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I tried this code for adding b to books:
IEnumerable<Book> books =null; foreach (Book b in context.Books.AsEnumerable()) if (someConditions) books = books.Concat(new[] {b}); but gives me this error on last line of code:
System.ArgumentNullException: Value cannot be null. Parameter name: first
it seems that null Collection could not concatenated. I use EF,so how should I initialize my Collection that have no thing in it and I could concatenate to it?
893
Empty<T> actually returns an empty array of T (T[0]), with the advantage that the same empty array is reused. Note that this approach is not ideal for non-empty arrays, because the elements can be modified (however an array can't be resized, resizing involves creating a new instance).
What you can do is use the Add extension method to create a new IEnumerable<T> with the added value. var items = new string[]{"foo"}; var temp = items; items = items. Add("bar");
IEnumerable in C# is an interface that defines one method, GetEnumerator which returns an IEnumerator interface. This allows readonly access to a collection then a collection that implements IEnumerable can be used with a for-each statement.
IEnumerable is an interface defining a single method GetEnumerator() that returns an IEnumerator interface. It is the base interface for all non-generic collections that can be enumerated. This works for read-only access to a collection that implements that IEnumerable can be used with a foreach statement.
It seams all you want to do is filter your context.Books by some criteria.
IEnumerable<Book> books = context.Books.Where(b => someConditions); If you still need the empty IEnumerable you can just call Enumerable.Empty():
IEnumerable<Book> books = Enumerable.Empty<Book>(); 
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