Question

I am going to pass an variable from asp.net server to the javascript but I got an exception.

The name ‘serializer’ does not exist in the current context
In my Admin.aspx.cs

 protected static string urlEdit;
    protected void Page_Load(object sender, EventArgs e)
    {
        // blah blah...
        var serializer = new System.Web.Script.Serialization.JavaScriptSerializer();
    }

Then in the markup code:

<script type="text/javascript">
            function Edit_Click() {
                var options = SP.UI.$create_DialogOptions();


                options.url = <%= serializer.Serialize(urlEdit) %>;

                              };

Answer 1

serializer only exists in Page_Load. Instead of accessing serializer from your markup, why not create public method or property that returns the serialized value instead?

protected static string urlEdit;
protected void Page_Load(object sender, EventArgs e)
{
    // blah blah...
    var serializer = new System.Web.Script.Serialization.JavaScriptSerializer();
}

public string GetSerializedUrl()
{
    return serializer.Serialize(urlEdit);
}

Then call this method in your markup:

options.url = "<%= GetSerializedUrl() %>";

Update

Actually … you don’t need to serialize a string to use it javascript. So your code could be simplified to the following:

protected static string urlEdit = "www.example.com";

And the simplified markup:

options.url = "<%= urlEdit  %>";