CODEWITHSUNDEEP
c#.netnull
Casper Dijkstra
Casper Dijkstra

Reputation: 1885

Practical purpose of the null-coalescing assignment operator in C#?

Nulls in C#

One of the new C# features allows us to get rid of nulls in our code with nullable reference types. We are encouraged to add

<Nullable>enable</Nullable>

to the project file due to problems like described here.

Of course, a lot of existing projects don't want to add this. Many, many errors need to be solved when enabling this functionality, so a lot of legacy nulls will still be around. But do we really need additional null-functionality in the language?

The confusion

In the same C# 8.0 release, the null-coalescing assignment operator (??=) has been introduced (see the docs). I understand the behavior, but which problem(s) does it solve for us? Why would we want to assign b to x when it's null x ??= b and have e.g. x = a when it's not null?

The examples I found are very theoretical, can someone give me a real-world application of this operator? Thanks in advance!

Upvotes: 8

Views: 4197

Answers (2)

Austin T French
Austin T French

Reputation: 5140

The simplest use case is that depending on C# version we previously would have done:

if (number == null)
{
    number = someVale;
}

or

number = number ?? someValue;

With the new assignment operator we can shorten it further to:

number ??= someNumber;

A real world example though:

public void WriteErrorToLog(string errorMessage, Exception ex)
{
    Log.Warn(errorMessage ??= AppConstants.UnhandledError, ex);
}

Not a perfect real world example. But lets say I want to log soft errors. Writing a template message, with an exception would be much preferable to generating a second error/ exception.

Upvotes: 6

JSteward
JSteward

Reputation: 7091

A real world example would be lazy loading a backing field on first access when that backing field is null. Something like this:

private string _dbQuery;
private string DbQuery => _dbQuery ??= GetQuery(queryName);

Upvotes: 17

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