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.NET Open Clients
Using the Open Client .NET OpenAPI to Directly Access the AppServer : Setting up parameters and return types : Defining the return type for a user-defined function : IsReturnUnknown property
 
IsReturnUnknown property
Set this property to TRUE in order to indicate that the user-defined function can return the Unknown value (?). This setting also sets the UnknownReturnType property to the enumeration value, UnknownType.HolderClass. If the function cannot return the Unknown value (?), set this property to FALSE, which also sets the UnknownReturnType property to the enumeration value, UnknownType.None.
Note: This property is supported for backward compatibility only. Progress Software recommends that you directly set and test the UnknownReturnType property instead. For more information, see UnknownReturnType property.
This is the syntax:

Syntax

public bool IsReturnUnknown
For example, if the ReturnType property is an INTEGER EXTENT, and this property is set to TRUE, the ReturnValue property returns an IntHolder[]. If it is set to FALSE, the ReturnValue property returns an Int32[]. The default is FALSE. For information on the ReturnValue property, see Accessing user-defined function return values.
If a scalar return value can be the Unknown value (?), check if the ReturnValue property is null. If it is not null, you can then unbox ReturnValue to get the value on a ParamArray object, parms, as follows:
int intRetVal = (int)parms.ReturnValue;