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Editorial Team
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Asked: 2026-05-27T08:12:05+00:00

public void Foo (IEnumerable<object> objects) { } var strings = new List<string>{first, second, third};

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public void Foo (IEnumerable<object> objects)
{
}

var strings = new List<string>{"first", "second", "third"};
Foo(strings); // Compilation Error.
Foo(strings.Cast<object>()); // O.k.
  1. Why the first call to Foo doesn’t compile? string derived from object.
  2. If I can cast the list to object and it’s compiled, why the compiler doesn’t do it by his own?
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1 Answer

  1. Editorial Team

    The first call compiles in .NET 4.0.

    In previous versions, the generic types have to match exactly.

    I suggest reading the blogs posts of Eric Lippert regarding variance (covariance and contravariance).

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