What's the strangest corner case you've seen in C# or .NET? [closed]

Answer 1

Consider this weird case:

public interface MyInterface {
  void Method();
}
public class Base {
  public void Method() { }
}
public class Derived : Base, MyInterface { }

If Base and Derived are declared in the same assembly, the compiler will make Base::Method virtual and sealed (in the CIL), even though Base doesn't implement the interface.

If Base and Derived are in different assemblies, when compiling the Derived assembly, the compiler won't change the other assembly, so it will introduce a member in Derived that will be an explicit implementation for MyInterface::Method that will just delegate the call to Base::Method.

The compiler has to do this in order to support polymorphic dispatch with regards to the interface, i.e. it has to make that method virtual.

Answer 2

Here's one I only found out about recently...

interface IFoo
{
   string Message {get;}
}
...
IFoo obj = new IFoo("abc");
Console.WriteLine(obj.Message);

The above looks crazy at first glance, but is actually legal.No, really (although I've missed out a key part, but it isn't anything hacky like "add a class called IFoo" or "add a using alias to point IFoo at a class").

See if you can figure out why, then: Who says you can’t instantiate an interface?

Answer 3

The scoping in c# is truly bizarre at times. Lets me give you one example:

if (true)
{
   OleDbCommand command = SQLServer.CreateCommand();
}

OleDbCommand command = SQLServer.CreateCommand();

This fails to compile, because command is redeclared? There are some interested guesswork as to why it works that way in this thread on stackoverflow and in my blog.

Answer 4

VB.NET, nullables and the ternary operator:

Dim i As Integer? = If(True, Nothing, 5)

This took me some time to debug, since I expected i to contain Nothing.

What does i really contain? 0.

This is surprising but actually "correct" behavior: Nothing in VB.NET is not exactly the same as null in CLR: Nothing can either mean null or default(T) for a value type T, depending on the context. In the above case, If infers Integer as the common type of Nothing and 5, so, in this case, Nothing means 0.

Answer 5

Public Class Item
   Public ID As Guid
   Public Text As String

   Public Sub New(ByVal id As Guid, ByVal name As String)
      Me.ID = id
      Me.Text = name
   End Sub
End Class

Public Sub Load(sender As Object, e As EventArgs) Handles Me.Load
   Dim box As New ComboBox
   Me.Controls.Add(box)          'Sorry I forgot this line the first time.'
   Dim h As IntPtr = box.Handle  'Im not sure you need this but you might.'
   Try
      box.Items.Add(New Item(Guid.Empty, Nothing))
   Catch ex As Exception
      MsgBox(ex.ToString())
   End Try
End Sub

The output is "Attempted to read protected memory. This is an indication that other memory is corrupt."

Answer 6

C# Accessibility Puzzler

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The following derived class is accessing a private field from its base class, and the compiler silently looks to the other side:

public class Derived : Base
{
    public int BrokenAccess()
    {
        return base.m_basePrivateField;
    }
}

The field is indeed private:

private int m_basePrivateField = 0;

Care to guess how we can make such code compile?

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Answer

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The trick is to declare Derived as an inner class of Base:

public class Base
{
    private int m_basePrivateField = 0;

    public class Derived : Base
    {
        public int BrokenAccess()
        {
            return base.m_basePrivateField;
        }
    }
}

Inner classes are given full access to the outer class members. In this case the inner class also happens to derive from the outer class. This allows us to "break" the encapsulation of private members.