Apex error when using try/catch blocks

Question

When I try to save my apex class with the following method in it, it is giving me an error "Error: Compile Error: Variable does not exist: Helper.prod_field1_api_name__c at line 37 column 41" (line 37 in my class is the highlighted line below)

global List<Related_Prods_Firms__c> start(Database.BatchableContext BC) 
    {
        try
        {
            Helper__c Helper = [Select firm_api_name__c,firm_field_api_name__c,
                                        Org_ID__c,Username__c,Password__c,prod_api_name__c,
                                        prod_field1_api_name__c,prod_field2_api_name__c
                                        from Helper__c];
        }
        catch(System.QueryException q)
        {
            String errMsg = q.getMessage();
            if(errMsg.contains('List has no rows for assignment to SObject'))
            {
                //add logic to be executed when there is no Helper__c record
            }
            if(errMsg.contains('List has more than 1 row for assignment to SObject'))
            {
                //add logic to be executed when there is more than one Helper__c record
            }
            return null;
        }
        List<Related_Prods_Firms__c> currRelatedProdFirmsList = new List<Related_Prods_Firms__c>();
        String query = 'Select Firm__c,Prod__c';
        if(Helper!=null)
        {
            if(Helper.prod_field1_api_name__c!=null&&Helper.prod_field1_api_name__c!='')
            {
                **query += ','+'Prod__r.'+Helper.prod_field1_api_name__c;**
            }
            if(Helper.prod_field2_api_name__c!=null&&Helper.prod_field2_api_name__c!='')
            {
                query += ','+'Prod__r.'+Helper.prod_field2_api_name__c;
            }
            if(Helper.firm_field_api_name__c!=null&&Helper.firm_field_api_name__c!='')
            {
                query += ','+'Firm__r.'+Helper.firm_field_api_name__c;
            }
        }
        query += ' from Related_Prods_Firms__c';
        if(Helper!=null)
        {
            query = String.escapeSingleQuotes(query);
            currRelatedProdFirmsList = Database.query(query);
        }
        return currRelatedProdFirmsList;
    }

If I repeat the "Helper" query part (as below), then it is saving the class!

global List<Related_Prods_Firms__c> start(Database.BatchableContext BC) 
    {
        try
        {
            **Helper__c Helper = [Select firm_api_name__c,firm_field_api_name__c,
                                        Org_ID__c,Username__c,Password__c,prod_api_name__c,
                                        prod_field1_api_name__c,prod_field2_api_name__c
                                        from Helper__c];**
        }
        catch(System.QueryException q)
        {
            String errMsg = q.getMessage();
            if(errMsg.contains('List has no rows for assignment to SObject'))
            {
                //add logic to be executed when there is no Helper__c record
            }
            if(errMsg.contains('List has more than 1 row for assignment to SObject'))
            {
                //add logic to be executed when there is more than one Helper__c record
            }
            return null;
        }
        **Helper__c Helper = [Select firm_api_name__c,firm_field_api_name__c,
                                        Org_ID__c,Username__c,Password__c,prod_api_name__c,
                                        prod_field1_api_name__c,prod_field2_api_name__c
                                        from Helper__c];**
        List<Related_Prods_Firms__c> currRelatedProdFirmsList = new List<Related_Prods_Firms__c>();
        String query = 'Select Firm__c,Prod__c';
        if(Helper!=null)
        {
            if(Helper.prod_field1_api_name__c!=null&&Helper.prod_field1_api_name__c!='')
            {
                query += ','+'Prod__r.'+Helper.prod_field1_api_name__c;
            }
            if(Helper.prod_field2_api_name__c!=null&&Helper.prod_field2_api_name__c!='')
            {
                query += ','+'Prod__r.'+Helper.prod_field2_api_name__c;
            }
            if(Helper.firm_field_api_name__c!=null&&Helper.firm_field_api_name__c!='')
            {
                query += ','+'Firm__r.'+Helper.firm_field_api_name__c;
            }
        }
        query += ' from Related_Prods_Firms__c';
        if(Helper!=null)
        {
            query = String.escapeSingleQuotes(query);
            currRelatedProdFirmsList = Database.query(query);
        }
        return currRelatedProdFirmsList;
    }

I don't understand why I have to define the "Helper" part twice! Can anyone please explain me this? Thanks in advance.

Answer 1

Your helper is getting scoped inside of the try block. If you instantiate it outside of the try block, then it will persist in a larger scope.

E.g. Where Helper is out of scope

try
{
    // Helper will only be in scope within this try block. 
    // You won't be able to reference it in code beyond the closing brace.
    Helper__c Helper = [Select firm_api_name__c from Helper__c];
}
catch(System.QueryException q)
{
    // You can't access Helper here
    String errMsg = q.getMessage();
    return null;
}
// You can't access Helper here

E.g. Where Helper has been scoped to the method

global List<Related_Prods_Firms__c> start(Database.BatchableContext BC)
{
    // Helper is now scoped within the method and can be used until the method is closed.
    // Note that is could be Null though, such as in the catch block. 
    Helper__c Helper = null;

    try
    {
        Helper = [Select firm_api_name__c from Helper__c];
    }
    catch(System.QueryException q)
    {
        // You can access Helper here, but it will be null.
        String errMsg = q.getMessage();
        return null;
    }
    // You can access Helper here (check for nulls depending on the exception handling above)
}