C#, null and using statement

csharp_null_and_using_statement.md

Do you know that C# supports using null values with using statements. For example the following program compiles and runs without any problems:

public class Program
{
	public static void Main()
	{
		using(var x = CreateIDisposable()) {
			// prints x = <null>
			Console.WriteLine("x = {0}", x?.ToString() ?? "<null>");	
		}
	}
	
	public static IDisposable CreateIDisposable() => null;
}

This behaviour is mandated by C# language specification. You can check that yourself by reading relevant sections of the spec.

This behaviour can have many interesting usages. For example to add an additional field to every log entry in Serilog you must write:

// borrowed from: https://github.com/serilog/serilog/wiki/Enrichment#the-logcontext
log.Information("No contextual properties");

using (LogContext.PushProperty("A", 1))
{
    log.Information("Carries property A = 1");

    using (LogContext.PushProperty("A", 2))
    using (LogContext.PushProperty("B", 1))
    {
        log.Information("Carries A = 2 and B = 1");
    }

    log.Information("Carries property A = 1, again");
}

Note that LogContext.PushProperty returns an object that must be disposed.

But what if you want to add a field to LogContext only if the field value is present? No problem, just use a method like this:

public static IDisposable AddUserNameToLogContext() {
	var userName = ExtractUserName();
	
	if (userName != null) {
		return LogContext.PushProperty("UserName", userName);	
	}
	
	return null; // null but it is not a problem in C#
}

// And usage...
using(AddUserNameToLogContext()) {
	log.Information("Attempt to open a document...");	
}