public IQueryable<mdm_DogovorAll> GetDogovorAll()
        {
            var query =
                   from c in this.Dogovor... select c;

            if (fOtborDoc["num"].active)
                query = (from c in query
                         where c.dg_num.Contains(fOtborDoc["dg_num"].value.ToString())
                         select c)
// и so on and so futh
}

Method 'otbor get_Item(System.String)' has no supported translation to SQL.

 public IQueryable<mdm_DogovorAll> GetDogovorAll(OtborDoc fOtborDoc )
        {
            var query =
                   from c in this.Dogovor... select c;

            if (fOtborDoc["num"].active)
                query = (from c in query
                         where c.dg_num.Contains(fOtborDoc["dg_num"].value.ToString())
                         select c)
// и so on and so futh
}

 DataContext db;
 private void Form1_Load(object sender, EventArgs e)
        {
            db = new DataContext(DBConnection);           
            ReadDoc();

        }

         void ReadDoc()
        {
            var q = db.GetDogovorAlll(db.fOtborDoc);// передаём как параметр!!!
            HdrbindingSource.DataSource = q;

        }

OtborDoc  FOtborDoc = new OtborDoc ();
internal class otbor_field
    {
        public bool active { get; set; }
        public object value { get; set; }
        public object value1 { get; set; }
    }

    internal class OtborDoc : Dictionary<string, otbor_field>
{
internal OtborPrihNakl()
        {
            this.Add("dg_num", new otbor_field { active = false, value = "" });
}
}

Method 'otbor get_Item(System.String)' has no supported translation to SQL.

publick IQueryable<Dogovor> GetDogovorAll() {
  var query = from c in this.Dogovor
                        select c;
            string snum = fOtborDoc["dg_num"].value.ToString();

            if (fOtborDoc["dg_num"].active)
                query = (query.Where(c => c.dg_num.Contains(snum)));

return query ;
}

 var query = from c in this.Dogovor
                        select c;

var query = db.Dogovor.AsQueryable();

  
var query = this.Вщпщмщк.AsQueryable();
             
            if (fOtborPrihNakl["dg_num"].active)
                query = (query.Where(c => c.dg_num.Contains(snum)));


            if (fOtborPrihNakl["dg_date"].active)
                query = query.Where(c => c.pn_date >= d1 && c.pn_date <= d2);

            return query;

foreach(var col in columns) {\n   if(col.Active)\n       query = query.Where(_expressions[col.Name](col.Value));\n}

class Person
{
	public string FirstName { get; set; }
	public string LastName { get; set; }
	public string Position { get; set; }
}

static List<Person> _persons = new List<Person>() {
	new Person() { FirstName = "Ivan", LastName="Ivanov", Position="Manager" },
	new Person() { FirstName = "Maxim", LastName="Ivanov", Position="Employee" },
	new Person() { FirstName = "Maxim", LastName="Petrov", Position="Employee" }
};

static Dictionary<string, Func<object, Func<Person, bool>>> expressions = 
	new Dictionary<string, Func<object, Func<Person, bool>>>() 
{
	{"first", (p) => ((a) => a.FirstName == p.ToString())},
	{"last", (p) => ((a) => a.LastName == p.ToString())},
	{"position", (p) => ((a) => a.Position == p.ToString())}
};