[Bug]: throw ：Sequence contains no elements，When parameter contains Flags enum .

[xingzhenlu]

Describe the bug 🐞

When requesting a webapi and the passed parameter contains an enum type with Flags Attribute,for example client.SomeMethod (MyEnum. Value1 | MyEnum. Value2 | MyEnum. Value3),throw InvalidOperationException: Sequence contains no elements.

Step to reproduce

for example client.SomeMethod (MyEnum. Value1 | MyEnum. Value2 | MyEnum. Value3)

Reproduction repository

Expected behavior

for example client.SomeMethod (MyEnum. Value1 | MyEnum. Value2 | MyEnum. Value3)

Screenshots 🖼️

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IDE

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Operating system

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Version

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Device

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ReactiveUI Version

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Additional information ℹ️

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[TheMaakarov]

A similar thing happend to me when passing an invalid enum value as a query parameter (i.e. (MyEnum)(-1)).

Seems that the exception is thrown at this line when calling the 'First' method.

Also, I noticed this issue is duplicated (although closed) at #794

[goors]

This is because you must define at least one flag in your request. Same thing happened to me.

[Liandrel]

This issue also occurs when passing an invalid enum value in the query. I encountered it while running integration tests for my application. It's a bit frustrating that the API gracefully returns a validation error, but the SDK throws an exception instead.

[Liandrel]

for all ppl in the future

that fixes the issue and exception is not beeing thrown anymore

[TimothyMakkison]

Thanks for raising this issue. I'll see if I can fix this tonight

[TimothyMakkison]

So I could add @Liandrel 's fix, but how should EnumMember be handled for flag enums? This comment suggest that refit used to comma/space separate enums with their EnumMemberAttribute. I could add that back but perhaps without the additional space.

[TheMaakarov]

Personally, I would rather that they were formatted as int values, but I guess that could break some more stuff...

Idk, I guess only trying and testing would show. Anyway, I guess it feels safer to implement your proposal by removing the whitespace

[rikjeremy]

I've just run into this while writing some API tests for our code.

Given the enum:

[Flags]
public enum Foo
{
    public Flag1 = 0x1,
    public Flag2 = 0x2
}

After playing around with the endpoint using Postman - for a query string, all those below correctly deserialise into the enum on the endpoint in .net8. I don't currently have an endpoint with that in a body so I can't see what's going on when the enum is inside json.

?flagField=flag1,flag2 ?flagField=3 ?flagField=flag1&flagField=flag2

From a debugging point of view it would be more friendly to have the names in the query string so we can see them in request logs without having to decode them every time, but if the int value is what is transmitted over the wire it's not the end of the world.

My solution (that works for my purposes) in the meantime was to handle the specific case in by inheriting the default handler and then handling the specific case of a bitmap enum and if those specific conditions aren't met passing it on to the base format function. It is worth noting that ToString on a bitmap enum produces a comma separated list of the set values. Is this easier than testing each value of the enum?

public class RefitFlagEnumFormatter : DefaultUrlParameterFormatter
{
    public override string? Format(object? value, ICustomAttributeProvider attributeProvider, Type type)
    {
        if (value != null && type.IsEnum && type.CustomAttributes.Any(o => o.AttributeType == typeof(FlagsAttribute)))
        {
            return value.ToString()?.Replace(" ", "");
        }

        return base.Format(value, attributeProvider, type);

    }
}

This just returns the set values as a comma separated list with the spaces removed. Refit handles the URL encoding of the comma.