difference var and Type. Why doesnt var work here?

Question

I dont understand why var m does not return Match. I havent checked but it seems to be returning an object.

        foreach (var m in Regex.Matches("dummy text", "(mm)"))
            var sz = m.Groups[1]; // error CS1061: 'object' does not contain a definition for 'Groups' and no extension method 'Groups' accepting a first argument of type 'object' could be found (are you missing a using directive or an assembly reference?)

        foreach (Match m in Regex.Matches("dummy text", "(mm)"))
            var sz = m.Groups[1]; //ok

Answer 1

Regex.Matches returns MatchCollection that implements IEnumerable and not IEnumerable<Match>.

Therefore the default item type is object. When using item type Match in the foreach you get the expected item type.

Answer 2

var infers the type of the variable at compile time from the type of the expression it's initialized to. Matches returns MatchCollection which implements the old school IEnumerable and not the generic IEnumerable<Match>. The type of the Current property returned by the enumerator returned from IEnumerable.GetEnumerator() is object. Thus, var will infer the type of m to be object and not Match.

When you explicitly specify the type in a foreach statement and the return type of the enumerator differs, the compiler will silently insert a cast instruction to make that work. No other compile time check can be performed in that case and it will throw an InvalidCastException at run time in case it fails.

Answer 3

MatchCollection implements IEnumerable rather than IEnumerable<Match> hence the compiler can only infer the type of the variable as object rather than Match.