IExpressionEval::EvalInContext
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Use this method to evaluate an expression against a pre-established context stored in the context cache.
Definition
HRESULT IExpressionEval::EvalInContext(longnExprID,IExprEvalContext*pContext,VARIANT* pvResult);
Parameters
nExprID
[in] A long that contains the unique identification of the expression in the expression store.
pContext
[in] A pointer to an IExprEvalContext interface of an ExprEvalContext object that identifies the specific context for this evaluation.
pvResult
[out, retval] A pointer to a VARIANT indicating the result of the expression evaluated in the specified context.
Return Values
This method returns an HRESULT indicating whether it completed successfully. See the Error Values section for more details. The result of evaluating the expression is contained in the pvResult parameter. This result may be TRUE, FALSE, or NULL. NULL indicates the expression could not be evaluated.
Error Values
This method returns S_OK (0x00000000) to indicate success and either standard or custom COM HRESULT error values to indicate failure. For more information about standard COM errors, see Standard COM Errors.
The following table shows the custom COM errors, and the standard COM errors with special meanings, that this method can return.
| Constant | Value | Description |
| E_EXPR_BADCONNECTION | 0x8110001F | Invalid connection |
| E_EXPR_BADEXPRID | 0x8110000A | Invalid expression ID specified |
| E_EXPR_NOCONNECTIONSTRING | 0x8110000D | No connection string specified |
| E_EXPR_MISSINGPROP | 0x8110001D | Invalid ExprEvalContext object |
| E_EXPR_NOTFOUND | 0x81100012 | Could not find the specified expression ID, nExprID, in the expression store |
Additional information may be available using the global Err object, which can be accessed using the API function GetErrorInfo. In particular, the GetDescription method of the IErrorInfo interface may return a text description of the error.
Remarks
The Eval method creates and discards an evaluation context for every evaluation. Evaluating against a stored context optimizes performance when multiple expressions need to be evaluated in the same context. Create the context for evaluation using the CreateEvalContext method.
Expression results are also stored in the context cache. If the result of a particular expression is requested multiple times from the same ExprEvalContext object using the EvalInContext method, the result is retrieved from the cache rather than by re-evaluating the expression, further increasing efficiency.
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Caution
- The state of the profile objects that comprise the context should not be externally changed during the lifetime of the ExprEvalContext object; doing so leads to non-deterministic evaluation results.
The Connect method must be called before using the EvalInContext method.
For more information about memory ownership issues related to COM property and method parameters, see Managing COM Parameter Memory.
The pvResult parameter contains valid data only if the method completes successfully.