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   How to: Access Data in SQL Query Results
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   How to: Implement a Custom Criteria Language Function Operator
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   How to: Implement Custom Functions and Criteria in LINQ to XPO
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How to: Implement a Custom Criteria Language Function Operator

While XPO supports a number of built-in criteria language operators, you may need additional function operators either for complex calculations or in cases when you want to exploit certain database-specific functions. This topic demonstrates how to implement the MyGetMonth function (similar to the built-in FunctionOperatorType.GetMonth function) based on the DATEPART function supported by the MS Access database engine. The MyGetMonth function returns the month part (as a number) of a specified date.

First, declare the MyGetMonthFunction class as the implementor of the ICustomFunctionOperatorFormattable interface.

Then, register the custom function. Previously, you had to pass a custom function to a data store provider's RegisterCustomFunctionOperator method and a dictionary's XPDictionary.CustomFunctionOperators collection to accomplish this. Starting with v2011 vol 1, you can simply call the CriteriaOperator.RegisterCustomFunction or CriteriaOperator.RegisterCustomFunctions method instead.

Modify the project's Main method as shown below. In Visual Basic, add a module to the project, create the Main method and set it as the startup object via the project's properties.

After a Data Access Layer has been instantiated, you can use the implemented custom function operator to build filter criteria and expressions.

To learn how to implement custom functions and criteria, and use them in LINQ to XPO expressions, see How to: Implement Custom Functions and Criteria in LINQ to XPO.

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