float resultF = Add(1.0f, 2.0f);
double resultD = Add(1.0, 2.0);

Console.WriteLine(resultF);
Console.WriteLine(resultD);

static T Add<T>(T left, T right) where T : struct
{
    unchecked
    {
        if      (left is short)  return (T)(ValueType)(short)  ( (short) (ValueType)left + (short) (ValueType)right );
        else if (left is int)    return (T)(ValueType)(int)    ( (int)   (ValueType)left + (int)   (ValueType)right );
        else if (left is long)   return (T)(ValueType)(long)   ( (long)  (ValueType)left + (long)  (ValueType)right );
        else if (left is ushort) return (T)(ValueType)(ushort) ( (ushort)(ValueType)left + (ushort)(ValueType)right );
        else if (left is uint)   return (T)(ValueType)(uint)   ( (uint)  (ValueType)left + (uint)  (ValueType)right );
        else if (left is ulong)  return (T)(ValueType)(ulong)  ( (ulong) (ValueType)left + (ulong) (ValueType)right );
        else if (left is byte)   return (T)(ValueType)(byte)   ( (byte)  (ValueType)left + (byte)  (ValueType)right );
        else if (left is sbyte)  return (T)(ValueType)(sbyte)  ( (sbyte) (ValueType)left + (sbyte) (ValueType)right );
        else if (left is float)  return (T)(ValueType)(float)  ( (float) (ValueType)left + (float) (ValueType)right );
        else if (left is double) return (T)(ValueType)(double) ( (double)(ValueType)left + (double)(ValueType)right );
        else throw new NotImplementedException();
    }
}