A model-defined function is a function that is defined in a conceptual model. The body of a model-defined function is expressed in Entity SQL, which allows for the function to be expressed independently of rules or languages supported in the data source.
A definition for a model-defined function contains the following information:
A function name. (Required)
The type of the return value. (Optional)
Note: If no return type is specified, the return value is void.
Parameter information. (Optional)
An Entity SQL expression that defines the body of the function.
Note that model-defined functions do not support output parameters. This restriction is in place so that model-defined functions can be composed.
The diagram below shows a conceptual model with three entity types: Book, Publisher, and Author.
The ADO.NET Entity Framework uses a domain-specific language (DSL) called conceptual schema definition language (CSDL) to define conceptual models. The following CSDL defines a function in the conceptual model that returns the numbers of years since an instance of a Book (in the diagram above) was published.