Enum.Format change in behavior between .Net Framework and .Net 5

[ChrisVicary]

Repro:

using System;

class Test
{
    [Flags]
    public enum TestEnum { None = 0, Item1 = 1, Item2 = 2, }

    static void Main()
    {
        Console.WriteLine(Enum.Format(typeof(TestEnum), (int)3, "G"));
    }
}

.NET Framework: Item1, Item2
.NET Core: 3

---

Description

While upgrading a project from .Net Framework 4.7.2 to .Net 5 and I noticed a difference in behavior of the EnumConverter.ConvertToString() method when passing an integer value that could represent a combination of flags enum values. Here is some code that reproduces the problem:

class Program
{
	[Flags]
	public enum TestEnum { None = 0, Item1 = 1, Item2 = 2, }
            
	static void Main(string[] args)
	{
		var none = TestEnum.None;
		var single = TestEnum.Item1;
		var multiple = TestEnum.Item1 | TestEnum.Item2;

		Console.WriteLine(none);
		Console.WriteLine(single);
		Console.WriteLine(multiple);

		var converter = new EnumConverter(typeof(TestEnum));
		Console.WriteLine(converter.ConvertToString(none));
		Console.WriteLine(converter.ConvertToString(single));
		Console.WriteLine(converter.ConvertToString(multiple));

		Console.WriteLine(converter.ConvertToString((int)none));
		Console.WriteLine(converter.ConvertToString((int)single));
		Console.WriteLine(converter.ConvertToString((int)multiple));
		Console.ReadLine();
	}
}

Creating a simple console app with the code above, if I compile and run the app against .Net Framework 4.8 the last line outputs:
Item1, Item 2
If I compile and run against .Net 5 the last line outputs:
3

I tried to have a look through the code here, compared to reference source and I see quite a few differences in there, but couldn't quite nail down the path that the code is taking to just return the int value as a string rather than calculating the enum value and converting that.

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Issue Details

---

Description

While upgrading a project from .Net Framework 4.7.2 to .Net 5 and I noticed a difference in behavior of the EnumConverter.ConvertToString() method when passing an integer value that could represent a combination of flags enum values. Here is come code that reproduces the problem:

class Program
{
	[Flags]
	public enum TestEnum { None = 0, Item1 = 1, Item2 = 2, }
            
	static void Main(string[] args)
	{
		var none = TestEnum.None;
		var single = TestEnum.Item1;
		var multiple = TestEnum.Item1 | TestEnum.Item2;

		Console.WriteLine(none);
		Console.WriteLine(single);
		Console.WriteLine(multiple);

		var converter = new EnumConverter(typeof(TestEnum));
		Console.WriteLine(converter.ConvertToString(none));
		Console.WriteLine(converter.ConvertToString(single));
		Console.WriteLine(converter.ConvertToString(multiple));

		Console.WriteLine(converter.ConvertToString((int)none));
		Console.WriteLine(converter.ConvertToString((int)single));
		Console.WriteLine(converter.ConvertToString((int)multiple));
		Console.ReadLine();
	}
}

Creating a simple console app with the code above, if I compile and run the app against .Net Framework 4.8 the last line outputs:
Item1, Item 2
If I compile and run against .Net 5 the last line outputs:
3

I tried to have a look through the code here, compared to reference source and I see quite a few differences in there, but couldn't quite nail down the path that the code is taking to just return the int value as a string rather than calculating the enum value and converting that.

Author:	ChrisVicary
Assignees:	-
Labels:	area-System.ComponentModel, untriaged
Milestone:	-

[KalleOlaviNiemitalo]

IIUC, both versions of EnumConverter call Enum.Format(type, value, "G"), where type is the enum type and value is (Int32)3:

runtime/src/libraries/System.ComponentModel.TypeConverter/src/System/ComponentModel/EnumConverter.cs

Line 132 in 35964c9

return Enum.Format(EnumType, value, "G");

https://github.com/microsoft/referencesource/blob/5697c29004a34d80acdaf5742d7e699022c64ecd/System/compmod/system/componentmodel/EnumConverter.cs#L170

In .NET 5.0.8, because value.GetType() is the underlying type rather than the enum type, Enum.Format calls GetEnumName, which returns null, and then falls back to value.ToString(), i.e. "3":

runtime/src/libraries/System.Private.CoreLib/src/System/Enum.cs

Lines 866 to 868 in 35964c9

	case 'G':
	case 'g':
	return GetEnumName(rtType, ToUInt64(value)) ?? value.ToString()!;

In Reference Source, Enum.Format instead calls InternalFormat, which checks for FlagsAttribute:

https://github.com/microsoft/referencesource/blob/5697c29004a34d80acdaf5742d7e699022c64ecd/mscorlib/system/enum.cs#L661-L663
https://github.com/microsoft/referencesource/blob/5697c29004a34d80acdaf5742d7e699022c64ecd/mscorlib/system/enum.cs#L138

AFAIK, TypeConverter.ConvertTo is not generally expected to support conversions from types other than the type for which the TypeConverter was constructed. EnumConverter however has deliberately supported conversions from the underlying type of the enum. (It has also supported conversions from other types that implement IConvertible.ToType to the underlying type of the enum, but I don't know whether that has been deliberate.)

I think Enum.Format was broken by dotnet/coreclr#6645. It is a public method, so the misbehaviour should be reproducible also as Enum.Format(typeof(TestEnum), 3, CultureInfo.InvariantCulture) without involving EnumConverter. From the Enum.Format documentation, this use seems to be valid.

[KalleOlaviNiemitalo]

Enum.Format was partially fixed in dotnet/coreclr#10610 for #20845 but that fix does not check for FlagsAttribute.

Tagging subscribers to this area: @dotnet/area-system-runtime
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

While upgrading a project from .Net Framework 4.7.2 to .Net 5 and I noticed a difference in behavior of the EnumConverter.ConvertToString() method when passing an integer value that could represent a combination of flags enum values. Here is some code that reproduces the problem:

class Program
{
	[Flags]
	public enum TestEnum { None = 0, Item1 = 1, Item2 = 2, }
            
	static void Main(string[] args)
	{
		var none = TestEnum.None;
		var single = TestEnum.Item1;
		var multiple = TestEnum.Item1 | TestEnum.Item2;

		Console.WriteLine(none);
		Console.WriteLine(single);
		Console.WriteLine(multiple);

		var converter = new EnumConverter(typeof(TestEnum));
		Console.WriteLine(converter.ConvertToString(none));
		Console.WriteLine(converter.ConvertToString(single));
		Console.WriteLine(converter.ConvertToString(multiple));

		Console.WriteLine(converter.ConvertToString((int)none));
		Console.WriteLine(converter.ConvertToString((int)single));
		Console.WriteLine(converter.ConvertToString((int)multiple));
		Console.ReadLine();
	}
}

Creating a simple console app with the code above, if I compile and run the app against .Net Framework 4.8 the last line outputs:
Item1, Item 2
If I compile and run against .Net 5 the last line outputs:
3

I tried to have a look through the code here, compared to reference source and I see quite a few differences in there, but couldn't quite nail down the path that the code is taking to just return the int value as a string rather than calculating the enum value and converting that.

Author:	ChrisVicary
Assignees:	-
Labels:	area-System.ComponentModel, area-System.Runtime, untriaged
Milestone:	-

[ChrisVicary]

From the comments by @KalleOlaviNiemitalo it looked like a pretty straight forward fix, so I've gone ahead and taken a stab at it. Added an extra test that failed without my change, ran the tests afterwards. This is my first PR for .net stuff, I've tried to follow all the guidelines, but please let me know if there is anything that I've missed or should be doing.