Question

I have a question on generics. I am trying to convert my code using generic so I don’t need to change my code again for every specific type like CompanyBO, EmployeeBO etc…

Here is my code:

public List<T1> GetAllEmployees<T1>()
{
    try
    {
        var objEmployeeList = new List<T1>();

        SqlConnection objSqlCon = new SqlConnection(ConfigurationManager.ConnectionStrings["AdventureWorksConnectionString"].ConnectionString);

        if (objSqlCon.State == System.Data.ConnectionState.Closed)
        {
            objSqlCon.Open();
        }

        SqlCommand objSqlCmd = new SqlCommand("SELECT EmployeeID, ContactID, LoginID, Title FROM HumanResources.Employee", objSqlCon);

        SqlDataReader objSqlDataRead;

        objSqlDataRead = objSqlCmd.ExecuteReader();

        while (objSqlDataRead.Read())
        {
            objEmployeeList.Add(FillDataRecord(objSqlDataRead));
        }

        return objEmployeeList;
    }
    catch (Exception ex)
    {

        throw ex;
    }
}


private EmployeeBO FillDataRecord(IDataRecord objDataRecord)
{
    try
    {
        EmployeeBO objEmployeeBO = new EmployeeBO();

        if (!objDataRecord.IsDBNull(objDataRecord.GetOrdinal("EmployeeID")))
        {
            objEmployeeBO.EmployeeID = objDataRecord.GetInt32(objDataRecord.GetOrdinal("EmployeeID"));
        }

        if (!objDataRecord.IsDBNull(objDataRecord.GetOrdinal("ContactID")))
        {
            objEmployeeBO.ContactID = objDataRecord.GetInt32(objDataRecord.GetOrdinal("ContactID"));
        }

        if (!objDataRecord.IsDBNull(objDataRecord.GetOrdinal("Title")))
        {
            objEmployeeBO.Title = objDataRecord.GetString(objDataRecord.GetOrdinal("Title"));
        }

        if (!objDataRecord.IsDBNull(objDataRecord.GetOrdinal("LoginID")))
        {
            objEmployeeBO.LoginID = objDataRecord.GetString(objDataRecord.GetOrdinal("LoginID"));
        }

        return objEmployeeBO;
    }
    catch (Exception ex)
    {
        throw ex;
    }
}

I am getting 1 error : cannot convert from ‘EmployeeBO’ to ‘T1’

Can any please help me to create this Generic Method…

Answer 1

Your FillDataRecord can only ever return an EmployeeBO, so it’s not clear why you want to use generics in the first place. Why would a name called GetAllEmployees want to return a List<CompanyBO> for example?

It looks like you should just change the signature to:

public List<EmployeeBO> GetAllEmployees()

and make the corresponding change within the method.

If you want to write a generic method, you should work out what would change between this method and the corresponding method for companies. I would suggest that in the current design, it would be:

• The SQL statement
• The method to use to convert an IDataRecord into an entity

So that would suggest you have something like:

public List<T> FetchEntities<T>(string sql, Func<IDataRecord, T> converter)

You’d then specify the converter using a method group conversion, e.g.

const string FetchAllEmployeesSql = "SELECT ...";

List<EmployeeBO> employees = FetchEntities(FetchAllEmployeesSql,
                                           FillEmployeeDataRecord);

Is there any reason you want to build your own sort of ORM for this rather than using some of the many existing ones, by the way?