How to check if IEnumerable is null or empty?

Sure you could write that:

public static class Utils {
    public static bool IsAny<T>(this IEnumerable<T> data) {
        return data != null && data.Any();
    }
}

however, be cautious that not all sequences are repeatable; generally I prefer to only walk them once, just in case.

Another way would be to get the Enumerator and call the MoveNext() method to see if there are any items:

if (mylist != null && mylist.GetEnumerator().MoveNext())
{
    // The list is not null or empty
}

This works for IEnumerable as well as IEnumerable<T>.

public static bool IsNullOrEmpty<T>(this IEnumerable<T> enumerable) {
    return enumerable == null || !enumerable.Any();
}

Here's a modified version of @Matt Greer's useful answer that includes a static wrapper class so you can just copy-paste this into a new source file, doesn't depend on Linq, and adds a generic IEnumerable<T> overload, to avoid the boxing of value types that would occur with the non-generic version. [EDIT: Note that use of IEnumerable<T> does not prevent boxing of the enumerator, duck-typing can't prevent that, but at least the elements in a value-typed collection will not each be boxed.]

using System.Collections;
using System.Collections.Generic;

public static class IsNullOrEmptyExtension
{
    public static bool IsNullOrEmpty(this IEnumerable source)
    {
        if (source != null)
        {
            foreach (object obj in source)
            {
                return false;
            }
        }
        return true;
    }

    public static bool IsNullOrEmpty<T>(this IEnumerable<T> source)
    {
        if (source != null)
        {
            foreach (T obj in source)
            {
                return false;
            }
        }
        return true;
    }
}