Github Link : Trace-Dapper.NET-Source-Code

6. Strongly Typed Mapping Part3: The important concept of dynamic create method "code from result" optimizes efficiency

Then use Expression to solve the Reflection version problem, mainly using Expression features: 「Methods can be dynamically created during Runtime」 to solve the problem.

Before this, we need to have an important concept: 「Reverse the most concise code from the result」 optimizing efficiency. For example: In the past, a classic topic of "printing regular triangle stars'' when learning a program to make a regular triangle of length 3, the common practice would be loop + recursion the way

void  Main ()
{
  Print(3,0);
}

static void Print(int length, int spaceLength)
{
  if (length < 0)
    return;
  else
    Print(length - 1, spaceLength + 1);
  for (int i = 0; i < spaceLength; i++)
    Console.Write(" ");
  for (int i = 0; i < length; i++)
    Console.Write("* ");
  Console.WriteLine("");
}

But in fact, this topic can be changed to the following code when the length is already known

Console.WriteLine("  * ");
Console.WriteLine(" * * ");
Console.WriteLine("* * * ");

This concept is very important, because the code is reversed from the result, so the logic is straightforward and efficient , and Dapper uses this concept to dynamically build methods.

Example:

• The Name property of User Class corresponds to Reader Index 0, the type is String, and the default value is null

• The Age attribute of User Class corresponds to Reader Index 1, the type is int, and the default value is 0
  

void  Main ()
{
  using (var cn = Connection)
  {
    var result = cn.Query<User>("select N'Wei' Name,26 Age").First();
  }
}

class User
{
  public string Name { get; set; }
  public int Age { get; set; }
}

If the system can help generate the following logical methods, then the efficiency will be the best

User dynamic method ( IDataReader  reader )
{
  var user = new User();
  var value = reader[0];
  if( !(value is System.DBNull) )
    user.Name = (string)value;
  value = reader[1];
  if( !(value is System.DBNull) )
    user.Age = (int)value;  
  return user;
}

In addition, the above example can be seen for Dapper SQL Select corresponds to the Class attribute order is very important , so the algorithm of Dapper in the cache will be explained later, which is specifically optimized for this.