"Object reference not set to an instance of an object" on predicate

Question

I'm trying to find clients which fields have or contain something. Everything works if I hard code predicate, but when I pass predicate it throws a NullReferenceException.

Repo code:

public IEnumerable<Contractor> Find(Func<Contractor, bool> predicate)
{
    var test = db.Contractors.Where(x => x.NIP.Contains("7822574676")).ToList(); //this is correct
    try
    {
        return db.Contractors.Where(predicate).ToList(); // this gives exception
    }
    catch (Exception ex)
    {
        return null;
    }  
}

Service code:

public IEnumerable<ContractorShortDataDTO> FindByNIP(string NIP)
{
    try {
        return Database.Contractors.Find(x => x.NIP.Contains(NIP)).Select(x =>
            new ContractorShortDataDTO()
            {
                NIP = x.NIP,
                CompanyName = x.CompanyName,
                ID = x.ID
            }).AsEnumerable();
    } catch(Exception ex) {
            return null;
    }
}

What is wrong with that code?

Answer 1

Change the type of the argument in the method from

Func<Contractor, bool> predicate

to

Expression<Func<Contractor, bool>> predicate

See also related question Why would you use Expression<Func<T>> rather than Func<T>?