Data Paths Within Data Sources in .NET framework

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Data Paths Within Data Sources
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Data sources specified in Windows Forms data binding are treated as hierarchical sources The top-level object is a container for other collections of data (for example, data tables in a data set), and the collections themselves are containers for individual data items (the rows within a data table in a data set) Each data item then has values for each property in the object (the column values within the row) The data source can be even simpler: it could be an object containing properties, or it could be a simple array of objects When dealing with custom object collections as your sources of data, the levels in the hierarchy can be arbitrarily deep You could have a top-level object, Company, that contains a collection ofEmployees; each Employee could contain collections of Jobs, Tasks, and Contacts, as well as data items specific to the Employee; and each of the child collections could contain collections or data items themselves To handle all these situations, data sources are treated as hierarchies, and a path syntax is used to specify data sources and data members This accommodates specifying the exact location in a hierarchy of data where the desired collection, item, or value resides When I bound the TextBox controls to the data source inListing 34, I specified the data source as the Customers table within the data set, and then I specified the data member as the column name within that table You could also set the data source to the data set itself instead of the contained data table But if you did that, you would also need to modify the way you specified the data member, because the data member represents a relative path within the data source hierarchy to the particular data object or value to which you are binding So an alternate approach to setting the text box data bindings could look like the code in Listing 35
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Listing 35 Modified TextBox Data Bindings
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private void AddTextBoxDataBindings(CustomersDataSet nwData) { // Alternative approach:
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m_CustomerIDTextBoxDataBindingsAdd("Text", nwData, "CustomersCustomerID", true); m_CompanyNameTextBoxDataBindingsAdd("Text", nwData, "CustomersCompanyName", true); m_ContactNameTextBoxDataBindingsAdd("Text", nwData, "CustomersContactName", true); m_PhoneTextBoxDataBindingsAdd("Text", nwData, "CustomersPhone", true);
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In this case, since the data source is the data set itself, you have to specify the column to bind to by prefacing the column name with the table name, separated with a dot in the same way you use a dot with members of types in NET code If you made just this change in the SimpleNorthwindClient sample application and ran it again, the data in the text boxes would initially present the data for the first row just fine But if you clicked on another row in the grid, you would see that the text boxes no longer updated properlybecause even though both the grid and the text boxes are working against the same underlying data source (the instance of the CustomersDataSet), the way that data source has been specified to the data-binding mechanisms was different for each of the two controls For the grid, the data source was specified to be the Customers table within the data set Effectively, the data
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This document was created by an unregistered ChmMagic, please go to http://wwwbisentercom to register it Thanks member for the grid is empty or null because it wasn't specified, but now the data source for the text boxes is the data set itself, not the table within it This is an important concept that has tripped up a lot of developers when dealing with complex data-binding scenarios and trying to keep bound controls synchronized with one another The data binding mechanisms create a separate synchronization object (either a CurrencyManager for list data sources or aPropertyManager for individual object data sources) for each data source that is specified But even if two bindings use the same underlying logical data source, if that data source is specified differently between two bindings, then they will have separate synchronization objects created for each way the data source is specified, and you won't get synchronization as you might expect it To remedy that problem for the situation just described, all you need to do is be consistent in the way you specify your data sources and data members, and you should get nice synchronized behavior between controls that are bound to the same data source However, the recommended practice is to set your data source at the list level and to avoid compound data members (eg, CustomersPhone) One exception to this would be if you are setting up data binding to a complex object that holds a single complex child object For example, consider a Customer object that has an Address property on it The Address property exposes a complex type instance with properties of City, State, and so on for that instance In this case, you might use a compound data member for binding to a single Customer object's address information You would probably set the data source to theCustomer object, but then you might set a data member to AddressCity if you were data binding to that piece of information within the Customer object
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