JavaScript function return boolean in C#

Question

How can I return a true/false from a JavaScript function to C# code?

What is the problem with my code?

ASP.NET

< asp:HiddenField ID="hiddenFieldResult" runat="server" />

JavaScript

< script type="text/javascript">
< !--
function deleteUser(userName) {
var retVal = confirm("Delete " + userName + "?");
if (retVal == true) {
  document.getElementById('<% =hiddenFieldResult.ClientID %>').value = true;
  return true;
 }
else {
  document.getElementById('<% =hiddenFieldResult.ClientID %>').value = false;
  return false;
 }
}
//-->
</script>

C#

Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "deleteUser('" + user.userName + "');", true);
Label1.Text = "User is deleted? " + hiddenFieldResult.Value;

You need to understand that difference between client-side code and server-side code. That's totally impossible; you need multiple HTTP requests. — SLaks
– SLaks, Commented Feb 6, 2014 at 17:11
Run it in the debugger - you'll see that those two C# lines are executed before the page is shown in the browser and the script executes. — Hans Keﬆing
– Hans Keﬆing, Commented Feb 6, 2014 at 18:33

Yushell · Accepted Answer · 2014-02-06 18:35:58Z

SLaks is right. You can't just pass values from JavaScript to C# that way. What I usually do for simple cases when I need to share data between JavaScript (client-side code) and C# (server-side code) is create an ASPX file that will recieve the JavaScript variables.

For example, in my HTML I have this:

var Id = $(this).data("Id");
$.ajax({
    type: "POST",
    url: "catcher.aspx/doSomething",
    data: '{ Id: ' + Id + ' }',
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    async: false,
    success: function (data) {
        // Do something on success
        console.log(data.d);
        // data.d returns "Hello World!"
    },
    failure: function (response) {
        // Do something on failure
    }
});

With this I'm sending and AJAX POST to a file called catcher.aspx, which has a WebMethod called doSomething in it's codebehind file. Which looks like this:

namespace myNamespace
{
    public partial class functions : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
        }

        [WebMethod()]
        public static string doSomething(string Id)
        {
            string something = "Hello World!";
            return something;
        }
    }
}

To wrap up. The JavaScript calls the method in C# through an AJAX POST. It sends and Id and gets a string in return. You can tweak both the JavaScript and C# to your needs.

If you're trying yo send a boolean to C#, POST it.

function deleteUser(userName) {
    var retVal = confirm("Delete " + userName + "?");
    if (retVal == true) {
        document.getElementById('<% =hiddenFieldResult.ClientID %>').value = true;
        // POST true to example.aspx doSomething method
    } else {
        document.getElementById('<% =hiddenFieldResult.ClientID %>').value = false;
        // POST false to example.aspx doSomething method
    }
}

Catch it in example.aspx.cs

namespace myNamespace
{
    public partial class functions : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
        }

        [WebMethod()]
        public static string doSomething(boolean Id)
        {
            string something = "Hello World!";
            return something;
        }
    }
}

By the way, this requires jQuery. Hope it helped.

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