Relationship between extension generic parameters and Func argument generic parameters

Question

So I guess I don't really understand the above. Like suppose you have an extension on an enumerable type like so....

public static TElement StringMatch<TElement, TData>(
        this IEnumerable<TElement> source,
        Func<TElement, TData> selector)

All is fine here, but I've been assuming that the StringMatch generic parameters mirror the Func generic parameters, since the Func<> is what appears to the user to be passed.

But say I want to specify that the return type of Func<> is a specific parameter, maybe like Func<TElement, string>

Now my thought would be to alter the signature like so...

public static TElement StringMatch<TElement, string>(
            this IEnumerable<TElement> source,
            Func<TElement, string> selector)

...again, to mirror the passed Func<>. But if I try to invoke this on something like Books.StringMatch(b => b.Title), I get an error like...

'Book' does not contain a definition for 'StringMatch' and no extension method 'StringMatch' accepting a first argument of type 'Book' could be found (are you missing a using directive or an assembly reference?) 

So what's the deal here? What exactly do the generic parameters in an extension method specify?

Answer 1

Once you have

public static TElement StringMatch<TElement, TData>(
    this IEnumerable<TElement> source,
    Func<TElement, TData> selector)

it covers the case where the caller passes a Func where TData is string.

If you want TData to always be the specific type string then simply remove that as a formal generic parameter in the method:

public static TElement StringMatch<TElement>(
    this IEnumerable<TElement> source,
    Func<TElement, string> selector)

You can, of course, implement both. The compiler will select the most specific one. The caller can also specify the type parameters explicitly to leave only one choice.