A small demo const problem

using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Text;
  
  namespace ConsoleApplication1
  {
      class Program
      {
  
          static void Main(string[] args)
          {
              TwoTuple<Boolean, Double> two = new TwoTuple<Boolean, Double>(true, 56.00);
              Boolean bo = two.first;
              Double st = two.second;
              Console.WriteLine(bo+st);
              Console.Read();
          }
      }
      public class TwoTuple<A,B> {
          public const A first=null;
          public const B second=null;
     
          public TwoTuple(A  a,B b){
            first=a;
            second=b;
  }
      public String toString(){
         return ")"+first+", " +second+")";
  }
      }
  }
  

public const A first = null;
public const B second = null;
This is because the two types of error does not recognize it ?

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that mistake compile it ? For C do not understand !
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  public A first = default(A);
  public B second = default(B);
  

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sixth floor is that you type is not clear , there may be a value type will not be null
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public readonly A first;
public readonly B second;
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compiled tips have been very clear:
Null can not be converted to a type parameter "A", as it may be not be a null value type. Please consider using the "default (A)".
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pursuing big god answers
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did not help me answer the next Mody. . .
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own Dinga . . . . . . . . . . . . . . .