Evaluate

Runs a compiled expression and returns its current value.

Syntax: expression.Evaluate()

expression
required An object expression that evaluates to an ITbExpression, typically the value returned by Compile.

The return value is a Variant holding the result of the expression. Its subtype reflects the natural type of the value — for example, Double for a numeric expression, String for a text-producing one, Boolean for a comparison.

Each call re-runs the expression against the current state of its bindings. If the bound objects expose properties whose values can change between calls — host application state, the current row of a recordset, configurable parameters — evaluating the same compiled expression twice may legitimately return different values.

A run-time error raised inside the expression — division by zero, type mismatch, an invalid call into a bound object — propagates out of Evaluate like any other run-time error.

Note BETA 1042

Evaluate raises -2147467259 for any error inside the expression: the error’s own number and description are lost. "1 / 0" raises -2147467259, not 11, and a Scripting.Dictionary’s Add of a key it holds raises -2147467259, not 457.

Note BETA 1042

In a built program, the first error the expression service raises has the description Automation error, and every later one Unspecified error. In a run from the IDE, every one has the description Automation error. An error of a bound object’s member has the description Automation error in both.

Common Errors

Error When
-2147467259 (&H80004005), Unspecified error evaluating the expression raises an error, whatever its number: division by zero, a type mismatch, an argument a function refuses
the expression calls a member of a bound object that the object does not have, or with the wrong number of arguments
-2147467259 (&H80004005), Automation error a member of a bound object that the expression calls raises an error, whatever its number

Example

This example compiles an expression that references a property on the host object via AddCustomBinderObject, then evaluates it twice with the property having different values.

' In the host class:
Public Counter As Long

Public Sub EvaluateTwice()
    Dim Service As TbExpressionService = New TbExpressionService
    Service.AddStdLibraryBinder()
    Service.AddCustomBinderObject "State", Me, IsAppObject

    Dim Expr As ITbExpression = Service.Compile("Counter * 2")

    Me.Counter = 1 : Debug.Print Expr.Evaluate()    ' 2
    Me.Counter = 5 : Debug.Print Expr.Evaluate()    ' 10
End Sub

See Also