ExpressionVisitor Class

A base class for implementing a custom visitor to traverse the expression tree.

Namespace: DevExpress.Spreadsheet.Formulas

Assembly: DevExpress.Spreadsheet.v17.2.Core.dll

Declaration

C#

public abstract class ExpressionVisitor :
    IExpressionVisitor

VB.NET

Public MustInherit Class ExpressionVisitor
    Implements IExpressionVisitor

Remarks

This example demonstrates how to use the FormulaEngine.Parse method to parse an expression and create an expression tree. The expression tree is available by using the ParsedExpression.Expression property.

You can use an expression tree visitor to traverse an existing expression tree and to perform required actions when it visits a specific node. To implement a visitor, create a descendant from the ExpressionVisitor class and override the ExpressionVisitor.Visit method which corresponds to the node type you wish to visit.

In this code snippet, the MyVisitor visitor changes the range reference to shift it down by one row.

After the visitor finishes, the ParsedExpression.ToString method is used to assemble a valid formula from the expression tree.

Note

A complete sample project is available at https://github.com/DevExpress-Examples/formula-engine-examples-of-use-t110344

Form1.cs

using DevExpress.Spreadsheet;
using DevExpress.Spreadsheet.Formulas;
using DevExpress.Spreadsheet.Functions;
            FormulaEngine engine = spreadsheetControl1.Document.FormulaEngine;
            if (spreadsheetControl1.ActiveCell != null){
                // Get the formula for parsing. 
                string formula = spreadsheetControl1.ActiveCell.Formula;
                if (formula != string.Empty)
                {
                    // Parse the formula
                    ParsedExpression p_expr = engine.Parse(formula);
                    // Traverse the expression tree and modify it.
                    MyVisitor visitor = new MyVisitor();
                    p_expr.Expression.Visit(visitor);
                    // Reconsitute the formula.
                    string formula1 = p_expr.ToString();
                    // Place the formula back to the cell.
                    spreadsheetControl1.ActiveCell.Formula = formula1;
                }

MyVisitor.cs

public class MyVisitor : DevExpress.Spreadsheet.Formulas.ExpressionVisitor
{
    public override void Visit(DevExpress.Spreadsheet.Formulas.CellReferenceExpression expression)
    {
        base.Visit(expression);

        expression.CellArea.TopRowIndex++;
        expression.CellArea.BottomRowIndex++;
    }
}

MyVisitor.vb

Public Class MyVisitor
    Inherits DevExpress.Spreadsheet.Formulas.ExpressionVisitor

    Public Overrides Sub Visit(ByVal expression As DevExpress.Spreadsheet.Formulas.CellReferenceExpression)
        MyBase.Visit(expression)

        expression.CellArea.TopRowIndex += 1
        expression.CellArea.BottomRowIndex += 1
    End Sub
End Class

Form1.vb

Imports DevExpress.Spreadsheet
Imports DevExpress.Spreadsheet.Formulas
Imports DevExpress.Spreadsheet.Functions
            Dim engine As FormulaEngine = spreadsheetControl1.Document.FormulaEngine
            If spreadsheetControl1.ActiveCell IsNot Nothing Then
                ' Get the formula for parsing. 
                Dim formula As String = spreadsheetControl1.ActiveCell.Formula
                If formula <> String.Empty Then
                    ' Parse the formula
                    Dim p_expr As ParsedExpression = engine.Parse(formula)
                    ' Traverse the expression tree and modify it.
                    Dim visitor As New MyVisitor()
                    p_expr.Expression.Visit(visitor)
                    ' Reconsitute the formula.
                    Dim formula1 As String = p_expr.ToString()
                    ' Place the formula back to the cell.
                    spreadsheetControl1.ActiveCell.Formula = formula1
                End If

Related GitHub Examples

The following code snippet (auto-collected from DevExpress Examples) contains a reference to the ExpressionVisitor class.

Note

The algorithm used to collect these code examples remains a work in progress. Accordingly, the links and snippets below may produce inaccurate results. If you encounter an issue with code examples below, please use the feedback form on this page to report the issue.

C#

formula-engine-examples-of-use-t110344/CS/FormulaEngineTest/MyVisitor.cs#L4

#region #myvisitor
public class MyVisitor : DevExpress.Spreadsheet.Formulas.ExpressionVisitor
{

VB.NET

formula-engine-examples-of-use-t110344/VB/FormulaEngineTest/MyVisitor.vb#L4

Public Class MyVisitor
    Inherits DevExpress.Spreadsheet.Formulas.ExpressionVisitor

Inheritance

Object

ExpressionVisitor