C# How to pass generic enumerable type parameter to another method expecting an IEnumerable?

Question

I have a method that accepts a basic generic variable in the method signature:

public void Foo<T>(T val) {

    if (val is IEnumerable) {
        Bar(val)
    }
}

...Logic if not enumerable

The signature for Bar looks like this:

private void Bar<T>(IEnumerable<T> vals) {
    ...Logic if enumerable
}

The logic is very lengthy and different based on whether the item passed in is enumerable or not. I can't get Bar(val) to work. I tried the following, let me know what I'm missing:

Bar(val)

Bar((IEnumerable<T>)val) (Compiles, but results in this error at run-time when tried with a List of ints:

Unable to cast object of type 'System.Collections.Generic.List`1[System.Int32]' to type 'System.Collections.Generic.IEnumerable`1[System.Collections.Generic.List`1[System.Int32]]'.'

Bar((IEnumerable)val)

You are checking that something is `IEnumerable` and then passing it as `IEnumerable`. — GSerg, Dec 27 '22 at 23:33
`if (val is IEnumerable e) { Bar(e); }`, but like I said, that implies `Bar(IEnumerable vals)`, not `Bar(IEnumerable vals)`. — GSerg, Dec 27 '22 at 23:46
Simplest way is probably `Bar((dynamic) val)`, although some people will lynch me for even mentioning `dynamic`. The alternative is getting tricksy with `MakeGenericMethod`, which is going to be very involved since you also need to get the generic type `U` of the `IEnumerable` that is `val` (which is not the same as `T`, as you've discovered). Something like `Expression e = () => Bar — Jeroen Mostert, Dec 27 '22 at 23:47
I am able to retrieve the `T` in the `IEnumerable` if it helps. I should have put this in the main post, but I've tried `Bar((IEnumerable<[valInnerType]>)val)` where `[valInnerType]` is the type being enumerated. Doesn't compile. — Brettyoke49, Dec 27 '22 at 23:50
If you have the type at the call site it's easier; `Bar((IEnumerable) (object) val)` will compile and do the conversion at runtime. — Jeroen Mostert, Dec 27 '22 at 23:51
I hope the accepted answer will explain why the intuitive `if (val is IEnumerable e) Bar(e)` does not work and why the cast throws an exception. — Wyck, Dec 28 '22 at 00:14
@Wyck Why that would not work? I just tried and it worked fine without any problems (indeed class need to implement iterator, but that's quite obvious - `class MyClass : IEnumerable {...}`) — Alexei Levenkov, Dec 28 '22 at 01:17

Guru Stron · Accepted Answer · 2022-12-28T00:18:39.877

The only option without changing the signature and/or using dynamic is to dive into reflection. You need to get the closed generic type of IEnumerable<TInner> (where T : IEnumerable<TInner>), construct corresponding closed version of Bar and invoke it. Something along this lines:

void Foo<T>(T val)
{
    var genericEnumInterface = typeof(T).GetInterfaces()
        .FirstOrDefault(t => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(IEnumerable<>));
    if (genericEnumInterface is not null)
    {
        MethodInfo mi = ...; // get Bar via reflection
        var constructedMethod = mi.MakeGenericMethod(genericEnumInterface);
        constructedMethod.Invoke(this, new[]{(object)val}); // assuming Bar is in the same class as Foo
    }
}

Note that there are types which will implement IEnumerable<T> which you possibly will not want to process via Bar like string which is IEnumerable<char>. Another concern is that reflection can cause noticable performance hit in some cases so you can try caching it (see for example this or this answer).

Also personally I would try to avoid such generic method as Foo due to possible misuse and corner cases like string.

score 0 · Answer 2 · answered Dec 28 '22 at 00:10

0

If you don't want to go down the reflection route there is another option:

if (val is IEnumerable) 
{
    Bar(val as dynamic);
}

You may not want to go down that rabbit hole though.

answered Dec 28 '22 at 00:10

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C# How to pass generic enumerable type parameter to another method expecting an IEnumerable?

2 Answers2