Compile

Parses an expression string and returns it as a compiled ITbExpression.

Syntax: service.Compile( expression )

service
required An object expression that evaluates to a TbExpressionService object.
expression
required A String containing a twinBASIC-syntax expression — for example, "Sqr(2) + 1" or "UCase(FirstName) & "" "" & UCase(LastName)".

The return value is an ITbExpression. Calling Evaluate on it runs the expression and produces the current value; the same instance can be evaluated as many times as needed.

Symbols referenced in expression — function names, object members, properties — are resolved against the binders registered with service at the time of the call. Without a binder only an expression that names nothing compiles, such as "1 + 1"; the most common starting point is AddStdLibraryBinder, which exposes the standard runtime library.

Compilation is the relatively expensive step; evaluation reuses the compiled form. When a piece of source text will be used for repeated evaluation — a formula column refreshed every row, a watch expression sampled in a debugger — compile it once and keep the ITbExpression around.

An expression that compiles can still fail when it is evaluated: "1 / 0" compiles, and Evaluate raises the error.

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.

Common Errors

Error When
-2147467259 (&H80004005), Unspecified error expression is a zero-length string or is not an expression
expression names something that no registered binder resolves (any name, when no binder is registered), or calls a function with the wrong number of arguments
94 (&H5E), Invalid use of Null expression is Null

Example 

Dim Service As TbExpressionService = New TbExpressionService
Service.AddStdLibraryBinder()

Dim Square As ITbExpression = Service.Compile("Sqr(2)")
Debug.Print Square.Evaluate()    ' 1.4142135623731
Debug.Print Square.Evaluate()    ' Same compiled instance, evaluated again.

See Also