# How to Add Custom Calculations to Expression Fields

27 Mar 20183 minutes to read

Essential Grouping lets you add your own functions to a function library which can be used in an expression field. In this manner, you can do custom calculations in expressions. This is a two-step process which is given below:

1. Register the function name and a delegate with the grouping engine.
2. Implement a method that does your calculation.
In the code given below, we add a method named ‘New_Function’ that takes two arguments and performs a certain calculation on those arguments.
``````// Step 1
// Add function named New_Function that uses a delegate named ComputeFunction to define a custom calculation.
ExpressionFieldEvaluator evaluator = this.groupingEngine.TableDescriptor.ExpressionFieldEvaluator;

// Sample usage in an Expression column.
this.groupingEngine.TableDescriptor.ExpressionFields["test"].Expression = "New_Function([Col1], [Col2])";

//...

// Step 2
// Define ComputeFunction that returns the absolute value of the 1st arg minus 2 * the 2nd arg.
public string ComputeFunction(string s)
{
// Get the list delimiter (for en-us, it is a comma).
char comma = Convert.ToChar(this.gridGroupingControl1.Culture.TextInfo.ListSeparator);
string[] ss = s.Split(comma);
if(ss.GetLength(0) != 2)
throw new ArgumentException("Requires 2 arguments.");

double arg1, arg2;
if(double.TryParse(ss[0], System.Globalization.NumberStyles.Any, null, out arg1)
&& double.TryParse(ss[1], System.Globalization.NumberStyles.Any, null, out arg2))
{
return Math.Abs(arg1 - 2 * arg2).ToString();
}
return "";
}``````
``````' Step 1
' Add function named New_Function that uses a delegate named ComputeFunction to define a custom calculation.
Dim evaluator As ExpressionFieldEvaluator = Me.groupingEngine.TableDescriptor.ExpressionFieldEvaluator

' Sample usage in an Expression column.
Me.groupingEngine.TableDescriptor.ExpressionFields("test").Expression = "New_Function([Col1], [Col2])"

' Step 2
' Define ComputeFunction that returns the absolute value of the 1st arg minus 2 * the 2nd arg.
Public Function ComputeFunction(ByVal s As String) As String

' Get the list delimiter (for en-us, it is a comma).
Dim comma As Char = Convert.ToChar(Me.gridGroupingControl1.Culture.TextInfo.ListSeparator)
Dim ss As String() = s.Split(comma)
If ss.GetLength(0) <> 2 Then
Throw New ArgumentException("Requires 2 arguments.")
End If
Dim arg1, arg2 As Double
If Double.TryParse(ss(0), System.Globalization.NumberStyles.Any, Nothing, arg1) AndAlso _
Double.TryParse(ss(1), System.Globalization.NumberStyles.Any, Nothing, arg2) Then
Return Math.Abs((arg1 - 2 * arg2)).ToString()
End If
Return ""

' ComputeFunction
End Function``````