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How to: Modify Expression Trees (C# and Visual Basic)

This topic shows you how to modify an expression tree. Expression trees are immutable, which means that they cannot be modified directly. To change an expression tree, you must create a copy of an existing expression tree and when you create the copy, make the required changes. You can use the ExpressionVisitor class to traverse an existing expression tree and to copy each node that it visits.


You can find the source code for the ExpressionVisitor class on the CodePlex Web site.

To modify an expression tree

  1. In Visual Studio, create a new Console Application project.

  2. Add the AndAlsoModifier class to your project.

    This class inherits the ExpressionVisitor class and is specialized to modify expressions that represent conditional AND operations. It changes these operations from a conditional AND to a conditional OR. To do this, the class overrides the VisitBinary method of the base type, because conditional AND expressions are represented as binary expressions. In the VisitBinary method, if the expression that is passed to it represents a conditional AND operation, the code constructs a new expression that contains the conditional OR operator instead of the conditional AND operator. If the expression that is passed to VisitBinary does not represent a conditional AND operation, the method defers to the base class implementation. The base class methods construct nodes that are like the expression trees that are passed in, but the nodes have their sub trees replaced with the expression trees that are produced recursively by the visitor.

    Add a using directive (or Imports statement in Visual Basic) to the file for the System.Linq.Expressions namespace.

    Public Class AndAlsoModifier
        Inherits ExpressionVisitor
        Public Function Modify(ByVal expr As Expression) As Expression
            Return Visit(expr)
        End Function
        Protected Overrides Function VisitBinary(ByVal b As BinaryExpression) As Expression
            If b.NodeType = ExpressionType.AndAlso Then
                Dim left = Me.Visit(b.Left)
                Dim right = Me.Visit(b.Right)
                ' Make this binary expression an OrElse operation instead 
                ' of an AndAlso operation.
                Return Expression.MakeBinary(ExpressionType.OrElse, left, right, _
                                             b.IsLiftedToNull, b.Method)
            End If
            Return MyBase.VisitBinary(b)
        End Function
    End Class
  3. Add code to the Main method in the Program.cs (Module1.vb in Visual Basic) file to create an expression tree and pass it to the method that will modify it.

    The following code creates an expression that contains a conditional AND operation. It then creates an instance of the AndAlsoModifier class and passes the expression to the Modify method of this class. Both the original and the modified expression trees are outputted to show the change.

    Add a using directive (or Imports statement in Visual Basic) to the file for the System.Linq.Expressions namespace.

            Dim expr As Expression(Of Func(Of String, Boolean)) = _
                Function(name) name.Length > 10 AndAlso name.StartsWith("G")
            Dim modifier As New AndAlsoModifier()
            Dim modifiedExpr = modifier.Modify(CType(expr, Expression))
            ' This code produces the following output:
            ' name => ((name.Length > 10) && name.StartsWith("G"))
            ' name => ((name.Length > 10) || name.StartsWith("G"))
  4. Compile and run the application.