How to get the first element of IEnumerable

问题

Is there a better way getting the first element of IEnumerable type of this:

foreach (Image image in imgList)
{
     picture.Width = (short)image.Columns;
     picture.Height = (short)image.Rows;
     break;
}

This is the exact declaration of the type:

public class ImageList : IEnumerable, IDisposable

回答1:

var firstImage = imgList.Cast<Image>().First();

回答2:

If you can't use LINQ you could also get the enumerator directly by imgList.GetEnumerator() And then do a .MoveNext() to move to the first element. .Current will then give you the first element.

回答3:

The extension .First() will grab the first item in an enumerable. If the collection is empty, it will throw an exception. .FirstOrDefault() will return a default value for an empty collection (null for reference types). Choose your weapon wisely!

回答4:

Might be slightly irrelevant to your current situation, but there is also a .Single() and a .SingleOrDefault() which returns the first element and throws an exception if there isn't exactly one element in the collection (.Single()) or if there are more than one element in the collection (.SingleOrDefault()).

These can be very useful if you have logic that depends on only having a single (or zero) objects in your list. Although I suspect they are not what you wanted here.

回答5:

I had an issue where I changed my datasource from a bindingsource to an entity framework query.

var query = dataSource as IQueryable;
var value = query.Where("prop = @0", value).Cast<object>().SingleOrDefault();

With entity framework this throw an exception `Unable to cast the type 'customer' to type 'object'. LINQ to Entities only supports casting EDM primitive or enumeration types.

The class where my code was did not have a reference to the lib with the model so ...Cast<customer> was not possible.

Anyway I used this approach

var query = dataSource as IQueryable;
var targetType = query.GetType().GetGenericArguments()[0];
var value = query.Where("prop = @0", value).SingleOrDefault(targetType);

in conjunction with an IEnumerable extension which uses reflection

    public static object SingleOrDefault(this IEnumerable enumerable, Type type)
    {
        var method = singleOrDefaultMethod.Value.MakeGenericMethod(new[] { type });
        return method.Invoke(null, new[] { enumerable });
    }

    private static Lazy<MethodInfo> singleOrDefaultMethod 
         = new Lazy<MethodInfo>(() =>
             typeof(Extensions).GetMethod(
                 "SingleOrDefault", BindingFlags.Static | BindingFlags.NonPublic));

    private static T SingleOrDefault<T>(IEnumerable<T> enumerable)
    {
        return enumerable.SingleOrDefault();
    }

feel free to implement caching per type to improve performance.

来源：https://stackoverflow.com/questions/2317108/how-to-get-the-first-element-of-ienumerable

标签

.net

ienumerable

linq-to-objects

html-lists