Question

I have these Extension methods:

public static void Replace<T>(this IList list, T newItem)

public static void Replace<T>(this IList<T> list, IEnumerable newItems)

public static void Replace<T>( this IList<T> list, IEnumerable<T> newItems )

I have a Linq statement that produces an IList<IWell> called, wells. (I confirm at runtime that wells is IEnumerable<IWell>.)

However, the statement

SelectedValues.Replace( wells );

always hits the first extension method, not the second or third. (I confirm at runtime that SelectedValues is IList<IWell>.)

Is it obvious what I am doing wrong?

Answer 1

What is the declared type of SelectedValues and of wells? Extension methods are bound at compile-time, not at runtime so it is the compile-time types that matter.

Edit: Since you said that SelectedValues is declared as type IList, the only possible candidate for use as an extension method on SelectedValues of the three you provided is

public static void Replace<T>(this IList list, T newItem)

The compiler then realizes that it can consider wells as a T with T being the declared type of wells and then can invoke the method

public static void Replace<T>(this IList list, T newItem)

where SelectedValues fills in for the parameter list and wells fills in for the parameter newItem, and the declared type of wells fills in for the type parameter T. This is why that extension method is invoked.

Again, extension methods are bound at compile-time. If you want to invoke a different method, you need to use a different declared type for SelectedValues.

So, this is not a case of the compiler “Matching wrong Extension method,” this is a case of the compiler matching the only possible extension method. This behavior is by design; it is a feature, not a bug.