Question

it will give me error Unable to cast object of type.

  public class CusInfomration
   {
      public string CustomerName { get; set; }
      public string CustomerID { get; set; }
      public string OrderDate { get; set; }
      public string OrderId { get; set; }
   }

  var CustomerFromWash =  from p in _NorthWindDataContext.Customers
                          join q in _NorthWindDataContext.Orders 
                          on p.CustomerID   equals q.CustomerID
                          where p.Region == "WA"
                          select new
                         {                                                   
                             CustomerName =Convert.ToString(p.CompanyName),
                             CustomerID = Convert.ToString(p.CustomerID),
                             OrderId = Convert.ToString(q.OrderID),
                             OrderDate = Convert.ToString(q.OrderDate),
                          };

  List<cusinfomration> lstCust = (List<cusinfomration>)CustomerFromWash;

Answer 1

That LINQ query returns an IQueryable<T>. IQueryable<T> is an interface and List<T> does not implement it, so there is no way that the underlying, concrete implementation is a List<T> (it does however implement IEnumerable<T>, so a cast to that would be valid). Even if it were it would not be a safe cast to make as the .NET folks may want to change out the underlying implementation someday and your cast would be broken. Not a good idea even if it did work initially.

You can call ToList() on the return value of the LINQ query:

var CustomerFromWash = (from p in _NorthWindDataContext.Customers
                        join q in _NorthWindDataContext.Orders 
                        on p.CustomerID   equals q.CustomerID
                        where p.Region == "WA"
                        select new
                        {                                                   
                           CustomerName =Convert.ToString(p.CompanyName),
                           CustomerID = Convert.ToString(p.CustomerID),
                           OrderId = Convert.ToString(q.OrderID),
                           OrderDate = Convert.ToString(q.OrderDate),
                        }).ToList();

Even then though you are selecting a collection of anonymous types, not CustInformation objects. If you want CustInformation objects to be returned then you will need to:

select new CustInformation 
{
    // set properties here
}