IEnumerable object 是否需要检查 null？

Question

var selectedRows = from drow in ugTable.Rows
                         .Cast<Infragistics.Win.UltraWinGrid.UltraGridRow>()
                         .Where(drow => drow != null && drow.Selected) 
                   select drow;

if(selectedRows.Count()==1){//do something with selected rows}

From the above statement, do i need to check Null for the selectedRows variable? selectedRows is an IEnumerable variable.

Answer 1

You do not need to check if selectedRows is null . The returned IEnumerable<> might be empty, but it will never be null .

As an aside, I'd suggest you simplify your code by writing:

var selectedRows
    = ugTable.Rows.Cast<Infragistics.Win.UltraWinGrid.UltraGridRow>()
                  .Where(drow => drow != null && drow.Selected);

Which is shorter and equivalent.

Answer 2

The LINQ query will return an empty list (0 items), if there are no matches on the where.

So, no need to check for null .

Answer 3

In your example you'll be fine using the extension method. But if you were to implement your own method that returned an IEnumerable, the answer depends on how you return your result.

The following method returns an empty enumerable:

IEnumerable<object> Correct()
{
    yield break;
}

The following method just returns null:

IEnumerable<object> Incorrect()
{
    return null;
}

Calling these methods will give the following results:

Correct().Any(); // returns false
Incorrect().Any(); // throws ArgumentNullException

So be careful when you return IEnumerable. Try to use the yield keyword and following the correct pattern.

Answer 4

My initial feeling is no, you don't but it certainly can't hurt.

I have, from I think Phil Haack, a useful extension method that checks to see if an IEnumerable is null or empty...

    /// <summary>
    /// Determines whether the collection is either null or empty.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="source">The source collection.</param>
    /// <returns>
    ///     <c>true</c> if the collection is null or empty; otherwise, <c>false</c>.
    /// </returns>
    public static bool IsNullOrEmpty<T>(this IEnumerable<T> source)
    {
        return source == null || !source.Any();
    }

.Any() is much more efficient for checking if not empty than .Count()

Answer 5

Linq wont retrun NULL. If you want to check some data is there you can go with Any()

var selectedRows = from drow in ugTable.Rows
                         .Cast<Infragistics.Win.UltraWinGrid.UltraGridRow>()
                         .Where(drow => drow != null && drow.Selected) 
                   select drow;
if(selectedRows .Any())
{
//your code
}