As String

    Dim C As Double
    C = (F - 32) / 1.8

    Return C.ToString

End Function

So, you CAN have a web method in the ascx control, but you can never call it directly - you have to "add" a method to the EXISTING page, and then call the ascx code from the current page's web method.

The result looks like this: