Does C# have IsNullOrEmpty for List/IEnumerable?

问题:

I know generally empty List is more prefer than NULL. But I am going to return NULL, for mainly two reasons

1. I have to check and handle null values explicitly, avoiding bugs and attacks.
2. It is easy to perform ?? operation afterwards to get a return value.

For strings, we have IsNullOrEmpty. Is there anything from C# itself doing the same thing for List or IEnumerable?

回答1:

nothing baked into the framework, but it's a pretty straight forward extension method.

See here

/// <summary>
    /// Determines whether the collection is null or contains no elements.
    /// </summary>
    /// <typeparam name="T">The IEnumerable type.</typeparam>
    /// <param name="enumerable">The enumerable, which may be null or empty.</param>
    /// <returns>
    ///     <c>true</c> if the IEnumerable is null or empty; otherwise, <c>false</c>.
    /// </returns>
    public static bool IsNullOrEmpty<T>(this IEnumerable<T> enumerable)
    {
        if (enumerable == null)
        {
            return true;
        }
        /* If this is a list, use the Count property for efficiency. 
         * The Count property is O(1) while IEnumerable.Count() is O(N). */
        var collection = enumerable as ICollection<T>;
        if (collection != null)
        {
            return collection.Count < 1;
        }
        return !enumerable.Any(); 
    }

Daniel Vaughan takes the extra step of casting to ICollection (where possible) for performance reasons. Something I would not have thought to do.

回答2:

Late update: since C# 6.0, the null-propagation operator may be used to express concise like this:

if (enumerable?.Any() ?? false)

---

Note 1: ?? false is necessary, because of the following reason (summary/quote from this post):

?. operator will return null if a child member is null. But [...] if we try to get a non-Nullable member, like the Any() method, that returns bool [...] the compiler will "wrap" a return value in Nullable<>. For example, Object?.Any() will give us bool? (which is Nullable<bool>), not bool. [...] Since it can't be implicitly casted to bool this expression cannot be used in the if

Note 2: as a bonus, the statement is also "thread-safe" (quote from answer of this question):

In a multithreaded context, if [enumerable] is accessible from another thread (either because it's a field that's accessible or because it's closed over in a lambda that is exposed to another thread) then the value could be different each time it's computed [i.e.prior null-check]

回答3:

There is nothing built in.

It is a simple extension method though:

public static bool IsNullOrEmpty<T>(this IEnumerable<T> enumerable)
{
  if(enumerable == null)
    return true;

  return !enumerable.Any();
}

回答4:

var nullOrEmpty = list == null || !list.Any();

回答5:

If you need to be able to retrieve all of the elements in the case of it not being empty, then some of the answers here won't work, because the call to Any() on a non-rewindable enumerable will "forget" an element.

You could take a different approach and turn nulls into empties:

bool didSomething = false;
foreach(var element in someEnumeration ?? Enumerable.Empty<MyType>())
{
  //some sensible thing to do on element...
  didSomething = true;
}
if(!didSomething)
{
  //handle the fact that it was null or empty (without caring which).
}

Likewise (someEnumeration ?? Enumerable.Empty<MyType>()).ToList() etc. can be used.

回答6:

As everyone else has said, nothing is built into the framework, but if you are using Castle then Castle.Core.Internal has it.

using Castle.Core.Internal;

namespace PhoneNumbers
{
    public class PhoneNumberService : IPhoneNumberService
    {
        public void ConsolidateNumbers(Account accountRequest)
        {
            if (accountRequest.Addresses.IsNullOrEmpty()) // Addresses is List<T>
            {
                return;
            }
            ...

回答7:

Putting together the previous answers into a simple extension method for C# 6.0+:

    public static bool IsNullOrEmpty<T>(this IEnumerable<T> me) => !me?.Any() ?? true;

回答8:

I modified the suggestion from Matthew Vines to avoid the "Possible multiple enumeration of IEnumerable" - problem. (see also the comment from Jon Hanna)

public static bool IsNullOrEmpty(this IEnumerable items)
    => items == null
    || (items as ICollection)?.Count == 0
    || !items.GetEnumerator().MoveNext();

... and the unit test:

[Test]
public void TestEnumerableEx()
{
    List<int> list = null;
    Assert.IsTrue(list.IsNullOrEmpty());

    list = new List<int>();
    Assert.IsTrue(list.IsNullOrEmpty());

    list.AddRange(new []{1, 2, 3});
    Assert.IsFalse(list.IsNullOrEmpty());

    var enumerator = list.GetEnumerator();
    for(var i = 1; i <= list.Count; i++)
    {
        Assert.IsFalse(list.IsNullOrEmpty());
        Assert.IsTrue(enumerator.MoveNext());
        Assert.AreEqual(i, enumerator.Current);
    }

    Assert.IsFalse(list.IsNullOrEmpty());
    Assert.IsFalse(enumerator.MoveNext());
}

回答9:

var nullOrEmpty = !( list?.Count > 0 );