ASP.NET Core 5: Make your options immutable


If you’ve been using ASP.NET Core 5 for a while, you’ll be delighted to learn that a major improvement of C# 9, in other words C# 9 (delivered with .NET 5) that allows the creation of immutable objects through Init-only properties. As a result, you’ll have the ability to set up your application options so that they’re immutable. After all, these are a kind of constants that you don’t necessarily want to change after initialization, or even ban it. So it’s possible with ASP.NET Core 5 and C# 9.

If you don’t know what are Init-only properties, you can read my article here:


For now, .NET 5 and ASP.NET Core 5 are still in preview, if you want to be able to reuse the code I will show you you’ll need to install:

Configure options as immutable

I have already written an article you can read for more details here if you want to configure your options in a simpler way:

I will take the same sample to show you how to make your options immutable, you just need to replace in your options class set by init:

Once done, you’ll need to register your options:

And now you are ready to consume them, and you won’t be able to modify your options, example:

As you can see you are not able to modifie your options, you’ll get a compiler exception: Error CS8852 Init-only property or indexer ‘{your property}’ can only be assigned in an object initializer, or on ‘this’ or ‘base’ in an instance constructor or an ‘init’ accessor.

Very practical feature isn’t it ? 😉

Written by


Anthony is a specialist in Web technologies (14 years of experience), in particular Microsoft .NET and learns the Cloud Azure platform. He has received twice the Microsoft MVP award and he is also certified Microsoft MCSD and Azure Fundamentals.
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