http://localhost:4567/Test/Callback#state=test&access_token=....

http://localhost:4567/Test/Callback

var Uri = new Uri("http://localhost:4567/Test/Callback#state=test&access_token=....");

// Contains the query
Uri.Fragment

#state=test&access_token=....

Request.Url.ToString();

http://localhost:4567/Test/Callback?state=test&access_token=...

var url=@"http://localhost:4567/Test/Callback#state=test";
var uri = new Uri(url);
var result = uri.Fragment;

string queryString = Request.Url.Query; // gives you "state=test&access_token=...."

string state = Request.QueryString["state"]; // gives you "test"

<script language="javascript" type="text/javascript">

    function JavaScriptFunction() {
        document.getElementById('<%= hdnResultValue.ClientID %>').value = document.URL;
    }

</script>

<asp:HiddenField ID="hdnResultValue" Value="0" runat="server" />
        <asp:Button ID="Button_Get" runat="server" Text="run" OnClick="Button_Get_Click" OnClientClick="JavaScriptFunction();" />

protected void Button_Get_Click(object sender, EventArgs e)
{
   string fullURL = hdnResultValue.Value;
   string URl = fullURL .Substring(fullURL .IndexOf('#') + 1);
}