Programming a Calculated Field

After you define a calculated field, you must write code to calculate its value. Otherwise, it always has a null value. Code for a calculated field is placed in the OnCalcFields event for its dataset.

To program a value for a calculated field

Select the dataset component from the Object Inspector drop-down list.
Choose the Object Inspector Events page.
Double-click the OnCalcFields property to bring up or create a CalcFields procedure for the dataset component.
Write the code that sets the values and other properties of the calculated field as desired.

For example, suppose you have created a CityStateZip calculated field for the Customers table on the CustomerData data module. CityStateZip should display a company's city, state, and zip code on a single line in a data-aware control.

To add code to the CalcFields procedure for the Customers table, select the Customers table from the Object Inspector drop-down list, switch to the Events page, and double-click the OnCalcFields property.

The TCustomerData.CustomersCalcFields procedure appears in the unit's source code window. Add the following code to the procedure to calculate the field:

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CustomersCityStateZip.Value := CustomersCity.Value + ', ' + CustomersState.Value  + ' ' + CustomersZip.Value;

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CustomersCityStateZip->Value = CustomersCity->Value + AnsiString(", ") +

Note: When writing the OnCalcFields event handler for an internally calculated field, you can improve performance by checking the client dataset's State property and only recomputing the value when State is dsInternalCalc. See Using internally calculated fields in client datasets for details.