Difference between IEnumerable and IQueryable

Here is the small sample that explains the difference between IEnumerable<T> and IQueryable<T>.

When we perform the IEnumerable <T> the data is first loaded into the memory and then the conditional Query is then performed on it, where as in IQueryable the query is executed in the database if possible .

consider the example..

        NorthwindEntities ne = new NorthwindEntities();
        protected void Page_Load(object sender, EventArgs e)
        {
           IQueryable<Employee> emp = ne.Employees.Take(10);
          
           List<Employee> empList = emp.ToList();

        }

When you look at the emp when you are in debug mode you could see in the  LOCALS window the emp is value is  
emp         --     {System.Data.Objects.ObjectQuery<JsonApplication2.Employee>}
this simply specifies it the just the Query and when you execute emp.ToList() , then the data is retrieved and assigned to empList.

and major advantage of IQuerayable is that the query is Executed in the database if possible . So most of the data is filtered there itself.

Coming to the IEnumerable<T>

public partial class TabLazyLoadingSample : System.Web.UI.Page
    {
        NorthwindEntities ne = new NorthwindEntities();
        protected void Page_Load(object sender, EventArgs e)
        {
           IEnumerable<Employee> emp = ne.Employees.Take(10);          
           List<Employee> empList = emp.ToList();

        }
     }
In this case the main Query is loaded into the memory and then there the execution for ISActive Employees is performed.

This is quite an important difference, and working on IQueryable<T> can in many cases save you from returning too many rows from the database. Another prime example is doing paging: If you use Take and Skip on IQueryable, you will only get the number of rows requested; doing that on an IEnumerable<T> will cause all of your rows to be loaded in memory.