IEnumerable and IQueryable + Entity Framework

Linq to SQL and Linq to Objects queries are not the same.

LINQ to Objects queries operate on IEnumerable collections. The query iterates through the collection and executes a sequence of methods (for example, Contains, Where etc) against the items in the collection.

LINQ to SQL queries operate on IQueryable collections. The query is converted into an expression tree by the compiler and that expression tree is then translated into SQL and passed to the database.

IQueryable inherits from IEnumerable

All LINQ to Objects queries return IEnumerable or a derivative of IEnumerable, all IEnumerable expressions and executed in memory against the full dataset

IQueryable uses a DbQueryProvider (IQueryProvider) to translate the expression (the chained extension methods) into a single database query (in this case, it generates T-SQL to run against the database). Once the query is invoked (by say, enumerating it), the query is executed against the database and the results are returned back to be consumed

All of your queries for data when using Entity Framework are written against DbSet
public class DbSet : DbQuery, IDbSet, IQueryable, IEnumerable, IQueryable, IEnumerable where TEntity : class



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