Compiler Error CS8170

Struct members cannot return 'this' or other instance members by reference

Example

The following sample generates CS8170:

Value types (i.e. structs), are most commonly allocated on the stack. A value type allocated on the stack becomes invalid leaving the scope in which it was declared. The compiler avoids a reference to a variable that becomes invalid leaving the scope by generating this error.

// CS8170.cs (8,14)

struct Program
{
    public int d;

    public ref int M()
    {
        return ref d;
    }
}

public class Other
{
    public void Method()
    {
        var p = new Program();
        ref int d = ref p.M();
    }
}

To correct this error

Changing the method to not ref-return corrects this error:

delegate void D();

struct Program
{
    public event D d;

    public D M()
    {
        return d;
    }
}

If a reference to a member is required, consider extending the scope of the value. For example:

public struct Program
{
    public int d;
}

public static class Extensions
{
    public static ref readonly int RefD(this in Program program)
    {
        return ref program.d;
    }
}

public class Other
{
    public void Method()
    {
        var p = new Program();
        var d = p.RefD();
    }
}

Another approach is to use the System.Diagnostics.CodeAnalysis.UnscopedRefAttribute attribute. It will mark the reference to be allowed to escape the scope.
Use this only when you know that it is safe for the reference to leave the scope. Below is the example of applying System.Diagnostics.CodeAnalysis.UnscopedRefAttribute to int M() method, which fixes the CS8170:

using System.Diagnostics.CodeAnalysis;

struct Program
{
    public int d;

    [UnscopedRef]
    public ref int M()
    {
        return ref d;    // No error - ref is valid to escape the scope in this line of that method
    }
}

public class Other
{
    public void Method()
    {
        var p = new Program();
        ref int d = ref p.M();
    }
}