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var fa = new FA<char,string>(); // instantiate the specialization than
var fa = new CharFA<string>(); // <-- what i have to do now

void DoSomething<T>(T value) {
  switch (typeof(T)) {
    case typeof(double): DoDoubleThing((double)value); break;
    case typeof(int): DoIntThing((int)value); break;
    // ... all common base type
    default: DoDefaultThing(value); break;
  }
}

class A<T>
{
    public virtual void Do(T value)
    {
        Console.WriteLine("Value: " + value);
    }
}
class B : A<int>
{
    public override void Do(int value)
    {
        Console.WriteLine("Int: " + value);
    }
}
class Program
{
    static void Main(string[] args)
    {
        A<int> a = new B();
        a.Do(1);
    }
}

class A<T>
{
    public void Do(T value)
    {
        if (value is int intV) Do(intV);
        else DoDefault(value);
    }
    void DoDefault(T value)
    {
        Console.WriteLine("Value: " + value);
    }
    void Do(int value)
    {
        Console.WriteLine("Int: " + value);
    }
}
class Program
{
    static void Main(string[] args)
    {
        A<int> a = new A<int>();
        a.Do(1);
    }
}

void Do<int>(int value) {
  if (value is double d) { // dead end code prune by the compiler
  }
  if (value is int ii) Do (ii);
}

void Do<int>(int value) {
  Do (value);
}

void Do<int>(int value) {
  // I would wage $5 the compiler remove the if statement and execute the nested statement always
  if (value is int ii) { 
  }
  // I would wage 5 dollar this is removed by the compiler
  if (value is double dd) { 
  }
}

class A<T>
{
    public void Do(T value)
    {
        if (value is int intV) Do(intV);
        else DoDefault(value);
    }
    void DoDefault(T value)
    {
        Console.WriteLine("Value: " + value);
    }
    void Do(int value)
    {
        Console.WriteLine("Int: " + value);
    }
}
class Program
{
    static void Main(string[] args)
    {
        A<int> a = new A<int>();
        a.Do(1);

        A<double> b = new A<double>();
        b.Do(1.0);
    }
}

            a.Do(1);
00F70898  mov         ecx,dword ptr [ebp-40h]<br />
00F7089B  mov         edx,1<br />
00F708A0  cmp         dword ptr [ecx],ecx<br />
00F708A2  call        00F70478<br />
00F708A7  nop  

            b.Do(1.0);
00F708C3  fld         qword ptr ds:[0F708E8h]<br />
00F708C9  sub         esp,8<br />
00F708CC  fstp        qword ptr [esp]<br />
00F708CF  mov         ecx,dword ptr [ebp-44h]<br />
00F708D2  cmp         dword ptr [ecx],ecx<br />
00F708D4  call        00F704A0<br />
00F708D9  nop  

.method public hidebysig instance void  Do(!T 'value') cil managed
{
  // Code size       58 (0x3a)
  .maxstack  2
  .locals init ([0] int32 intV,
           [1] bool V_1)
  IL_0000:  nop
  IL_0001:  ldarg.1
  IL_0002:  box        !T
  IL_0007:  isinst     [mscorlib]System.Int32
  IL_000c:  brfalse.s  IL_0022
  IL_000e:  ldarg.1
  IL_000f:  box        !T
  IL_0014:  isinst     [mscorlib]System.Int32
  IL_0019:  unbox.any  [mscorlib]System.Int32
  IL_001e:  stloc.0
  IL_001f:  ldc.i4.1
  IL_0020:  br.s       IL_0023
  IL_0022:  ldc.i4.0
  IL_0023:  stloc.1
  IL_0024:  ldloc.1
  IL_0025:  brfalse.s  IL_0031
  IL_0027:  ldarg.0
  IL_0028:  ldloc.0
  IL_0029:  call       instance void class ConsoleApp1.A`1<!T>::Do(int32)
  IL_002e:  nop
  IL_002f:  br.s       IL_0039
  IL_0031:  ldarg.0
  IL_0032:  ldarg.1
  IL_0033:  call       instance void class ConsoleApp1.A`1<!T>::DoDefault(!0)
  IL_0038:  nop
  IL_0039:  ret
} // end of method A`1::Do