Returning a value based on a lookup

Question

I am trying to return a value (a.tnAddress) from a custom class based on a lookup (foreach loop). Depending on the type of transaction, I will need to do the foreach loop based on different properties (sExecID, iMsgSeqNum, orsClOrderID). I prefer to not have 3 differentforeach` loops just but I am not sure how else to re-write this.

Keep in mind that the code is working fine; I just want to simplify it.

private TreeNode GetNodeAddress(cls_Transactions trPassedInTransaction)
{  
    switch (trPassedInTransaction.sMessageType)
    {
        case "Q":
        case "8b":
            foreach (cls_Transactions a in cls_GlobalVariables.transList)
            {
                if (trPassedInTransaction.sExecID == a.sExecID)
                {
                    return a.tnAddress;
                }
            }
            break;
        case "3":
            foreach (cls_Transactions a in cls_GlobalVariables.transList)
            {
                if (trPassedInTransaction.iMsgSeqNum == a.iMsgSeqNum)
                {
                    return a.tnAddress;
                }
            }
            break;
        default:
            foreach (cls_Transactions a in cls_GlobalVariables.transList)
            {
                if (trPassedInTransaction.sClOrderID == a.sClOrderID)
                {
                    return a.tnAddress;
                }
            }                    
            break;
    }

    return null;
}

Answer 1

This will add the use of LINQ to clean up the code the way you want:

private TreeNode GetNodeAddress(cls_Transactions trPassedInTransaction)
{
    //a predicate to pass to the FirstOrDefault method
    Func<cls_Transactions,Boolean> filter = null;
    switch (trPassedInTransaction.sMessageType)
    {

        case "Q":
        case "8b":
                 filter = x => trPassedInTransaction.sExecID == x.sExecID;
            break;
        case "3":
                 filter = x => trPassedInTransaction.iMsgSeqNum == x.iMsgSeqNum;
            break;
        default:
                 filter = x => trPassedInTransaction.sClOrderID == x.sClOrderID;                 
            break;
    }

    cls_Transactions result = cls_GlobalVariables.transList.FirstOrDefault(filter);
    return result != null ? result.tnAddress : null;
}

As an explanation, the switch statement has just been converted to use a predicate, which is a function type where it passes one parameter (in this case a cls_Transactions) and returns true/false.

The FirstOrDefault method is shorthand for the foreach loop and return, foreach-ing through the elements and using the predicate to determine if it meets the required condition, if none meet the required condition it will return a default value (in the case null).

You can use the First method also, which will throw an exception if nothing is found :)

Answer 2

I like using a dictionary because to me it looks like tighter cleaner code and you can easily add to the dictionary or if you want later pull the logic from another source and put into the dictionary.

        var dictionary = new Dictionary<string, Predicate<cls_Transactions>>();
        dictionary.Add("Q", x => x.sExecID == trPassedInTransaction.sExecID);
        dictionary.Add("8b", x => x.sExecID == trPassedInTransaction.sExecID);
        dictionary.Add("3", x => x.iMsgSeqNum == trPassedInTransaction.iMsgSeqNum);
        dictionary.Add("default", x => x.sClOrderID == trPassedInTransaction.sClOrderID);

        var switchValue = dictionary[trPassedInTransaction.sMessageType] != null
                              ? trPassedInTransaction.sMessageType
                              : "default";
        var result = cls_GlobalVariables.transList.FirstOrDefault(t => dictionary[switchValue](t));
        return result != null ? result.tnAddress : null;