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Asked: 2026-05-11T08:22:59+00:00

I have a routine public void SomeRoutine(List<IFormattable> list) { … } I then try

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I have a routine

public void SomeRoutine(List<IFormattable> list) { ... }

I then try to call this routine

List<Guid>list = new List<Guid>(); list.Add(Guid.NewGuid()); SomeRoutine(list);

And it fails with a compile-time error. System.Guid implements IFormattable, but the error I get is

cannot convert from ‘System.Collections.Generic.List’ to ‘System.Collections.Generic.List’

NOTE: You will get the same error if you just use an array of Guids. Generics is NOT the cause….

But! Given this

public void SomeRoutine2(IFormattable obj) { ... }

and this 

Guid a = Guid.NewGuid(); SomeRoutine2(a);

It compiles! So the question is WHY? Why am I able to pass a Guid object (which implements IFormattable) into a routine that accepts an object of IFormattable, but when I try to expand that to a collection (a generic list, or an array, or anything else), I get a conversion error?

I have had a heck of a time finding an answer, and I figured this would be the best place to go.

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