When I use foreach for regex.matches, var thinks its type is object. Why?

[zms9110750]

It can even use generic LINQ

[gfoidl]

It's because of

runtime/src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/MatchCollection.cs

Lines 71 to 73 in 6294858

	public IEnumerator GetEnumerator() => new Enumerator(this);
	IEnumerator<Match> IEnumerable<Match>.GetEnumerator() => new Enumerator(this);

and that C# binds to the instance method.
When you cast to IEnumerable<Match> first, then it bind to the explicit interface method.

foreach(var m in (IEnumerable<Match>)matches)
{
    // m is of type Match
}

Or if you use the explicit type instead of var...

I guess this is for historic reasons, as MatchCollection dates back to a time where generics weren't available.

[PathogenDavid]

As @gfoidl mentioned, MatchCollection predates generics so foreach (var m in r.Matches("...")) results in m being object. It's briefly mentioned here when IEnumerable<T> was added: #13933

However, LINQ should pick up the explicit generic implementation as long as you're on a modern version of .NET. If you're still on .NET Framework you should be able to use .Cast<Match>() to use LINQ.

[zms9110750]

What happens if you delete the old API or change to the generic version of the iterator? How much will this affect the previous code?

[stephentoub]

That would break every compiled binary using the existing API.