C#: How to return optional ref to struct

Question

In C# how do I write a function that may or may not return a ref to a struct?

What I basically want is a return type Nullable<ref OutboxSlot> but the compiler complains about ref beeing an unexpected token.

I want to do is something like this:

Nullable<ref SomeFieldStruct> GetCorrectField(ref SomeStruct s) {
  // Depending on some property of s either return null or a ref to a field of s
}

In something like Rust I could easily declare a return type like Option<&mut SomeFieldStruct>.

PS: No I cannot use classes.

Answer 1

In project properties, in Compilation, you must check "Allow unsafe code":

Don't forget set in Debug and Release.

I'm going to use this structs:

public struct SomeStruct
{
    public int Value;
    public SomeFieldStruct Field;
}

public struct SomeFieldStruct
{
    public double Value;
}

Then, create your method:

public unsafe static SomeFieldStruct* GetCorrectField(ref SomeStruct s)
{
    // Depending on some property of s either return null or a ref to a field of s
    if (s.Value == 0)
    {
        fixed (SomeFieldStruct* field = &s.Field)
        {
            return field;
        }
    }

    return null;
}

If Value is zero, you return the address of Field property of your struct. Check it:

public unsafe static void Test()
{
    SomeStruct obj;
    obj.Value = 0;
    obj.Field.Value = 1.2;

    SomeFieldStruct* field = GetCorrectField(ref obj);
    if (field != null)
    {
        field->Value *= 10.0;
    }
}

With pointers, you must use arrow -> instead of dot. All your unsafe methods must include unsafe keyword.