C# 在Linq to实体和Linq to对象之间共享表达式
我试图在LINQtoEntities调用和其他一些代码之间“共享”一组条件，以减少两个调用之间可能存在的条件不匹配 我首先声明了我的条件：C# 在Linq to实体和Linq to对象之间共享表达式,c#,linq-to-entities,linq-to-objects,func,C#,Linq To Entities,Linq To Objects,Func,我试图在LINQtoEntities调用和其他一些代码之间“共享”一组条件，以减少两个调用之间可能存在的条件不匹配 我首先声明了我的条件： private Func<DateTime, Status, bool> _submissionDateExpiredCondition = (submissionDate, status) => submissionDate < DateTime.Now && status == Status.OK; privat
private Func<DateTime, Status, bool> _submissionDateExpiredCondition =
(submissionDate, status) => submissionDate < DateTime.Now && status == Status.OK;
private Func<DateTime, Status, bool> _submissionDateWithinOneWeekCondition =
(submissionDate, status) => DateTime.Now < DbFunctions.AddDays(submissionDate, -7) && status == Status.Pending;
private Func<DateTime?, Status, bool> _bidValidityEndPeriodWithinThirtyDaysCondition =
(bidValidityEndPeriod, status) => bidValidityEndPeriod.HasValue && DateTime.Now < DbFunctions.AddDays(bidValidityEndPeriod.Value, -30) && (status == Status.OK);
private void应用程序条件（列表项）{
foreach（项目中的var x）{
if（_submissionDateExpiredCondition（x.Data.Timestamp，x.Data.Status））{
x、 Property=“条件1”；
}
否则如果（_submissiondatewithinneweekcondition（x.Data.Timestamp，x.Data.Status））
{
x、 Property=“条件2”；
}
else if（_bidvalidity和period，在第三天条件下（x.Data.ValidityStamp，x.Data.Status））
{
x、 Property=“条件3”；
}
}
}
LINQ to实体中不支持LINQ表达式节点类型“Invoke”。执行存储库查询时 我曾尝试使用谓词生成器（如所示）构建谓词，但没有成功
有人能给我指出正确的方向吗？晚会迟到了，但有人可能会发现我解决这个问题的方法很有用。 但是，如果没有一些表达式操作，就很难实现 主要问题是：在
.Where调用表达式（即编译代码）。EF无法找出委托的逻辑是什么，因此无法将其转换为SQL。这就是异常产生的地方
目标是得到一个.Where
谓词lambda表达式，该表达式在逻辑上与您的表达式等价，但EF可以理解。这意味着我们必须从
Expression<Func<EntityType, bool>> xPredicate = x =>
x.Property == "value" &&
x.Data.AnotherProperty == true &&
_submissionDateExpiredCondition(x.Data.Timestamp, x.Data.Status)
|| ...;
，其中，EntityType
是由Find
返回的可查询元素类型，该元素类型不同于MyCustomObject
请注意，委托的调用被其定义表达式（lambda body）替换，其中（lambda）参数submissionDate
和status
被调用的各个参数表达式替换
如果将条件定义为委托，则它们的内部逻辑将在编译代码中丢失，因此我们必须从lambda表达式而不是委托开始：
private Expression<Func<DateTime, Status, bool>> _xSubmissionDateExpiredCondition = (submissionDate, status) => submissionDate < DateTime.Now && status == Status.OK;
// getting the delegate as before (to be used in ApplyConditions) is trivial:
private Func<DateTime, Status, bool> _submissionDateExpiredCondition = _xSubmissionDateExpiredCondition.Compile();
// ... other conditions here
MAGIC
的作用是：查找委托的InvocationExpression
，该委托是对lambda表达式进行Compile（）
方法调用的结果，并将其替换为lambda的主体，但确保将主体中的lambda参数替换为调用的参数表达式
这里是我的实现。实际上，MAGIC
被称为Express.Prepare
在这里，这稍微不太具体
/// <summary>
/// Helps in building expressions.
/// </summary>
public static class Express
{
#region Prepare
/// <summary>
/// Prepares an expression to be used in queryables.
/// </summary>
/// <returns>The modified expression.</returns>
/// <remarks>
/// The method replaces occurrences of <see cref="LambdaExpression"/>.Compile().Invoke(...) with the body of the lambda, with it's parameters replaced by the arguments of the invocation.
/// Values are resolved by evaluating properties and fields only.
/// </remarks>
public static Expression<TDelegate> Prepare<TDelegate>(this Expression<TDelegate> lambda) => (Expression<TDelegate>)new PrepareVisitor().Visit(lambda);
/// <summary>
/// Wrapper for <see cref="Prepare{TDelegate}(Expression{TDelegate})"/>.
/// </summary>
public static Expression<Func<T1, TResult>> Prepare<T1, TResult>(Expression<Func<T1, TResult>> lambda) => lambda.Prepare();
/// <summary>
/// Wrapper for <see cref="Prepare{TDelegate}(Expression{TDelegate})"/>.
/// </summary>
public static Expression<Func<T1, T2, TResult>> Prepare<T1, T2, TResult>(Expression<Func<T1, T2, TResult>> lambda) => lambda.Prepare();
// NOTE: more overloads of Prepare here.
#endregion
/// <summary>
/// Evaluate an expression to a simple value.
/// </summary>
private static object GetValue(Expression x)
{
switch (x.NodeType)
{
case ExpressionType.Constant:
return ((ConstantExpression)x).Value;
case ExpressionType.MemberAccess:
var xMember = (MemberExpression)x;
var instance = xMember.Expression == null ? null : GetValue(xMember.Expression);
switch (xMember.Member.MemberType)
{
case MemberTypes.Field:
return ((FieldInfo)xMember.Member).GetValue(instance);
case MemberTypes.Property:
return ((PropertyInfo)xMember.Member).GetValue(instance);
default:
throw new Exception(xMember.Member.MemberType + "???");
}
default:
// NOTE: it would be easy to compile and invoke the expression, but it's intentionally not done. Callers can always pre-evaluate and pass a captured member.
throw new NotSupportedException("Only constant, field or property supported.");
}
}
/// <summary>
/// <see cref="ExpressionVisitor"/> for <see cref="Prepare{TDelegate}(Expression{TDelegate})"/>.
/// </summary>
private sealed class PrepareVisitor : ExpressionVisitor
{
/// <summary>
/// Replace lambda.Compile().Invoke(...) with lambda's body, where the parameters are replaced with the invocation's arguments.
/// </summary>
protected override Expression VisitInvocation(InvocationExpression node)
{
// is it what we are looking for?
var call = node.Expression as MethodCallExpression;
if (call == null || call.Method.Name != "Compile" || call.Arguments.Count != 0 || call.Object == null || !typeof(LambdaExpression).IsAssignableFrom(call.Object.Type))
return base.VisitInvocation(node);
// get the lambda
var lambda = call.Object as LambdaExpression ?? (LambdaExpression)GetValue(call.Object);
// get the expressions for the lambda's parameters
var replacements = lambda.Parameters.Zip(node.Arguments, (p, x) => new KeyValuePair<ParameterExpression, Expression>(p, x));
// return the body with the parameters replaced
return Visit(new ParameterReplaceVisitor(replacements).Visit(lambda.Body));
}
}
/// <summary>
/// <see cref="ExpressionVisitor"/> to replace parameters with actual expressions.
/// </summary>
private sealed class ParameterReplaceVisitor : ExpressionVisitor
{
private readonly Dictionary<ParameterExpression, Expression> _replacements;
/// <summary>
/// Init.
/// </summary>
/// <param name="replacements">Parameters and their respective replacements.</param>
public ParameterReplaceVisitor(IEnumerable<KeyValuePair<ParameterExpression, Expression>> replacements)
{
_replacements = replacements.ToDictionary(kv => kv.Key, kv => kv.Value);
}
protected override Expression VisitParameter(ParameterExpression node)
{
Expression replacement;
return _replacements.TryGetValue(node, out replacement) ? replacement : node;
}
}
}
//
///帮助构建表达式。
///
公共静态类快车
{
#区域准备
///
///准备要在查询文件中使用的表达式。
///
///修改后的表达式。
///
///该方法将.Compile（）.Invoke（…）的实例替换为lambda的主体，其参数替换为调用的参数。
///仅通过计算属性和字段来解析值。
///
公共静态表达式Prepare（此表达式lambda）=>（表达式）new PrepareVisitor（）。访问（lambda）；
///
///包装纸。
///
公共静态表达式Prepare（表达式lambda）=>lambda.Prepare（）；
///
///包装纸。
///
公共静态表达式Prepare（表达式lambda）=>lambda.Prepare（）；
//注意：这里有更多的Prepare重载。
#端区
///
///将表达式求值为简单值。
///
私有静态对象GetValue（表达式x）
{
开关（x.NodeType）
{
大小写表达式类型。常量：
返回（（恒压）x）值；
case ExpressionType.MemberAccess：
var xMember=（MemberExpression）x；
var实例=xMember.Expression==null？null:GetValue（xMember.Expression）；
开关（xMember.Member.MemberType）
{
案例成员类型。字段：
return（（FieldInfo）xMember.Member）.GetValue（实例）；
案例成员类型。属性：
return（（PropertyInfo）xMember.Member）.GetValue（实例）；
违约：
抛出新异常（xMember.Member.MemberType+“？？？”）；
}
违约：
//注意：编译和调用表达式很容易，但这不是有意的。调用者总是可以预先计算并传递捕获的成员。
抛出新的NotSupportedException（“仅支持常量、字段或属性”）；
}
}
///
///因为。
///
私人密封类准备人：ExpressionVisitor
{
///
///将lambda.Compile（）.Invoke（…）替换为lambda的主体，其中参数替换为调用的参数。
///
受保护的重写表达式VisitInvocation（调用表达式节点）
{
//这就是我们要找的吗？
var call=node.Expression作为MethodCallExpression；
如果（call==null | | | call.Method.Name！=“Compile”| | call.Arguments.Count！=0 | | call.Object==null | |！typeof（LambdaExpression）.IsAssignableFrom（call.Object.Type））
返回基地。访问职业（节点）；
//去拿兰姆达
var lambda=call.Object作为LambdaExpression？？？（LambdaExpression）GetValue（call.Object）；
//获取lambda参数的表达式
var replacements=lambda.Parameters.Zip（node.Arguments，（p，x）=>newkeyv
Expression<Func<EntityType, bool>> xPredicate = x =>
x.Property == "value" &&
x.Data.AnotherProperty == true &&
x.Data.Timestamp < DateTime.Now && x.Data.Status == Status.OK
|| ...;
myRepository.FindAll().Where(xPredicate)
private Expression<Func<DateTime, Status, bool>> _xSubmissionDateExpiredCondition = (submissionDate, status) => submissionDate < DateTime.Now && status == Status.OK;
// getting the delegate as before (to be used in ApplyConditions) is trivial:
private Func<DateTime, Status, bool> _submissionDateExpiredCondition = _xSubmissionDateExpiredCondition.Compile();
// ... other conditions here
Expression<Func<EntityType, bool>> xPredicate = x =>
x.Property == "value" &&
x.Data.AnotherProperty == true &&
_xSubmissionDateExpiredCondition.Compile()(x.Data.Timestamp, x.Data.Status)
|| ...;
xPredicate = MAGIC(xPredicate);
/// <summary>
/// Helps in building expressions.
/// </summary>
public static class Express
{
#region Prepare
/// <summary>
/// Prepares an expression to be used in queryables.
/// </summary>
/// <returns>The modified expression.</returns>
/// <remarks>
/// The method replaces occurrences of <see cref="LambdaExpression"/>.Compile().Invoke(...) with the body of the lambda, with it's parameters replaced by the arguments of the invocation.
/// Values are resolved by evaluating properties and fields only.
/// </remarks>
public static Expression<TDelegate> Prepare<TDelegate>(this Expression<TDelegate> lambda) => (Expression<TDelegate>)new PrepareVisitor().Visit(lambda);
/// <summary>
/// Wrapper for <see cref="Prepare{TDelegate}(Expression{TDelegate})"/>.
/// </summary>
public static Expression<Func<T1, TResult>> Prepare<T1, TResult>(Expression<Func<T1, TResult>> lambda) => lambda.Prepare();
/// <summary>
/// Wrapper for <see cref="Prepare{TDelegate}(Expression{TDelegate})"/>.
/// </summary>
public static Expression<Func<T1, T2, TResult>> Prepare<T1, T2, TResult>(Expression<Func<T1, T2, TResult>> lambda) => lambda.Prepare();
// NOTE: more overloads of Prepare here.
#endregion
/// <summary>
/// Evaluate an expression to a simple value.
/// </summary>
private static object GetValue(Expression x)
{
switch (x.NodeType)
{
case ExpressionType.Constant:
return ((ConstantExpression)x).Value;
case ExpressionType.MemberAccess:
var xMember = (MemberExpression)x;
var instance = xMember.Expression == null ? null : GetValue(xMember.Expression);
switch (xMember.Member.MemberType)
{
case MemberTypes.Field:
return ((FieldInfo)xMember.Member).GetValue(instance);
case MemberTypes.Property:
return ((PropertyInfo)xMember.Member).GetValue(instance);
default:
throw new Exception(xMember.Member.MemberType + "???");
}
default:
// NOTE: it would be easy to compile and invoke the expression, but it's intentionally not done. Callers can always pre-evaluate and pass a captured member.
throw new NotSupportedException("Only constant, field or property supported.");
}
}
/// <summary>
/// <see cref="ExpressionVisitor"/> for <see cref="Prepare{TDelegate}(Expression{TDelegate})"/>.
/// </summary>
private sealed class PrepareVisitor : ExpressionVisitor
{
/// <summary>
/// Replace lambda.Compile().Invoke(...) with lambda's body, where the parameters are replaced with the invocation's arguments.
/// </summary>
protected override Expression VisitInvocation(InvocationExpression node)
{
// is it what we are looking for?
var call = node.Expression as MethodCallExpression;
if (call == null || call.Method.Name != "Compile" || call.Arguments.Count != 0 || call.Object == null || !typeof(LambdaExpression).IsAssignableFrom(call.Object.Type))
return base.VisitInvocation(node);
// get the lambda
var lambda = call.Object as LambdaExpression ?? (LambdaExpression)GetValue(call.Object);
// get the expressions for the lambda's parameters
var replacements = lambda.Parameters.Zip(node.Arguments, (p, x) => new KeyValuePair<ParameterExpression, Expression>(p, x));
// return the body with the parameters replaced
return Visit(new ParameterReplaceVisitor(replacements).Visit(lambda.Body));
}
}
/// <summary>
/// <see cref="ExpressionVisitor"/> to replace parameters with actual expressions.
/// </summary>
private sealed class ParameterReplaceVisitor : ExpressionVisitor
{
private readonly Dictionary<ParameterExpression, Expression> _replacements;
/// <summary>
/// Init.
/// </summary>
/// <param name="replacements">Parameters and their respective replacements.</param>
public ParameterReplaceVisitor(IEnumerable<KeyValuePair<ParameterExpression, Expression>> replacements)
{
_replacements = replacements.ToDictionary(kv => kv.Key, kv => kv.Value);
}
protected override Expression VisitParameter(ParameterExpression node)
{
Expression replacement;
return _replacements.TryGetValue(node, out replacement) ? replacement : node;
}
}
}