如何使用LINQ Lambda从IEnumerable集合中获取对象？

Question

storageColl is having a IStorage with property "Id" as "Test".

What I am doing-

string id="Test";
IEnumerable<IStorageObject> storageColl = getStorageCollection();
IStorageObject storageObject = storageColl.ToList().Where(m => m.Properties["Id"] == id)
.ToList()
.Cast<IStorageObject>().ToArray()[0];

Is there a better way to do this. As this may throw array out of bound exception if the storageColl will not have that "Test".

Answer 1

You can use FirstOrDefault on the IEnumerable .

var storageObject = storageCol1.Where(m => m.Properties["Id"] == id).FirstOrDefault();

Or as David Hedlund pointed out, use the predicate overload on FirstOrDefault and remove the Where.

var storageObject = storageCol1.FirstOrDefault(m => m.Properties["Id"] == id);

Answer 2

Your storageColl is a sequence of objects that implement IStorageObject. The use of the Where only limits the elements you get when you enumerate over the sequence, it does not change them.

It is a waste of processing power to convert the sequence to a list when you only need the first element of the sequence or the a subset of it.

Familiarize yourself with the following Ling functions:

• Any() returns true if the sequence contains at least one element
• Any( item => ....) return true if any of the elements in the sequence meets the requirement
• First() returns the first element of the sequence. Exception if not Any()
• FirstOrDefault returns the first element of the sequence or the default (usually null) if not Any()

The nice thing about these functions is that they don't have to enumerate over all elements in the sequence, but can stop as soon as they found something.

If you use ToList() the code enumerates over all elements, throws most of them away and uses only the first element. FirstOrDefault() would have stopped after the first enumeration.

Answer 3

您可以通过以下方式简单地实现

var result = storageColl.Where(m => m.Properties["Id"] == id);

Answer 4

since the collection is implement IStorageObject you don't neet to cast them and for get item by index you can use any class that utilizes Array or IList

since LINQ operates on IEnumerable (Array itself is enumerable->to iterate) you don't need to cast them to array.you can utilize ElementAt method or Use IList classes (as List)

IStorageObject storageObject = storageColl.Where(m => m.Properties["Id"] == id).First();

Answer 5

You should check firstOrDefault because can return null.

var Object = storageCol.Where(p => p.Properties["Id"] == id).FirstOrDefault();

if(Object != null)
{
    // Do some Work
}