LINQ: Cannot get more than two Where clauses to work - linq

Question

I cannot get the following to work properly. I want to search my database using more than 2 conditions where it will be possible to search only on one, two or all three, thus narrowing the search. At the moment, I can only get it to work with one condition at a time, and it only evaluates the first condition when having more than 3.
How can I Include more than 2 conditions? I have tried several proposed solutions from other sources, that uses if statements and where clauses, but so far none of them have worked.
This is my controller
public IActionResult Index(string searchString, int searchProject, string searchTitel)
{
var Projektdokumenter = from p in _context.ProjekterDokutypeDokuundertype
.Include(p => p.Dokumenttype)
.Include(p => p.Dokuundertype)
.Include(p => p.Forfatter)
.Include(p => p.P)
.Include(p => p.Sprog)
select p;
if (!String.IsNullOrEmpty(searchString) ^ searchProject != null ^ !String.IsNullOrEmpty(searchTitel))
{
if (!String.IsNullOrEmpty(searchString))
{
var c = Projektdokumenter.Where(p => p.Forfatter.Initialer.Contains(searchString)).ToList();
return View(c);
}
{
if (searchProject != 0)
{
var d = Projektdokumenter.Where(p => p.P.ProjektId.Equals(searchProject)).ToList();
return View(d);
}
{
if(!String.IsNullOrEmpty(searchTitel))
{
var f = Projektdokumenter.Where(p => p.Titel.Contains(searchTitel)).ToList();
return View(f);
}
}
}
}
var e = Projektdokumenter.Where(c.Contains(searchString) && p.P.ProjektId.Equals(searchProject) && p.Titel.Contains(searchTitel)).ToList();
return View(e);
}

Answer 1

IQueryable can be combined, so just write in native way:
{
var query = _context.ProjekterDokutypeDokuundertype
.Include(p => p.Dokumenttype)
.Include(p => p.Dokuundertype)
.Include(p => p.Forfatter)
.Include(p => p.P)
.Include(p => p.Sprog)
.AsQueryable();
if (!string.IsNullOrEmpty(searchString))
{
query = query.Where(p => p.Forfatter.Initialer.Contains(searchString));
}
if (searchProject != 0)
{
query = query.Where(p => p.P.ProjektId == searchProject);
}
if (!string.IsNullOrEmpty(searchTitel))
{
query = query.Where(p => p.Titel.Contains(searchTitel))
}
return View(query.ToList());
}