Marking the return value of a property in a method signature as readonly in C#

Question

Visual studio is recommending I add the readonly keyword to the return signature of some methods and properties (IDE 0251). IDE 0251 has no title or description, which looks like it has been reported here (https://github.com/dotnet/roslyn/issues/67949). I listed this for context, but this issue is more about the underlying meaning of how readonly can be applied to a method or property return value.

Below is a sample of the recommendation with the readonly applied. I didn't realize it was even a feature of the language to allow placing readonly before the return type of a method, but this compiles and runs fine.

In property form:

public readonly bool IsInitialized => component != null;

In Method (Function) form:

public readonly bool IsInitialized()
{
    return component != null;
}

I spent some time in Microsoft's documentation to understand what readonly means in this context. I see where Microsoft provided documentation related to marking a return as ref readonly here: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/readonly#ref-readonly-return-example. However, I couldn't find an example of just the readonly term by itself attached to a method return.

The example of ref readonly makes sense, in that "The readonly modifier on a ref return indicates that the returned reference can't be modified."

What does it mean if I apply readonly to a non-ref return, such as the Boolean value in this example? If you had asked me before I saw this message in Visual Studio, I would have assumed this wouldn't even compile.

Note, the message in Visual Studio is IDE0251, which provides a blank description and provides a link to a 404 page (https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/style-rules/ide0251). While this missing documentation is contributing to the general confusion, my question is about the language syntax, not the missing documentation.

I am using Visual Studio 2022, Version 17.6.4.

EDIT: It looks like this code provides an compilation error in a fresh console application project targeting .net 6.0, so it only allows compiling on my mature project and not a fresh console application. Thank you Peter B for the feedback.

Answer 1

You squished a bunch of different questions in a single post, which you generally shouldn't do. Anyway, the answers are as such:

I didn't realize it was even a feature of the language to allow placing readonly before the return type of a method

That's correct, you can declare readonly member functions in value type structs. The meaning of the modifier is that your function won't change the state of the instance. You can read more about this here.

Note, the message in Visual Studio is IDE0251, which provides a blank description and provides a link to a 404 page

That is correct and is tracked as a bug here.

It looks like this code provides an compilation error in a fresh console application project targeting .net 6.0, so it only allows compiling on my mature project and not a fresh console application

You probably tried to reproduce it in a class object? That's my best guess, and why you should generally back your claims with code -- it would be immediately obvious what you're doing to cause the different results.