How to use Hwnd(c++) type in c#

In the absence of any other information, I'd try:

[DllImport("somedll.dll", CharSet = CharSet.Ansi)]
static extern IntPtr(IntPtr hwnd, [MarshalAs(UnmanagedType.LPStr)] string params);

However, as Sam said, if the return type is a C++ string, I doubt whether you're going to be able to do anything with the IntPtr you get back as it's not a type which the .NET marshaller supports.

Look for articles about p/invoke or pinvoke. Also articles about using DllImport or DllImportAttribute. They are all the same thing so if you find an article about one then it will be about the others too.

I am concerned about the return type "string". If it is a C++ string then you probably cannot use the DLL with any language other than C++.

If the return type is a .Net string type then you do not need to use p/invoke. So you need to determine for sure whether the DLL is unmanaged or managed. The best way to determine for sure whether the DLL is unmanaged or managed is to look at the documentation of the DLL.