Convert.ToString should not return nullable string

[nathan130200]

As title, Convert.ToString(object?) and its overload Convert.ToString(object?, IFormatProvider?) should not return string with nullable annotation:

public static string? ToString(object? value, IFormatProvider? provider)
{
    if (value is IConvertible ic) // already check not null and IConvertible and .ToString does not return: string?
        return ic.ToString(provider);
    if (value is IFormattable formattable) // already check: not null and IFormattable and .ToString does not return: string?
        return formattable.ToString(null, provider);
    return value == null ? string.Empty : value.ToString(); // if value is null, just returns an empty string instead.
}

I know the value.ToString() may return null string but instead maybe it should be replaced with:

return value?.ToString() ?? string.Empty;

[tannergooding]

This would be a breaking change, would modify a behavior we've had for 20 years, would make it no longer possible to discern when the input was itself null, and would deviate from calling value.ToString() directly in the case value was non-null.

All of these are undesirable changes and would remove part of the purpose/intent of the API.

If you have a need for null output to be normalized, you should create a wrapper or utility function that handles that and use it throughout your codebase.

[nathan130200]

The nullable annotations feature in .NET is not even 20 years old.

Even VS/VSCode or any C# language service when overriding Object.ToString call on custom types does not add nullable annotation to the string (aka string?). It also does not make sense to override ToString method to return only null.

SharpLab - Sample

But anyway... It was just an observation.

[tannergooding]

Edit: Minor fix to the behavior description below

The nullable annotations feature in .NET is not even 20 years old.

The behavior of Convert.ToString() is 20 years old, it's been around since .NET Framework 1.0 and has always returned string.Empty when value is null and null when value.ToString() returns null. This is what would be breaking.

The annotations are simply surfacing, to the consumer of the API, the behavior that we've always had.

It also does not make sense to override ToString method to return only null.

The consideration is that types and APIs do exist that return null for ToString(), thus Convert.ToString(object) may return null. They may not always return null, but they can return null and it would be a breaking change to force them to no longer do so.

[tannergooding]

Only return null if value.ToString() (return value is null) returns null.

Which is a breaking change to modify in one of our oldest APIs. It's not something we're interested in doing.

There are trivial workarounds if you need a different behavior for your code base.

[nathan130200]

When value is null it returns string.Empty and not null. That's i'm pointing. Only return null if value.ToString() (return value is null) returns null.