Release 10.1A: OpenEdge Development:
.NET Open Clients

Unknown value (?)

Unknown value (?) is mapped to a null value for a .NET reference type (class); however, the .NET value types (System.Boolean, System.Decimal, System.Int32, System.Int64, and System.DateTime) always have a default value, and the null value does not belong to the set of values they support. For each of these value types, OpenEdge provides a holder class, which can be set to a null value, in the Progress.Open4GL namespace. In ProxyGen, if you specify that one of these parameters can accept the Unknown value (?), the method is defined with the corresponding holder class; otherwise the parameter is defined using the .NET value type.

This is an example of a C# proxy method signature that does not support Unknown value (?) for the first and third parameters:

foo(int i, string s, bool b)

This is an example of a C# proxy method that allows all the parameter values to have Unknown value (?):

foo(IntHolder i, string s, BooleanHolder b)

Holder classes

For each Progress data type that maps to a .NET value type, there is a Holder class. These Holder classes belong to the Progress.Open4GL namespace and extend the Progress.Open4GL.Holder class, shown in Table 4–3.

Table 4–3: Holder class definitions 

Class	Constructors	Property
BooleanHolder	BooleanHolder() BooleanHolder(bool value)	System.Boolean    BooleanValue;
DateHolder	DateHolder() DateHolder(System.DateTime value)	System.DateTime    DateValue;
DecimalHolder	DecimalHolder(decimal value) DecimalHolder(double value)	System.Decimal    DecimalValue;
IntHolder	IntHolder() IntHolder(int value)	System.Int32 IntValue;
LongHolder	LongHolder() LongHolder(long value)	System.Int64 LongValue;

Table 4–4 shows 4GL-to-.NET value-type mappings for parameters that have a default mapping that cannot accept Unknown value (?). These 4GL types are mapped to classes in the Progress.Open4GL namespace, as shown in the table.

Table 4–4: Progress 4GL-to-.NET value-type mappings

Progress 4GL	.NET
DATE DATETIME DATETIME-TZ	Progress.Open4GL.DateHolder
DECIMAL	Progress.Open4GL.DecimalHolder
INTEGER	Progress.Open4GL.IntHolder
LOGICAL	Progress.Open4GL.BooleanHolder
RECID	Progress.Open4GL.LongHolder

Setting an INPUT or INPUT/OUTPUT parameter to unknown

For INPUT or INPUT-OUTPUT parameters, you can set an input value to the Unknown value (?) by setting Value=null on the holder object.

For example, in C#, to set the value of a holder object to unknown, use the Value property:

Holder.Value=null

Testing an INPUT/OUTPUT or OUTPUT parameter for unknown

For OUTPUT and INPUT-OUTPUT parameters, you can find out whether an unknown output value is returned by testing if the IsNull property is true or the Value property on the holder object equals null.

For example, in C#, to test the value of a holder object, check the isNULL or Value property:

if (Holder.isNull)         — OR — if (Holder.Value == null)