Como: Implementar um visitante de árvore de expressões
O exemplo de código deste tópico é uma implementação de um visitante da árvore de expressão.Essa classe foi projetada para ser herdada para criar classes mais especializadas cuja funcionalidade requer atravessando, examinar ou copiando uma árvore de expressão.
Os tópicos a seguir usam esta classe:
Estes tópicos contêm código que cria uma subclasse de visitante especializado do árvore de expressão classe base visitante apresentado neste tópico.
Exemplo
Nessa expressão árvore visitante implementação, a Visit método, que deve ser chamado pela primeira vez, envia a expressão, que ele é passado para um dos métodos mais especializados visitante na classe, com base no tipo da expressão. Os métodos especializados visitante visite sub-árvore da expressão que são passados.Se uma subexpressão for alterado depois que ela visitou, por exemplo por um método de substituição em uma classe derivada, os métodos especializados visitante criam uma nova expressão que inclui as alterações na subárvore.Caso contrário, elas retornam a expressão que eles foram passados.Esse comportamento recursiva permite que uma nova árvore de expressão a ser criado que ou igual a ou uma versão modificada da expressão original que foi passado para 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 "Observação") Observação: |
|---|
Nessa implementação, o Visit método, que é o ponto de partida para visitar uma árvore de expressão, tem o protected (Protected no Visual Basic) modificador de acesso. Isso significa que, para que esse método acessível de fora da classe ou de suas classes derivadas, você deve criar um public (Public no Visual Basic) método que chama Visit. Por ter esse um public (Public no Visual Basic) método no seu visitante, o ponto de entrada é mais óbvio para os chamadores. |
Compilando o código
Adicione uma referência a sistema.Core.dll se não forem citado já em seu projeto.
Add using diretivas (ou Imports instruções no Visual Basic) para os espaços para nome sistema.Collections.Generic sistema.Collections.ObjectModel e sistema.Linq.Expressions.
Consulte também
Tarefas
Como: Modificar árvores de expressão
Demonstra Passo a passo: Criar um provedor de LINQ IQueryable