c# - Why use .AsEnumerable() rather than casting to IEnumerable<T>?

Question

One of the extension methods on IEnumerable<T> is .AsEnumerable(). This method converts the enumerable object it was called on into an instance of IEnumerable<T>. However, since an object must implement IEnumerable<T> in order to apply to this extension method, converting to IEnumerable<T> is a simple matter of casting to IEnumerable<T>. My question is why does this method exist at all?

Example:

List<string> strings = new List<string>() { "test", "test2", "test3" };
IEnumerable<string> stringsEnum1 = strings.AsEnumerable();
IEnumerable<string> stringsEnum2 = (IEnumerable<string>)strings;

In the example above, stringsEnum1 and stringsEnum2 are equivalent. What's the point of the extension method?

Edit: As a corollary, why is there an .AsQueryable() method when casting to IQueryable<T> is equivalent?

Answer 1

Readability is the main issue here. Consider that

Table.AsEnumerable().Where(somePredicate)

is far more readable than

((IEnumerable<TableObject>)Table).Where(somePredicate).

Or imagine wanting to execute part of the query on the SQL Server and the rest in memory:

Table.Where(somePredicate)
     .Select(someProjection)
     .AsEnumerable()
     .SomethingElse()

versus

((IEnumerable<SomeProjectionType>)Table.Where(somePredicate)
                                       .Select(someProjection))
                                       .SomethingElse()

Now, as for why such a method is useful at all think of the example of a Table in a LINQ to SQL DataContext. As Table is an IQueryable it implements IEnumerable. When you invoke a Where method on such a Table and enumerate through the results, code is executed that eventually causes a SQL statement to be executed on a SQL Server. What AsEnumerable does is says, no, I don't want to use the LINQ to SQL provider to execute the Where, I want to use the LINQ to Objects implementation of Where.

Thus enumerating over

Table.Where(somePredicate)

causes a query to be executed on a SQL Server whereas enumerating over

Table.AsEnumerable().Where(somePredicate)

brings the table represented by Table into memory and executes the Where functionality in memory (and not on the SQL Server!)

This is the point of AsEnumerable: to allow you to hide a specific implementation of IEnumerable methods and instead use the standard implementation.