I want to send java script array values to C# code behind file

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I want to send Java script Array values to C# code behind file

I have Check Boxes in Dev Express gridview and and now on client click event I want to
take selected check box values in java script array. and then on button click I want
to send that values to code behind using json ajax call.

But It is giving Error Message.

What I have tried:

ASPX Code For Gridview With Check Box

<dx:ASPxGridView ID="ASPxGridView1" runat="server" AutoGenerateColumns="False" DataSourceID="SqlDataSource1" KeyFieldName="AccId"
    ClientInstanceName="grid">
    <Columns>
        <dx:GridViewCommandColumn ShowSelectCheckbox="true"></dx:GridViewCommandColumn>
        <%-- <dx:GridViewDataTextColumn>
            <DataItemTemplate>
                <dx:ASPxCheckBox ID="cbCheck" runat="server" Visible="true" AutoPostBack="true"></dx:ASPxCheckBox>
            </DataItemTemplate>
        </dx:GridViewDataTextColumn>--%>
        <dx:GridViewDataTextColumn FieldName="AccId" ReadOnly="True" VisibleIndex="1">
            <EditFormSettings Visible="False" />
        </dx:GridViewDataTextColumn>
        <dx:GridViewDataTextColumn FieldName="AccessName" VisibleIndex="0">
        </dx:GridViewDataTextColumn>
    </Columns>
    <ClientSideEvents SelectionChanged="grid_SelectionChanged" />
</dx:ASPxGridView>

Then Bellow this button control as

<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:DemoDBConnectionString %>"
           SelectCommand="GetUserAccess" SelectCommandType="StoredProcedure"></asp:SqlDataSource>
       <input id="Button1" type="button" value="GetData" onclick="myfun()" />

Java Script Function in Head Section as follows

<script type="text/JavaScript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
    <script type="text/javascript">
        // alert('c');
        var myval = [];
        function grid_SelectionChanged(s, e) {
            alert("Click");
            s.GetSelectedFieldValues("AccId", GetSelectedFieldValuesCallback);
        }
        function GetSelectedFieldValuesCallback(values) {
           alert(values);
            try {
                myval.push(values);
            }
            finally { }
            
        }

        function myfun() {
            $.ajax({
                type: "POST",
                url: "WebForm1.aspx/MyMethod", // the method we are calling
                contentType: "application/json; charset=utf-8",
                //data: { Param1: myval },
                data: "{ Param1: '" + myval + "'}",
                dataType: "json",
                success: function (result) {
                    alert('Yay! It worked!');
                },
                error: function (result) {
                    alert('Oh no :(');
                }
            });

        }
</script>

code behind file Method(.cs files) as

[WebMethod]
       public static void MyMethod(string []Param1)
       {
           try
           {
               //Do here server event
           }
           catch (Exception)
           {
               throw;
           }
       }

but it is giving Error as

Oh no

plese help me to solve this
Thanks in advance