What is ref struct in definition site

Question

I think I've heard a term "ref like struct" in GitHub some time ago.

Now that I have my hands on latest C# version (7.3), I could finally test it my self. So this seems to be a valid code:

public ref struct MyStruct
{
    int x;
}

I know what are ref locals and ref returns as there is documentation about that. But I could not find documentation about ref struct.

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Ref structs can not be used on auto properties or fields. They can not be cast to object either. These were empirical findings.

With "Span" background that new c# gave me recently I guessed that ref struct is a stack only struct. That is an struct that never goes on heap. But im not 100% sure.

Im pretty sure there should be a documentation about this but I failed to find it.

Answer 1

After some research, I stumbled upon this article on Compile time enforcement of safety for ref-like types in C# 7.2.

This C# feature is also known as “interior pointer” or “ref-like types”. The proposal is to allow the compiler to require that certain types such as Span<T> only appear on the stack.

The site also states the benefits of doing so, mainly concerning garbage collection and stack allocation.

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Using ref-like types also brings some restrictions with it such as:

• ref-like type cannot be a type of an array element
• ref-like type cannot be used as a generic type argument
• ref-like variable cannot be boxed
• ref-like type cannot be a field of ordinary not ref-like type
• ref-like types cannot implement interfaces
• indirect restrictions, such as disallowed use of ref-like types in async methods, which are really a result of disallowing ref-like typed fields.

This limits them to be used for parameters, local variables, and in some cases return values.

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There also exists an official documentation from Microsoft, as @UnholySheep pointed out in the comments.