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How to: Modify Expression Trees (Visual Basic)

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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.

To modify an expression tree

  1. Create a new Console Application project.

  2. Add an Imports statement to the file for the System.Linq.Expressions namespace.

  3. Add the AndAlsoModifier class to your project.

    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

    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.

  4. Add an Imports statement to the file for the System.Linq.Expressions namespace.

  5. Add code to the Main method in the Module1.vb file to create an expression tree and pass it to the method that will modify it.

    Dim expr As Expression(Of Func(Of String, Boolean)) = _
    Function(name) name.Length > 10 AndAlso name.StartsWith("G")

Console.WriteLine(expr)

Dim modifier As New AndAlsoModifier()
Dim modifiedExpr = modifier.Modify(CType(expr, Expression))

Console.WriteLine(modifiedExpr)

' This code produces the following output:
' name => ((name.Length > 10) && name.StartsWith("G"))
' name => ((name.Length > 10) || name.StartsWith("G"))

    The 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.

  6. Compile and run the application.

See also