Question

Below is some code that demonstrates I cannot declare and initialize a struct type as null. The Nullable type is a struct, so why am I able to set it to null?

Nullable<bool> b = null;
if (b.HasValue)
{
    Console.WriteLine("HasValue == true");
}

//Does not compile...
Foo f = null;
if (f.HasValue)
{
    Console.WriteLine("HasValue == true");
}

Where Foo is defined as

public struct Foo
{
    private bool _hasValue;
    private string _value;

    public Foo(string value)
    {
        _hasValue = true;
        _value = value;
    }

    public bool HasValue
    {
        get { return _hasValue; }
    }

    public string Value
    {
        get { return _value; }
    }
}

The question has been answered (see below). To clarify I’ll post an example. The C# code:

using System;

class Program
{
    static void Main(string[] args)
    {
        Nullable<bool> a;
        Nullable<bool> b = null;
    }
}

produces the following IL:

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       10 (0xa)
  .maxstack  1
  .locals init ([0] valuetype [mscorlib]System.Nullable`1<bool> a,
           [1] valuetype [mscorlib]System.Nullable`1<bool> b)
  IL_0000:  nop
  IL_0001:  ldloca.s   b
  IL_0003:  initobj    valuetype [mscorlib]System.Nullable`1<bool>
  IL_0009:  ret
} // end of method Program::Main

a and b are declared, but only b is initialized.

Answer 1

The C# compiler provides you with a bit of sugar so you really are doing this:

Nullable<bool> b = new Nullable<bool>();

Here is the syntactic sugar

bool? b = null;    
if (b ?? false) 
{
   b = true;
}