Nullable<T>.ToString() should return null when there is no value

[ehorrent]

Description

In the Nullable struct, we can find the following ToString() implementation:

public override string? ToString() => hasValue ? value.ToString() : "";

The return type is a nullable string, so when calling ToString() on a null value, I'm expecting to get null rather than an empty string.
It's a misleading behavior.

Reproduction Steps

using System.Diagnostics;

namespace MyApp;

class Program
{
    static void Main(string[] args)
    {
        int? nullValue = null;
        Debug.Assert(string.Empty == nullValue.ToString());
    }
}

Expected behavior

I would expect either:

1. Get a null value:

ToString() returns null when there is no value:

public override string? ToString() => hasValue ? value.ToString() : null;

2. A compilation warning so you can avoid unexpected behavior.

Actual behavior

ToString()is returning "" when called on a null value.

Regression?

No response

Known Workarounds

Use the null-conditional operator (but you can easily forget to do so):

using System.Diagnostics;

namespace MyApp;

class Program
{
    static void Main(string[] args)
    {
        int? nullValue = null;
        Debug.Assert(null == nullValue?.ToString());
    }
}

Configuration

Tested with .NET 9, windows, x64 but I suspect any .NET build.

Other information

No response

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[huoyaoyuan]

The behavior is more than 20 years-old and we can't change it. There can be thousands of codebases expect the return value to be non-null.

The return value is annotated as nullable because the ToString of inner struct can return null.

[ehorrent]

The behavior is more than 20 years-old and we can't change it. There can be thousands of codebases expect the return value to be non-null.

The return value is annotated as nullable because the ToString of inner struct can return null.

Yes, I imagine it cannot be changed easily :-)
A compilation warning would really help to avoid this misleading behavior (particulary when you are used to play with nullable references).

[huoyaoyuan]

A compilation warning would really help to avoid this misleading behavior

It can't help. Many libraries won't be recompiled and can't get any new warning. Such behavioral change in low-level component is impossible.

[stephentoub]

Thank you for the issue. This is not actionable, however.

[lechu445]

The documentation does not mention this behavior. It may be worth documenting it, as it is not going to change.

[stephentoub]

The documentation does not mention this behavior. It may be worth documenting it, as it is not going to change.

https://learn.microsoft.com/en-us/dotnet/api/system.nullable-1.tostring?view=net-9.0#returns