Compiler Breaking Changes Post DotNet 5

This document lists known breaking changes in Roslyn after .NET 5.

1. https://github.com/dotnet/roslyn/issues/46044 In C# 9.0 (Visual Studio 16.9), a warning is reported when assigning default to, or when casting a possibly null value to a type parameter type that is not constrained to value types or reference types. To avoid the warning, the type can be annotated with ?.

   C#Copy

   static void F1<T>(object? obj)
   {
       T t1 = default; // warning CS8600: Converting possible null value to non-nullable type
       t1 = (T)obj;    // warning CS8600: Converting possible null value to non-nullable type
   
       T? t2 = default; // ok
       t2 = (T?)obj;    // ok
   }
   

2. https://github.com/dotnet/roslyn/pull/50755 In .NET 5.0.200 (Visual Studio 16.9), if there is a common type between the two branches of a conditional expression, that type is the type of the conditional expression.

   This is a breaking change from 5.0.103 (Visual Studio 16.8) which due to a bug incorrectly used the target type of the conditional expression as the type even if there was a common type between the two branches.

   This latest change aligns the compiler behavior with the C# specification and with versions of the compiler before .NET 5.0.

   C#Copy

   static short F1(bool b)
   {
       // 16.7, 16.9          : CS0266: Cannot implicitly convert type 'int' to 'short'
       // 16.8                : ok
       // 16.8 -langversion:8 : CS8400: Feature 'target-typed conditional expression' is not available in C# 8.0
       return b ? 1 : 2;
   }
   
   static object F2(bool b, short? a)
   {
       // 16.7, 16.9          : int
       // 16.8                : short
       // 16.8 -langversion:8 : CS8400: Feature 'target-typed conditional expression' is not available in C# 8.0
       return a ?? (b ? 1 : 2);
   }
   

3. https://github.com/dotnet/roslyn/issues/52630 In C# 9 (.NET 5, Visual Studio 16.9), it is possible that a record uses a hidden member from a base type as a positional member. In Visual Studio 16.10, this is now an error:

record Base
{
    public int I { get; init; }
}
record Derived(int I) // The positional member 'Base.I' found corresponding to this parameter is hidden.
    : Base
{
    public int I() { return 0; }
}

4. In .NET 5 and Visual Studio 16.9 (and earlier), top-level statements could be used in a program containing a type named Program. In .NET 6 and Visual Studio 17.0, top-level statements generate a partial declaration of a Program class, so any user-defined Program type must also be a partial class.

System.Console.Write("top-level");
Method();

partial class Program
{
    static void Method()
    {
    }
}

5. https://github.com/dotnet/roslyn/issues/53021 C# will now report an error for a misplaced :: token in explicit interface implementation. In this example code:

   C#Copy

   void N::I::M()
   {
   }
   

   Previous versions of Roslyn wouldn't report any errors.

   We now report an error for a :: token before M.