What is the format specifier for string in globalisation in ASP.net?

Server Error in '/HINDITEST' Application.
Format specifier was invalid.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.FormatException: Format specifier was invalid.
Source Error:
Line 59:         txtLangName.Text = ddlLanguage.SelectedItem.Text;
Line 60:         txtLongDate.Text = DateTime.Now.ToLongDateString();
Line 61:         txtCurrency.Text = dblCurrency.ToString("s");
Line 62:         txtAmount.Text = dblAmount.ToString("c");
Line 63:     }

Source File: e:\HINDITEST\Default.aspx.cs    Line: 61
Stack Trace:
[FormatException: Format specifier was invalid.]
   System.Number.FormatDouble(Double value, String format, NumberFormatInfo info) +0
   System.Double.ToString(String format) +26
   _Default.ApplyLocalization() in e:\HINDITEST\Default.aspx.cs:61
   _Default.ddlLanguage_SelectedIndexChanged(Object sender, EventArgs e) in e:\HINDITEST\Default.aspx.cs:48
   System.Web.UI.WebControls.ListControl.OnSelectedIndexChanged(EventArgs e) +105
   System.Web.UI.WebControls.DropDownList.RaisePostDataChangedEvent() +134
   System.Web.UI.WebControls.DropDownList.System.Web.UI.IPostBackDataHandler.RaisePostDataChangedEvent() +7
   System.Web.UI.Page.RaiseChangedEvents() +117
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1646