C#: Different ways to Check for Null

What is the classic way to check if for example a parameter value is null? If you’ve developed with C# since a while, you might be familiar with this classic syntax:

public static int CountNumberOfSInName(string name)
  if (name == null)
    throw new ArgumentNullException(nameof(name));

  return name.Count(c => char.ToLower(c).Equals('s'));

Since C# version 7 you can use the is keyword for the null check like in the snippet below:

  if (name is null)
    throw new ArgumentNullException(nameof(name));

But with C# 7, there’s even a shorter syntax. Discards were also introduced. They are unused and ignored variables that are represented in your code with an underscore (_). In combination with the Null-coalescing operator (??) you can write the null check like this:

_ = name ?? throw new ArgumentNullException(nameof(name));

That means, the whole method looks just like this:

public static int CountNumberOfSInName(string name)
  _ = name ?? throw new ArgumentNullException(nameof(name));

  return name.Count(c => char.ToLower(c).Equals('s'));

To be honest, I really like the last approach using the discards, but maybe for some developers it’s too much. I think the is keyword is very clear and readable. It is my favorite.

The is keyword has also the big advantage that it ignores any ==/!= operator overloads on the specific class. It will do a null check, no matter if there’s an operator overload or not. That makes it better than just using ==. You can read more about this in this blog post.

The Is Keyword And the Not Pattern in C# 9.0

With C# 9.0, you can combine the is expression with the logical not pattern, which is powerful if you want to check if an object is NOT null. Before C# 9.0 you had to use the is expression like below to check if an object is not null:

if (!(name is null)) { }

Some developers preferred the following syntax to check if the name is not null:

if (name is object) { }

But the statements above are neither very readable nor easy to understand. That’s why many developers still prefer the classic way:

if (name != null) { }

But since C# 9.0, you can write that not null check like below, and I think that is really readable code:

if (name is not null) { }


So, with C# 9.0, you can write your null / not-nulll checks like below, and I think that’s readable:

if (name is null) { }

if (name is not null) { }

Happy coding,

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Comments (19)

  • Dew Drop – March 13, 2020 (#3153) | Morning Dew Reply

    […] C#: Different ways to Check for Null (Thomas Claudius Huber) […]

    March 13, 2020 at 12:03 pm
  • Joel Reply

    When you want to check for not-null, you can do..

    if (foo != null)
    if (!(foo is null))
    if (foo is object)

    I prefer the third because it’s clear and concise and doesn’t involve negation or nested parens.

    March 13, 2020 at 4:43 pm
    • Thomas Claudius Huber Reply

      Hi Joel,

      yes, that’s super cool. I really like “foo is object”, but I guess it’s not as obvious as !(foo is null)

      Best would be

      if(foo is not null)

      But we don’t have that (yet?).


      March 17, 2020 at 3:04 pm
  • The Morning Brew - Chris Alcock » The Morning Brew #2953 Reply

    […] C#: Different ways to Check for Null – Thomas Claudius Huber […]

    March 16, 2020 at 7:00 am
  • Dipo Reply

    Good use of _ = name ?? throw new ArgumentNullException(nameof(name));

    it’s sensible and neat

    March 16, 2020 at 9:11 am
  • Eric Bäckhage Reply

    I like the ’is’ variant. Hopefully, with the nullable reference types introduced in C#8 we’ll be able to get rid of null-guards like these.
    If there is a need to represent the possible abscence of a value I prefer the Option type. I wrote a blog post on this topic that you might find interesting, see https://ericbackhage.net/c/nullable-reference-types-compared-to-the-option-monad/

    March 17, 2020 at 6:25 am
  • Jamie Walters Reply

    Thanks Thomas. I had the following piece of code that does a null check within a Ternary Expression:

    private static string GetAttributeValue(Assembly assembly)
    where T : Attribute
    var type = typeof(T);
    var attribute = assembly
    .Where(x => x.AttributeType == type)
    .Select(x => x.ConstructorArguments.FirstOrDefault())
    return attribute == null ? string.Empty : attribute.Value?.ToString();

    When I updated this to change the line that was attempting a null check to

    return attribute is null ? string.Empty : attribute.Value?.ToString();

    The compiler explained to me that I am trying to do a null check against a non-nullable value type. By attempting this, I realized the null check was not necessary after confirming the type System.Reflection.CustomAttributeTypedArgument is a non-nullable struct.

    So the offending line can be simplified to

    return attribute.Value?.ToString();

    And this can be simplified to

    return attribute.Value.ToString();

    Since System.Reflection.CustomAttributeTypedArgument.Value cannot be null.

    Does this seem accurate to you? Point of sharing this is to demonstrate after going through a code base to update null checks, I learned something the code was doing that is not necessary: a null check against a non-nullable value type.

    September 11, 2020 at 5:00 pm
    • Thomas Claudius Huber Reply

      Hey Jamie, yes, your adjustments looks accurate to me. I see a lot of code that does null checks against value types that can not be null. :-) Usually you find these also in the warnings of the compiler.

      September 12, 2020 at 9:32 am
  • Avi Farah Reply

    Thank you

    September 13, 2020 at 3:21 pm
  • Olivier Jacot-Descombes Reply

    In C# 9.0 you can also test for not null with an empty property pattern. This is useful when you want to assign the result to a variable `if (GetValue() is { } value)`. (But `is not null value` does not compile).

    December 17, 2020 at 6:41 pm
    • Thomas Claudius Huber Reply

      Interesting. Seems to be a shortcut for if (GetValue() is [variabletype] value)

      December 21, 2020 at 3:15 pm
  • Tony L Reply

    OHHHH you need c# 9.0 for the “is not null” keywords to work.

    I was wondering why it was giving me weird results on my Unity project.

    Thanks for your insight. Cheers

    January 18, 2021 at 5:59 pm

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