C# for comparing two lists that may contain null values

Question

I am comparing two excel sheets using C#.

I have assigned the sheets to datatables and I hope to compare the values in each cell. I have converted the datatables to lists and I'm iterating through them comparing them. How do I avoid a NullReferenceException if one of the values contains a null?

I am hitting a NullReferenceException at this point in the code:

    if (lst1[i].ToString() != lst2[j].ToString())

Is there a way to guard against nulls here or do I need to handle null values earlier in the code?

Thanks

Could you try this aproach: https://stackoverflow.com/questions/12795882/quickest-way-to-compare-two-generic-lists-for-differences? — Jorge Duarte, May 27 '22 at 11:49
`i` and `j` look confusing. Are you comparing an item from first list to every item in your second list? — DotNet Developer, May 27 '22 at 12:01

score 1 · Answer 1 · answered May 27 '22 at 11:47

1

One good way to do this is to ask if the value is null before call .ToString().

So, would be like:

if (lst1[i] != null && lst2[j] != null && lst1[i].ToString() != lst2[j].ToString())

answered May 27 '22 at 11:47

Sergio Quinteiro

116
2

score 1 · Answer 2 · answered May 27 '22 at 11:48

1

if (lst1[i]?.ToString() != lst2[j]?.ToString())

More info on the ?. operator

answered May 27 '22 at 11:48

RandomSlav

507
3
13

score 1 · Answer 3 · answered May 27 '22 at 12:04

1

Just test for null before dereferencing lst1[i] and lst2[j]

if(lst1[i] is not null and lst2[j] is not null)
{
     if (lst1[i].ToString() != lst2[j].ToString())
     {
         ...
     }

}

answered May 27 '22 at 12:04

Vlad Radu

337
1
13

score 1 · Answer 4 · answered May 27 '22 at 13:17

Assuming

if both items are null then they are equal;
if both items are not null and their string representation are equal then they are equal;
in any other case they are not equal;

you can make comparison of your lists more readable With following LINQ query.

var query = list1
    .Zip(list2, (a, b) => new { a, b })
    .Any(m => !((m.a == null && m.b == null) || (m.a != null && m.b != null && m.a.ToString() == m.b.ToString())));

if (query)
{
    Console.WriteLine("Lists are not equal.");
}
else
{
    Console.WriteLine("Lists are equal.");
}

Scott Hannen · Answer 5 · 2022-05-27T20:36:44.093

Since you're doing ToString() it looks like both lists are typed as objects. Since they're coming from Excel they're presumably all either strings or simple value types.

In that case you can do

if(lst1.SequenceEquals(lst2))

That compares each item in both lists and ensure that they're equal. It accounts for nulls. You wouldn't need to compare each item one at a time. This compares the entire list. I'd write unit tests to make sure it doesn't throw any curves.

If for some reason you wanted to do a text-based comparison ("2" == 2) then you could create a comparer like this:

public class ObjectTextComparer : IEqualityComparer<object>
{
    public bool Equals(object x, object y)
    {
        return x?.ToString() == y?.ToString();
    }

    public int GetHashCode(object obj)
    {
        return obj?.GetHashCode() ?? 0;
    }
}

and then this would return true:

var x = new object[] { "x", 2, null, "y" };
var y = new object[] { "x", "2", null, "y" };
var comparer = new ObjectTextComparer();
var listsAreEqual = x.SequenceEqual(y, comparer);

That second option is a can of worms, though, if the values in one list are stored in Excel as text and the other ones as numbers or dates.

For example if one list contains a date and the other list contains the same date stored as text, the comparer is going to convert the first one to a string, but it might not be formatted the same way as the other one stored as text, and the two strings won't match.

C# for comparing two lists that may contain null values

5 Answers5