C# Error CS8333 – Cannot return {0} ‘{1}’ by writable reference because it is a readonly variable

C# Error

CS8333 – Cannot return {0} ‘{1}’ by writable reference because it is a readonly variable

Reason for the Error & Solution

Cannot return by writable reference because it is a readonly variable

Example

The following sample generates CS8333:

// CS8333.cs (3,36)
class Program
{
    static ref T F4<T>(in T t) => ref t;
}

To correct this error

To return a reference to a readonly variable, refactoring to return ref readonly will correct this error:

class Program
{
    static ref readonly T F4<T>(in T t) => ref t;
}