Gewusst wie: Implementieren eines Ausdrucksbaumstruktur-Besuchers
Aktualisiert: November 2007
Das Codebeispiel in diesem Thema ist eine Implementierung eines Ausdrucksbaumstruktur-Besuchers. Diese Klasse wurde für die Vererbung konzipiert, um spezialisiertere Klassen zu erstellen, deren Funktionalität das Durchlaufen, Prüfen oder Kopieren einer Ausdrucksbaumstruktur erfordert.
In den folgenden Themen wird diese Klasse verwendet:
Beide Themen enthalten Code, der eine spezialisierte Besucherunterklasse der in diesem Thema erläuterten Basisklasse des Ausdrucksbaumstruktur-Besuchers erstellt.
Beispiel
In dieser Implementierung des Ausdrucksbaumstruktur-Besuchers leitet die Visit-Methode, die zuerst aufgerufen werden sollte, den Ausdruck je nach Typ des Ausdrucks an eine der verschiedenen speziellen Besuchermethoden in der Klasse weiter. Die spezialisierten Besuchermethoden rufen die Teilstruktur des Ausdrucks auf, an den sie übergeben werden. Wenn sich ein Teilausdruck nach dem Aufruf ändert, z. B. durch eine überschreibende Methode in einer abgeleiteten Klasse, erstellt die spezialisierte Besuchermethode einen neuen Ausdruck, der die Änderungen in der Teilstruktur enthält. Andernfalls geben sie den Ausdruck zurück, an den sie übergeben wurden. Dieses rekursive Verhalten ermöglicht die Erstellung einer neuen Ausdrucksbaumstruktur, die dem ursprünglichen Ausdruck oder einer modifizierten Version des ursprünglichen Ausdrucks entspricht, der an Visit übergeben wurde.
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 "Hinweis") Hinweis: |
|---|
In dieser Implementierung verfügt die Visit-Methode, die der Ausgangspunkt für den Aufruf einer Ausdrucksbaumstruktur ist, über den protected-Zugriffsmodifizierer (Protected in Visual Basic). Das bedeutet, dass Sie eine public-Methode (Public in Visual Basic) erstellen müssen, die Visit aufruft, damit von außerhalb der Klasse oder der abgeleiteten Klassen auf diese Methode zugegriffen werden kann. Durch die Aufnahme dieser public-Methode (Public in Visual Basic) in Ihren Besucher ist der Einstiegspunkt für Aufrufer offensichtlicher. |
Kompilieren des Codes
Fügen Sie einen Verweis auf System.Core.dll hinzu, wenn dieser Verweis in Ihrem Projekt noch nicht vorhanden ist.
Fügen Sie using-Direktiven (oder Imports-Anweisungen in Visual Basic) für die Namespaces System.Collections.Generic, System.Collections.ObjectModel und System.Linq.Expressions hinzu.
Siehe auch
Aufgaben
Gewusst wie: Ändern von Ausdrucksbaumstrukturen
Exemplarische Vorgehensweise: Erstellen eines IQueryable-LINQ-Anbieters