[Proposal] enhanced yield pattern

[spaccabit]

public IEnumerable<string> GetStrings()
{
   /* the scope of the following variables is common
      to all blocks of enumeration code (yield begin, yield catch, yield end) */
   DBConnection connection;
   IDataReder dr;
   retryCount = 0;
   yield begin {
    /* this block of code is called before the start of the enumeration;
       the instruction "yield begin" is optional.
       the scope of the variables defined in this block of code is local. */
   }
    // normal yield instruction; required
   while(dr.Read()){
       var value = dr.GetString(0);
       if(value == "STOP")
          yield break;
       yield return value;
   }
   /* intercept exceptions; the instruction "yield catch" is optional,
      multiple blocks can be defined to catch different types of exception
      the scope of the variables defined in this block of code is local. */
   yield catch (SqlException sqlError) when (retryCount < 3){
      // manage exception e retry
      retryCount++;
      continue; // continue the enumeration
   }
   yield catch (ArgumentNullException ne)
      break; // break the enumeration
   }
   yield catch (Exception ex) {
      throw ex // break the enumeration and thorw exception;
   }
   yield end{ 
      /* if this block of code is defined, it is always invoked last indifferently 
         if the enumeration ended correctly or stopped due to an exception;
         the instruction "yield end" is optional. 
         the scope of the variables defined in this block of code is local. */
      if (dr != null){
         dr.Dispose();
      }
      if(connection != null && connection.State != ConnectionState.Closed){
         connection.Dispose();
      }
  }
}

[franchyd]

is this not just a try catch finally?

[HaloFour]

The idea of a context before iteration was discussed before and the approach accepted by the LDM was the use of a local function to contain the actual iteration.

[spaccabit]

The idea of a context before iteration was discussed before and the approach accepted by the LDM was the use of a local function to contain the actual iteration.

Can you share the discussion link?

[HaloFour]

I found dotnet/roslyn#2930 which isn't so much a discussion of lifecycles within an iterator but argument validation was listed as a specific motivation for local functions. The original proposal was added relatively early to Roslyn (dotnet/roslyn#259) so the original discussion might be buried in the CodePlex archives.