Cómo: Implementar un visitante de árbol de expresión
Actualización: noviembre 2007
El ejemplo de código que se incluye en este tema es una implementación de un visitante de árbol de expresión. Esta clase se ha diseñado para que se herede y se creen clases más especializadas cuya funcionalidad requiera el recorrido, el análisis o la copia de un árbol de expresión.
Los temas siguientes utilizan esta clase:
Estos dos temas contienen código que crea una subclase especializada de la clase base del visitante de árbol de expresión que se presenta en este tema.
Ejemplo
En esta implementación del visitante de árbol de expresión, el método Visit, al que debe llamarse en primer lugar, distribuye la expresión, que se pasa a uno o varios métodos de visitante más especializados de la clase, en función del tipo de expresión. Los métodos de visitante especializados visitan el subárbol de la expresión que les pasan. Si una subexpresión cambia una vez que se ha visitado, por ejemplo, por un método de invalidación de una clase derivada, los métodos de visitante especializados crean una nueva expresión que incluye los cambios en el subárbol. De lo contrario, devuelven la expresión que se les pasó. Este comportamiento recursivo permite generar un nuevo árbol de expresión que o bies es igual o bien es una versión modificada de la expresión original que se pasó a Visit.
Public MustInherit Class ExpressionVisitor
Protected Sub New()
End Sub
Protected Overridable Function Visit(ByVal exp As Expression) As Expression
If exp Is Nothing Then
Return exp
End If
Select Case exp.NodeType
Case ExpressionType.Negate, _
ExpressionType.NegateChecked, _
ExpressionType.Not, _
ExpressionType.Convert, _
ExpressionType.ConvertChecked, _
ExpressionType.ArrayLength, _
ExpressionType.Quote, _
ExpressionType.TypeAs
Return Me.VisitUnary(CType(exp, UnaryExpression))
Case ExpressionType.Add, _
ExpressionType.AddChecked, _
ExpressionType.Subtract, _
ExpressionType.SubtractChecked, _
ExpressionType.Multiply, _
ExpressionType.MultiplyChecked, _
ExpressionType.Divide, _
ExpressionType.Modulo, _
ExpressionType.And, _
ExpressionType.AndAlso, _
ExpressionType.Or, _
ExpressionType.OrElse, _
ExpressionType.LessThan, _
ExpressionType.LessThanOrEqual, _
ExpressionType.GreaterThan, _
ExpressionType.GreaterThanOrEqual, _
ExpressionType.Equal, _
ExpressionType.NotEqual, _
ExpressionType.Coalesce, _
ExpressionType.ArrayIndex, _
ExpressionType.RightShift, _
ExpressionType.LeftShift, _
ExpressionType.ExclusiveOr
Return Me.VisitBinary(CType(exp, BinaryExpression))
Case ExpressionType.TypeIs
Return Me.VisitTypeIs(CType(exp, TypeBinaryExpression))
Case ExpressionType.Conditional
Return Me.VisitConditional(CType(exp, ConditionalExpression))
Case ExpressionType.Constant
Return Me.VisitConstant(CType(exp, ConstantExpression))
Case ExpressionType.Parameter
Return Me.VisitParameter(CType(exp, ParameterExpression))
Case ExpressionType.MemberAccess
Return Me.VisitMemberAccess(CType(exp, MemberExpression))
Case ExpressionType.Call
Return Me.VisitMethodCall(CType(exp, MethodCallExpression))
Case ExpressionType.Lambda
Return Me.VisitLambda(CType(exp, LambdaExpression))
Case ExpressionType.New
Return Me.VisitNew(CType(exp, NewExpression))
Case ExpressionType.NewArrayInit, _
ExpressionType.NewArrayBounds
Return Me.VisitNewArray(CType(exp, NewArrayExpression))
Case ExpressionType.Invoke
Return Me.VisitInvocation(CType(exp, InvocationExpression))
Case ExpressionType.MemberInit
Return Me.VisitMemberInit(CType(exp, MemberInitExpression))
Case ExpressionType.ListInit
Return Me.VisitListInit(CType(exp, ListInitExpression))
Case Else
Throw New Exception("Unhandled expression type: '" & exp.NodeType & "'")
End Select
End Function
Protected Overridable Function VisitBinding(ByVal binding As MemberBinding) As MemberBinding
Select Case binding.BindingType
Case MemberBindingType.Assignment
Return Me.VisitMemberAssignment(CType(binding, MemberAssignment))
Case MemberBindingType.MemberBinding
Return Me.VisitMemberMemberBinding(CType(binding, MemberMemberBinding))
Case MemberBindingType.ListBinding
Return Me.VisitMemberListBinding(CType(binding, MemberListBinding))
Case Else
Throw New Exception("Unhandled binding type '" & binding.BindingType & "'")
End Select
End Function
Protected Overridable Function VisitElementInitializer(ByVal initializer As ElementInit) _
As ElementInit
Dim arguments = Me.VisitExpressionList(initializer.Arguments)
If arguments IsNot initializer.Arguments Then
Return Expression.ElementInit(initializer.AddMethod, arguments)
End If
Return initializer
End Function
Protected Overridable Function VisitUnary(ByVal u As UnaryExpression) As Expression
Dim operand = Me.Visit(u.Operand)
If operand IsNot u.Operand Then
Return Expression.MakeUnary(u.NodeType, operand, u.Type, u.Method)
End If
Return u
End Function
Protected Overridable Function VisitBinary(ByVal b As BinaryExpression) As Expression
Dim left = Me.Visit(b.Left)
Dim right = Me.Visit(b.Right)
Dim conversion = Me.Visit(b.Conversion)
If left IsNot b.Left Or right IsNot b.Right Or conversion IsNot b.Conversion Then
If b.NodeType = ExpressionType.Coalesce And b.Conversion IsNot Nothing Then
Return Expression.Coalesce(left, right, _
TryCast(conversion, LambdaExpression))
Else
Return Expression.MakeBinary(b.NodeType, left, right, _
b.IsLiftedToNull, b.Method)
End If
End If
Return b
End Function
Protected Overridable Function VisitTypeIs(ByVal b As TypeBinaryExpression) As Expression
Dim expr = Me.Visit(b.Expression)
If expr IsNot b.Expression Then
Return Expression.TypeIs(expr, b.TypeOperand)
End If
Return b
End Function
Protected Overridable Function VisitConstant(ByVal c As ConstantExpression) As Expression
Return c
End Function
Protected Overridable Function VisitConditional(ByVal c As ConditionalExpression) As Expression
Dim test = Me.Visit(c.Test)
Dim ifTrue = Me.Visit(c.IfTrue)
Dim ifFalse = Me.Visit(c.IfFalse)
If test IsNot c.Test Or ifTrue IsNot c.IfTrue Or ifFalse IsNot c.IfFalse Then
Return Expression.Condition(test, ifTrue, ifFalse)
End If
Return c
End Function
Protected Overridable Function VisitParameter(ByVal p As ParameterExpression) As Expression
Return p
End Function
Protected Overridable Function VisitMemberAccess(ByVal m As MemberExpression) As Expression
Dim exp = Me.Visit(m.Expression)
If exp IsNot m.Expression Then
Return Expression.MakeMemberAccess(exp, m.Member)
End If
Return m
End Function
Protected Overridable Function VisitMethodCall(ByVal m As MethodCallExpression) As Expression
Dim obj = Me.Visit(m.Object)
Dim args = Me.VisitExpressionList(m.Arguments)
If obj IsNot m.Object Or args IsNot m.Arguments Then
Return Expression.Call(obj, m.Method, args)
End If
Return m
End Function
Protected Overridable Function VisitExpressionList( _
ByVal original As ReadOnlyCollection(Of Expression)) As ReadOnlyCollection(Of Expression)
Dim list As List(Of Expression) = Nothing
Dim n = original.Count
For i = 0 To n - 1
Dim p = Me.Visit(original(i))
If list IsNot Nothing Then
list.Add(p)
ElseIf p IsNot original(i) Then
list = New List(Of Expression)(n)
For j = 0 To i - 1
list.Add(original(j))
Next j
list.Add(p)
End If
Next i
If list IsNot Nothing Then
Return list.AsReadOnly()
End If
Return original
End Function
Protected Overridable Function VisitMemberAssignment(ByVal assignment As MemberAssignment) _
As MemberAssignment
Dim e = Me.Visit(assignment.Expression)
If e IsNot assignment.Expression Then
Return Expression.Bind(assignment.Member, e)
End If
Return assignment
End Function
Protected Overridable Function VisitMemberMemberBinding(ByVal binding As MemberMemberBinding) _
As MemberMemberBinding
Dim bindings = Me.VisitBindingList(binding.Bindings)
If bindings IsNot binding.Bindings Then
Return Expression.MemberBind(binding.Member, bindings)
End If
Return binding
End Function
Protected Overridable Function VisitMemberListBinding(ByVal binding As MemberListBinding) _
As MemberListBinding
Dim initializers = Me.VisitElementInitializerList(binding.Initializers)
If initializers IsNot binding.Initializers Then
Return Expression.ListBind(binding.Member, initializers)
End If
Return binding
End Function
Protected Overridable Function VisitBindingList( _
ByVal original As ReadOnlyCollection(Of MemberBinding)) As IEnumerable(Of MemberBinding)
Dim list As List(Of MemberBinding) = Nothing
Dim n = original.Count
For i = 0 To n - 1
Dim b = Me.VisitBinding(original(i))
If list IsNot Nothing Then
list.Add(b)
ElseIf b IsNot original(i) Then
list = New List(Of MemberBinding)(n)
For j = 0 To i - 1
list.Add(original(j))
Next j
list.Add(b)
End If
Next i
If list IsNot Nothing Then
Return list
End If
Return original
End Function
Protected Overridable Function VisitElementInitializerList( _
ByVal original As ReadOnlyCollection(Of ElementInit)) As IEnumerable(Of ElementInit)
Dim list As List(Of ElementInit) = Nothing
Dim n = original.Count
For i = 0 To n - 1
Dim init = Me.VisitElementInitializer(original(i))
If list IsNot Nothing Then
list.Add(init)
ElseIf init IsNot original(i) Then
list = New List(Of ElementInit)(n)
For j = 0 To i - 1
list.Add(original(j))
Next j
list.Add(init)
End If
Next i
If list IsNot Nothing Then
Return list
End If
Return original
End Function
Protected Overridable Function VisitLambda(ByVal lambda As LambdaExpression) As Expression
Dim body = Me.Visit(lambda.Body)
If body IsNot lambda.Body Then
Return Expression.Lambda(lambda.Type, body, lambda.Parameters)
End If
Return lambda
End Function
Protected Overridable Function VisitNew(ByVal nex As NewExpression) As NewExpression
Dim args = Me.VisitExpressionList(nex.Arguments)
If args IsNot nex.Arguments Then
If nex.Members IsNot Nothing Then
Return Expression.[New](nex.Constructor, args, nex.Members)
Else
Return Expression.[New](nex.Constructor, args)
End If
End If
Return nex
End Function
Protected Overridable Function VisitMemberInit(ByVal init As MemberInitExpression) As Expression
Dim n = Me.VisitNew(init.NewExpression)
Dim bindings = Me.VisitBindingList(init.Bindings)
If n IsNot init.NewExpression Or bindings IsNot init.Bindings Then
Return Expression.MemberInit(n, bindings)
End If
Return init
End Function
Protected Overridable Function VisitListInit(ByVal init As ListInitExpression) As Expression
Dim n = Me.VisitNew(init.NewExpression)
Dim initializers = Me.VisitElementInitializerList(init.Initializers)
If n IsNot init.NewExpression Or initializers IsNot init.Initializers Then
Return Expression.ListInit(n, initializers)
End If
Return init
End Function
Protected Overridable Function VisitNewArray(ByVal na As NewArrayExpression) As Expression
Dim exprs = Me.VisitExpressionList(na.Expressions)
If exprs IsNot na.Expressions Then
If na.NodeType = ExpressionType.NewArrayInit Then
Return Expression.NewArrayInit(na.Type.GetElementType(), exprs)
Else
Return Expression.NewArrayBounds(na.Type.GetElementType(), exprs)
End If
End If
Return na
End Function
Protected Overridable Function VisitInvocation(ByVal iv As InvocationExpression) As Expression
Dim args = Me.VisitExpressionList(iv.Arguments)
Dim expr = Me.Visit(iv.Expression)
If args IsNot iv.Arguments Or expr IsNot iv.Expression Then
Return Expression.Invoke(expr, args)
End If
Return iv
End Function
End Class
public abstract class ExpressionVisitor
{
protected ExpressionVisitor()
{
}
protected virtual Expression Visit(Expression exp)
{
if (exp == null)
return exp;
switch (exp.NodeType)
{
case ExpressionType.Negate:
case ExpressionType.NegateChecked:
case ExpressionType.Not:
case ExpressionType.Convert:
case ExpressionType.ConvertChecked:
case ExpressionType.ArrayLength:
case ExpressionType.Quote:
case ExpressionType.TypeAs:
return this.VisitUnary((UnaryExpression)exp);
case ExpressionType.Add:
case ExpressionType.AddChecked:
case ExpressionType.Subtract:
case ExpressionType.SubtractChecked:
case ExpressionType.Multiply:
case ExpressionType.MultiplyChecked:
case ExpressionType.Divide:
case ExpressionType.Modulo:
case ExpressionType.And:
case ExpressionType.AndAlso:
case ExpressionType.Or:
case ExpressionType.OrElse:
case ExpressionType.LessThan:
case ExpressionType.LessThanOrEqual:
case ExpressionType.GreaterThan:
case ExpressionType.GreaterThanOrEqual:
case ExpressionType.Equal:
case ExpressionType.NotEqual:
case ExpressionType.Coalesce:
case ExpressionType.ArrayIndex:
case ExpressionType.RightShift:
case ExpressionType.LeftShift:
case ExpressionType.ExclusiveOr:
return this.VisitBinary((BinaryExpression)exp);
case ExpressionType.TypeIs:
return this.VisitTypeIs((TypeBinaryExpression)exp);
case ExpressionType.Conditional:
return this.VisitConditional((ConditionalExpression)exp);
case ExpressionType.Constant:
return this.VisitConstant((ConstantExpression)exp);
case ExpressionType.Parameter:
return this.VisitParameter((ParameterExpression)exp);
case ExpressionType.MemberAccess:
return this.VisitMemberAccess((MemberExpression)exp);
case ExpressionType.Call:
return this.VisitMethodCall((MethodCallExpression)exp);
case ExpressionType.Lambda:
return this.VisitLambda((LambdaExpression)exp);
case ExpressionType.New:
return this.VisitNew((NewExpression)exp);
case ExpressionType.NewArrayInit:
case ExpressionType.NewArrayBounds:
return this.VisitNewArray((NewArrayExpression)exp);
case ExpressionType.Invoke:
return this.VisitInvocation((InvocationExpression)exp);
case ExpressionType.MemberInit:
return this.VisitMemberInit((MemberInitExpression)exp);
case ExpressionType.ListInit:
return this.VisitListInit((ListInitExpression)exp);
default:
throw new Exception(string.Format("Unhandled expression type: '{0}'", exp.NodeType));
}
}
protected virtual MemberBinding VisitBinding(MemberBinding binding)
{
switch (binding.BindingType)
{
case MemberBindingType.Assignment:
return this.VisitMemberAssignment((MemberAssignment)binding);
case MemberBindingType.MemberBinding:
return this.VisitMemberMemberBinding((MemberMemberBinding)binding);
case MemberBindingType.ListBinding:
return this.VisitMemberListBinding((MemberListBinding)binding);
default:
throw new Exception(string.Format("Unhandled binding type '{0}'", binding.BindingType));
}
}
protected virtual ElementInit VisitElementInitializer(ElementInit initializer)
{
ReadOnlyCollection<Expression> arguments = this.VisitExpressionList(initializer.Arguments);
if (arguments != initializer.Arguments)
{
return Expression.ElementInit(initializer.AddMethod, arguments);
}
return initializer;
}
protected virtual Expression VisitUnary(UnaryExpression u)
{
Expression operand = this.Visit(u.Operand);
if (operand != u.Operand)
{
return Expression.MakeUnary(u.NodeType, operand, u.Type, u.Method);
}
return u;
}
protected virtual Expression VisitBinary(BinaryExpression b)
{
Expression left = this.Visit(b.Left);
Expression right = this.Visit(b.Right);
Expression conversion = this.Visit(b.Conversion);
if (left != b.Left || right != b.Right || conversion != b.Conversion)
{
if (b.NodeType == ExpressionType.Coalesce && b.Conversion != null)
return Expression.Coalesce(left, right, conversion as LambdaExpression);
else
return Expression.MakeBinary(b.NodeType, left, right, b.IsLiftedToNull, b.Method);
}
return b;
}
protected virtual Expression VisitTypeIs(TypeBinaryExpression b)
{
Expression expr = this.Visit(b.Expression);
if (expr != b.Expression)
{
return Expression.TypeIs(expr, b.TypeOperand);
}
return b;
}
protected virtual Expression VisitConstant(ConstantExpression c)
{
return c;
}
protected virtual Expression VisitConditional(ConditionalExpression c)
{
Expression test = this.Visit(c.Test);
Expression ifTrue = this.Visit(c.IfTrue);
Expression ifFalse = this.Visit(c.IfFalse);
if (test != c.Test || ifTrue != c.IfTrue || ifFalse != c.IfFalse)
{
return Expression.Condition(test, ifTrue, ifFalse);
}
return c;
}
protected virtual Expression VisitParameter(ParameterExpression p)
{
return p;
}
protected virtual Expression VisitMemberAccess(MemberExpression m)
{
Expression exp = this.Visit(m.Expression);
if (exp != m.Expression)
{
return Expression.MakeMemberAccess(exp, m.Member);
}
return m;
}
protected virtual Expression VisitMethodCall(MethodCallExpression m)
{
Expression obj = this.Visit(m.Object);
IEnumerable<Expression> args = this.VisitExpressionList(m.Arguments);
if (obj != m.Object || args != m.Arguments)
{
return Expression.Call(obj, m.Method, args);
}
return m;
}
protected virtual ReadOnlyCollection<Expression> VisitExpressionList(ReadOnlyCollection<Expression> original)
{
List<Expression> list = null;
for (int i = 0, n = original.Count; i < n; i++)
{
Expression p = this.Visit(original[i]);
if (list != null)
{
list.Add(p);
}
else if (p != original[i])
{
list = new List<Expression>(n);
for (int j = 0; j < i; j++)
{
list.Add(original[j]);
}
list.Add(p);
}
}
if (list != null)
{
return list.AsReadOnly();
}
return original;
}
protected virtual MemberAssignment VisitMemberAssignment(MemberAssignment assignment)
{
Expression e = this.Visit(assignment.Expression);
if (e != assignment.Expression)
{
return Expression.Bind(assignment.Member, e);
}
return assignment;
}
protected virtual MemberMemberBinding VisitMemberMemberBinding(MemberMemberBinding binding)
{
IEnumerable<MemberBinding> bindings = this.VisitBindingList(binding.Bindings);
if (bindings != binding.Bindings)
{
return Expression.MemberBind(binding.Member, bindings);
}
return binding;
}
protected virtual MemberListBinding VisitMemberListBinding(MemberListBinding binding)
{
IEnumerable<ElementInit> initializers = this.VisitElementInitializerList(binding.Initializers);
if (initializers != binding.Initializers)
{
return Expression.ListBind(binding.Member, initializers);
}
return binding;
}
protected virtual IEnumerable<MemberBinding> VisitBindingList(ReadOnlyCollection<MemberBinding> original)
{
List<MemberBinding> list = null;
for (int i = 0, n = original.Count; i < n; i++)
{
MemberBinding b = this.VisitBinding(original[i]);
if (list != null)
{
list.Add(b);
}
else if (b != original[i])
{
list = new List<MemberBinding>(n);
for (int j = 0; j < i; j++)
{
list.Add(original[j]);
}
list.Add(b);
}
}
if (list != null)
return list;
return original;
}
protected virtual IEnumerable<ElementInit> VisitElementInitializerList(ReadOnlyCollection<ElementInit> original)
{
List<ElementInit> list = null;
for (int i = 0, n = original.Count; i < n; i++)
{
ElementInit init = this.VisitElementInitializer(original[i]);
if (list != null)
{
list.Add(init);
}
else if (init != original[i])
{
list = new List<ElementInit>(n);
for (int j = 0; j < i; j++)
{
list.Add(original[j]);
}
list.Add(init);
}
}
if (list != null)
return list;
return original;
}
protected virtual Expression VisitLambda(LambdaExpression lambda)
{
Expression body = this.Visit(lambda.Body);
if (body != lambda.Body)
{
return Expression.Lambda(lambda.Type, body, lambda.Parameters);
}
return lambda;
}
protected virtual NewExpression VisitNew(NewExpression nex)
{
IEnumerable<Expression> args = this.VisitExpressionList(nex.Arguments);
if (args != nex.Arguments)
{
if (nex.Members != null)
return Expression.New(nex.Constructor, args, nex.Members);
else
return Expression.New(nex.Constructor, args);
}
return nex;
}
protected virtual Expression VisitMemberInit(MemberInitExpression init)
{
NewExpression n = this.VisitNew(init.NewExpression);
IEnumerable<MemberBinding> bindings = this.VisitBindingList(init.Bindings);
if (n != init.NewExpression || bindings != init.Bindings)
{
return Expression.MemberInit(n, bindings);
}
return init;
}
protected virtual Expression VisitListInit(ListInitExpression init)
{
NewExpression n = this.VisitNew(init.NewExpression);
IEnumerable<ElementInit> initializers = this.VisitElementInitializerList(init.Initializers);
if (n != init.NewExpression || initializers != init.Initializers)
{
return Expression.ListInit(n, initializers);
}
return init;
}
protected virtual Expression VisitNewArray(NewArrayExpression na)
{
IEnumerable<Expression> exprs = this.VisitExpressionList(na.Expressions);
if (exprs != na.Expressions)
{
if (na.NodeType == ExpressionType.NewArrayInit)
{
return Expression.NewArrayInit(na.Type.GetElementType(), exprs);
}
else
{
return Expression.NewArrayBounds(na.Type.GetElementType(), exprs);
}
}
return na;
}
protected virtual Expression VisitInvocation(InvocationExpression iv)
{
IEnumerable<Expression> args = this.VisitExpressionList(iv.Arguments);
Expression expr = this.Visit(iv.Expression);
if (args != iv.Arguments || expr != iv.Expression)
{
return Expression.Invoke(expr, args);
}
return iv;
}
}
.gif "Nota") Nota: |
|---|
En esta implementación, el método Visit, que es el punto inicial para visitar un árbol de expresión, tiene el modificador de acceso protected (Protected en Visual Basic). Esto significa que para que este método sea accesible desde fuera de la clase o sus clases derivadas, es necesario crear un método public (Public en Visual Basic) que llame a Visit. Al tener este método public (Public en Visual Basic) en el visitante, el punto de entrada resulta más obvio para los llamadores. |
Compilar el código
Agregue una referencia a System.Core.dll si ésta aún no existe en el proyecto.
Agregue directivas using (o instrucciones Imports en Visual Basic) para los espacios de nombres System.Collections.Generic, System.Collections.ObjectModel y System.Linq.Expressions.
Vea también
Tareas
Cómo: Modificar árboles de expresiones
Tutorial: Crear un proveedor LINQ IQueryable