Returning IEnumerable vs. IQueryable

Question

What is the difference between returning IQueryable vs. IEnumerable, when should one be preferred over the other?

Answer 1

There is a blog post with brief source code sample about how misuse of IEnumerable can dramatically impact LINQ query performance: Entity Framework: IQueryable vs. IEnumerable.

If we dig deeper and look into the sources, we can see that there are obviously different extension methods are perfomed for IEnumerable:

// Type: System.Linq.Enumerable
// Assembly: System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
// Assembly location: C:\Windows\Microsoft.NET\Framework\v4.0.30319\System.Core.dll
public static class Enumerable
{
    public static IEnumerable Where(
        this IEnumerable source, 
        Func predicate)
    {
        return (IEnumerable) 
            new Enumerable.WhereEnumerableIterator(source, predicate);
    }
}

and IQueryable:

// Type: System.Linq.Queryable
// Assembly: System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
// Assembly location: C:\Windows\Microsoft.NET\Framework\v4.0.30319\System.Core.dll
public static class Queryable
{
    public static IQueryable Where(
        this IQueryable source, 
        Expression> predicate)
    {
        return source.Provider.CreateQuery(
            Expression.Call(
                null, 
                ((MethodInfo) MethodBase.GetCurrentMethod()).MakeGenericMethod(
                    new Type[] { typeof(TSource) }), 
                    new Expression[] 
                        { source.Expression, Expression.Quote(predicate) }));
    }
}

The first one returns enumerable iterator, and the second one creates query through the query provider, specified in IQueryable source.