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public class SomeParsingData {}

public class FA<T> 
{
  // ...
  public virtual void MyOperation(SomeParsingData data) {}
}
public class CharFA : FA<char> 
{
  public override void MyOperation(SomeParsingData data) {}
}

static class FAUtil
{
  public static void MyOperation<T>(FA<T> target, SomeParsingData data) => target.MyOperation(data);
}

class FA<TInput,TAccept> {
}

class CharFA<TAccept> : FA<char,TAccept> {
}

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