Change the contents of a file ProjectItem within a Project in a Visual Studio 2015 Extension?

Question

How do I change the contents of a file/ProjectItem within a given Project? I would like to replace all of its contents with a string.

Answer 1

there is have a resolution of this question?

I want to make some changes in ProjectItem (CS file) by using VSIX package, and an only single way to do this that I see right now - it's read this file by FullPath, make changes, and then save changes. I know that in ProjectItem have FileCodeModel property, but it is null.