C# Language Tutorial => Extension methods dispatch based on static...

Example

The static (compile-time) type is used rather than the dynamic (run-time type) to match parameters.

public class Base 
{ 
    public virtual string GetName()
    {
        return "Base";
    }
}

public class Derived : Base
{ 
    public override string GetName()
    {
        return "Derived";
    }
}

public static class Extensions
{
    public static string GetNameByExtension(this Base item)
    {
        return "Base";
    }

    public static string GetNameByExtension(this Derived item)
    {
        return "Derived";
    }
}

public static class Program   
{
    public static void Main()
    {
        Derived derived = new Derived();
        Base @base = derived;

        // Use the instance method "GetName"
        Console.WriteLine(derived.GetName()); // Prints "Derived"
        Console.WriteLine(@base.GetName()); // Prints "Derived"

        // Use the static extension method "GetNameByExtension"
        Console.WriteLine(derived.GetNameByExtension()); // Prints "Derived"
        Console.WriteLine(@base.GetNameByExtension()); // Prints "Base"
    }
}

Live Demo on .NET Fiddle

Also the dispatch based on static type does not allow an extension method to be called on a dynamic object:

public class Person
{
    public string Name { get; set; }
}

public static class ExtenionPerson
{
    public static string GetPersonName(this Person person)
    {
        return person.Name;
    }
}

dynamic person = new Person { Name = "Jon" };
var name = person.GetPersonName(); // RuntimeBinderException is thrown

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