c# - Why does a property inherited from an interface become virtual?

Question

Say I have one interface and two classes, and one of the classes implement this interface:

interface IAAA
{
    int F1 { get; set; }
}

class AAA1
{
    public int F1 { get; set; }
    public int F2 { get; set; }
}

class AAA2 : IAAA
{
    public int F1 { get; set; }
    public int F2 { get; set; }
}

In class AAA2, property F1 is 'inherited' (I'm not sure) from interface IAAA, then I use reflection to check whether a property is virtual:

Console.WriteLine("AAA1 which does not implement IAAA");
foreach (var prop in typeof(AAA1).GetProperties())
{
    var virtualOrNot = prop.GetGetMethod().IsVirtual ? "" : " not";
    Console.WriteLine($@"{prop.Name} is{virtualOrNot} virtual");
}

Console.WriteLine("AAA2 which implements IAAA");
foreach (var prop in typeof(AAA2).GetProperties())
{
    var virtualOrNot = prop.GetGetMethod().IsVirtual ? "" : " not";
    Console.WriteLine($"{prop.Name} is{virtualOrNot} virtual");
}

The output is:

AAA1 which does not implement IAAA
F1 is not virtual
F2 is not virtual
AAA2 which implements IAAA
F1 is virtual
F2 is not virtual

Any reason for this?

question from:https://stackoverflow.com/questions/53492538/why-does-a-property-inherited-from-an-interface-become-virtual

Answer 1

As from remarks section of MS docs:

A virtual member may reference instance data in a class and must be referenced through an instance of the class... The common language runtime requires that all methods that implement interface members must be marked as virtual; therefore, the compiler marks the method virtual final

If you need to determine whether this method is overridable then checking IsVirtual is not enough and you need to also check that IsFinal is false.

Here is an extension method that do this check:

public static bool IsOverridable(this MethodInfo method)
    => method.IsVirtual && !method.IsFinal;