Update Web.config file Dynamically using DTE in VSExtention

Hi Team,

I have update Web.config file Dynamically using DTE but its Work only When Web.config file open. if Web.config File Close its throw Argument incrorrect Exception.

doc = dte.Documents.Item(dte.Solution.FindProjectItem("web.config").Name);

 doc.ReplaceText("assemblyName","newAssemblyName");