You can use a function in an expression to perform actions on data in data regions, groups and datasets. You can access these functions in the Expression Editor dialog. In any property that accepts expressions, you can drop down the property and select <Expression...> to open the dialog.
Within the Expression Editor dialog, there is a tree view of Fields. Expand the Common Functions node to view the available functions. The following tables contain details about each of the functions included in ActiveReports for use in property expressions.
Date & Time
These are all methods from the DateAndTime class in Visual Basic. Please see the msdn DateAndTime Class topic for information on overloads for each method.


Math
These are all methods and fields from the System.Math class. Please see the msdn Math Class topic for information on overloads for each method.



Inspection
These are all methods from the Information class in Visual Basic. Please see the msdn Information Class topic for more information.


Program Flow
These are all methods from the Interaction class in Visual Basic. Please see the msdn Interaction Class topic for more information.


Aggregate
You can use aggregate functions within report control value expressions to accrue data. ActiveReports supports aggregate functions from RDL 2005, plus some proprietary extended set of functions. For all of the functions, you can add an optional <Scope> parameter.
These are all the available aggregate functions:
Function  Description  Example 

AggregateIf  Decides whether to calculate a custom aggregate from the data provider of the values returned by the expression based on a Boolean expression.  =AggregateIf(Fields!Discontinued.Value=True, Sum, Fields!InStock.Value) 
Avg  Calculates the average of the nonnull values returned by the expression.  =Avg(Fields!Cost.Value, Nothing) 
Count  Calculates the number of nonnull values returned by the expression.  =Count(Fields!EmployeeID.Value, Nothing) 
CountDistinct  Calculates the number of nonrepeated values returned by the expression.  =CountDistinct(Fields!ManagerID.Value, "Department") 
CountRows  Calculates the number of rows in the scope returned by the expression.  =CountRows("Department") 
CumulativeTotal  Calculates the sum of pagelevel aggregates returned by the expression for current and previous pages.  =CumulativeTotal(Fields!OrderID.Value, Count) 
DistinctSum  Calculates the sum of the values returned by an expression using only the rows when the value of another expression is not repeated.  =DistinctSum(Fields!OrderID.Value, Fields!OrderFreight.Value, "Order") 
First  Shows the first value returned by the expression.  =First(Fields!ProductNumber.Value, "Category") 
Last  Shows the last value returned by the expression.  =Last(Fields!ProductNumber.Value, "Category") 
Max  Shows the largest nonnull value returned by the expression.  =Max(Fields!OrderTotal.Value, "Year") 
Median  Shows the value that is the midpoint of the values returned by the expression. Half of the values returned will be above this value and half will be below it.  =Median(Fields!OrderTotal.Value) 
Min  Shows the smallest nonnull value returned by the expression  =Min(Fields!OrderTotal.Value) 
Mode  Shows the value that appears most frequently in the values returned by the expression.  =Mode(Fields!OrderTotal.Value) 
RunningValue  Shows a running aggregate of values returned by the expression (Takes one of the other aggregate functions as a parameter),  =RunningValue(Fields!Cost.Value, Sum, Nothing) 
StDev  Calculates the dispersion (standard deviation) of all nonnull values returned by the expression.  =StDev(Fields!LineTotal.Value, "Order") 
StDevP  Calculates the population dispersion (population standard deviation) of all nonnull values returned by the expression.  =StDevP(Fields!LineTotal.Value, "Order") 
Sum  Calculates the sum of the values returned by the expression.  =Sum(Fields!LineTotal.Value, "Order") 
Var  Calculates the variance (standard deviation squared) of all nonnull values returned by the expression.  =Var(Fields!LineTotal.Value, "Order") 
VarP  Calculates the population variance (population standard deviation squared) of all nonnull values returned by the expression.  =VarP(Fields!LineTotal.Value, "Order") 
Conversion
These are all methods from the Convert class in the .NET Framework. Please see the msdn Convert Class topic for more information.


Miscellaneous
ActiveReports also offers several functions which do not aggregate data, but which you can use with an IIf function to help determine which data to display or how to display it.
The first four are miscellaneous functions from the RDL 2005 specifications. Please see the msdn Builtin Functions for ReportViewer Reports topic for more information. GetFields is a proprietary function to extend RDL specifications.
Function  Description  Syntax and Example  

InScope  Determines whether the current value is in the indicated scope. 
=InScope(<Scope>) 

Level  Returns the level of the current value in a recursive hierarchy. 
=Level(optional <Scope>) 

Previous  Returns the previous value within the indicated scope. 
=Previous(<Expression>) 

RowNumber  Shows a running count of all the rows in the scope returned by the expression. 
=RowNumber(optional <Scope>) 

GetFields 
Returns an IDictionary<string,Field> object that contains the current contents of the Fields collection. Only valid when used within a data region. This function makes it easier to write code that deals with complex conditionals. To write the equivalent function without GetFields() would require passing each of the queried field values into the method which could be prohibitive when dealing with many fields. 
=GetFields()


Lookup  Returns the first matching value for the specified name from the dataset with pairs of name and value. More pair ore information, see Report Builder Functions  Lookup Function. 
=Lookup(<SourceExpression>, <DestinationExpression>, <ResultExpression>, <LookupDataset>) =Lookup(Fields!ProductID.Value, Fields!ProductID.Value, Fields!Quantity.Value, "DataSet2") 

MapPoint  Allows displaying simple data directly on the Map as a map Point Layer.  =MapPoint(<Latitude>, <Longitude>) 
All functions have a Scope parameter which determines the grouping, data region, or dataset to be considered when calculating the aggregate or other function. Within a data region, the Scope parameter's default value is the innermost grouping to which the report control belongs. Alternately, you can specify the name of another grouping, dataset, or data region, or you can specify Nothing, which sets it to the outermost data region to which the report control belongs.
The Scope parameter must be a data region, grouping, or dataset that directly or indirectly contains the report control using the function in its expression. If the report control is outside of a data region, the Scope parameter refers to a dataset. If there is only one dataset in the report, you can omit the Scope parameter. If there are multiple datasets, you must specify which one to use to avoid ambiguity.