带有导致错误的表达式的规范模式 - 从范围 '' 引用的类型为 '' 的变量 'x'，但未定义

Question

我正在使用规范类来封装在我的域中使用的各种查询。

规范继承自 Specification<T> 基类：

public abstract class Specification<T> : ISpecification<T>
    where T : class
{
    public abstract Expression<Func<T, bool>> ToExpression();

    public virtual bool IsSatisfiedBy(T candidate)
    {
        var predicate = ToExpression().Compile();
        return predicate(candidate);
    }

    public Specification<T> And(Specification<T> specification)
    {
        return new AndSpecification<T>(this, specification);
    }

    public Specification<T> Or(Specification<T> specification)
    {
        return new OrSpecification<T>(this, specification);
    }
}

AndSpecification<T> 定义为：

public class AndSpecification<T> : Specification<T>
    where T : class
{
    private readonly Specification<T> _left;
    private readonly Specification<T> _right;

    public AndSpecification(Specification<T> left, Specification<T> right)
    {
        _right = right;
        _left = left;
    }

    public override Expression<Func<T, bool>> ToExpression()
    {
        var leftExpression = _left.ToExpression();
        var rightExpression = _right.ToExpression();

        var andExpression = Expression.AndAlso(
            leftExpression.Body, rightExpression.Body);

        var param = leftExpression.Parameters.Single();

        return Expression.Lambda<Func<T, bool>>(
            andExpression, param);
    }
}

Specification<T> 类可以链接在一起，例如：

firstSpecification.And(secondSpecification).IsSatisfiedBy(candidate);

一个现实生活中的规范类的例子是：

public class IsAssignmentSetForStudentOverdueSpecification : Specification<Student>
{
    private readonly Assignment _assignment;

    public IsAssignmentSetForStudentOverdueSpecification(Assignment assignment)
    {
        _assignment = assignment
    }

    public override Expression<Func<Student, bool>> ToExpression()
    {

        var isStudentSetAssignment = new IsStudentSetAssignmentSpecification(_assignment);

        var hasAssignmentBeenCompletedByStudent = new HasAssignmentBeenCompletedByStudentSpecification(_assignment);

        return isStudentSetAssignment.And(hasAssignmentBeenCompletedByStudent).ToExpression();
    }
}

IsStudentSetAssignmentSpecification 的ToExpression() 方法如下（实际情况稍微复杂一些，但为简洁起见已简化）：

public override Expression<Func<Student, bool>> ToExpression()
    {
        return x =>  _assignment.Students.Contains(x);
    }

'HasAssignmentBeenCompletedByStudentSpecificationhas aToExpression()`方法如下：

public override Expression<Func<Student, bool>> ToExpression()
    {
        return x =>  _assignment.StudentSubmissions.Contains(x);
    }

一个使用例子是：

var overdue = new IsAssignmentSetForStudentOverdueSpecification(assignment).IsSatisfiedBy(student);

其中assignment 和student 是域类。但是，当我尝试使用它时，我得到以下System.InvalidOperationException：

从范围“”引用的“学生”类型的变量“x”，但未定义

我该如何解决这个问题？

堆栈跟踪是：

在 System.Linq.Expressions.Expression.Property（表达式表达式，PropertyInfo 属性）\r\n 在 System.Linq.Expressions.ExpressionVisitor.VisitBinary（BinaryExpression 节点）\r\n 在 System.Linq.Expressions.ExpressionVisitor .VisitLambda[T](Expression1 node)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitArguments(IArgumentProvider nodes)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitMethodCall(MethodCallExpression node)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitBinary(BinaryExpression node)\r\n at Beehive.Domain.Specifications.AndSpecification1.ToExpression() in C:\Beehive\2.0\Beehive.Domain\Specifications\AndSpecification.cs:第 35 行\r\n 在 Beehive.Domain.Planner.Assignments.Specifications。 IsAssignmentSetForStudentOverdueSpecification.ToExpression() 在 C:\Beehive\2.0\Beehive.Domain\Planner\Assignments\Specifications\IsAssignmentSetForStudentOverdueSpecification.cs:line 28\r\n at Beehive.Domain.Specifications.Specification1.IsSatisfiedBy(T candidate) in C:\\Beehive\\2.0\\Beehive.Domain\\Specifications\\Specification.cs:line 19\r\n at Beehive.Services.Shared.Planner.Assignments.AssignmentToAssignmentListViewMapHelper.Map(Assignment assignment, Student student) in C:\\Beehive\\2.0\\Beehive.Services\\Shared\\Planner\\Assignments\\AssignmentToAssignmentListViewMapHelper.cs:line 22\r\n at Beehive.Services.QueryHandlers.Planner.Assignments.AssignmentsForUserQueryHandler.<>c__DisplayClass5_0.<CreateMap>b__0(Assignment x) in C:\\Beehive\\2.0\\Beehive.Services\\QueryHandlers\\Planner\\Assignments\\AssignmentsForUserQueryHandler.cs:line 70\r\n at System.Linq.Enumerable.WhereSelectListIterator2.MoveNext()\ r\n 在 System.Collections.Generic.List1..ctor(IEnumerable1 集合）\r\n 在 C:\Beehive\2.0\Beehive 中的 System.Linq.Enumerable.ToList[TSource](IEnumerable1 source)\r\n at Beehive.Services.QueryHandlers.Planner.Assignments.AssignmentsForUserQueryHandler.CreateMap(AssignmentToAssignmentListViewMapHelper mapHelper, Student student, IRepositoryCollection1 集合)。 Services\QueryHandlers\Planner\Assignments\AssignmentsForUserQueryHandler.cs:line 70\r\n at Beehive.Services.QueryHandlers.Planner.Assignmen ts.AssignmentsForUserQueryHandler.Handle(AssignmentsForUserQuery 查询) 在 C:\Beehive\2.0\Beehive.Services\QueryHandlers\Planner\Assignments\AssignmentsForUserQueryHandler.cs:line 59\r\n 在 System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1, TRet]（CallSite 站点，T0 arg0，T1 arg1）\r\n 在 Beehive.DI.QueryProcessor.Process[TResult]（IQuery1 query) in C:\\Beehive\\2.0\\Beehive.DI\\QueryProcessor.cs:line 26\r\n at Beehive.API.Controllers.Planner.AssignmentsController.<>c__DisplayClass1_0.<GetAssignments>b__0() in C:\\Beehive\\2.0\\Beehive.API\\Controllers\\Planner\\AssignmentsController.cs:line 26\r\n at Beehive.API.Controllers.ControllerBase.ExecuteQuery[TResult](Func1 操作）在 C:\Beehive\2.0\Beehive.API\Controllers\ControllerBase .cs:line 20\r\n 在 Beehive.API.Controllers.Planner.AssignmentsController.GetAssignments(Guid userId) 在 C:\Beehive\2.0\Beehive.API\Controllers\Planner\AssignmentsController.cs:line 26\r\ n 在 lambda_method(Closure , Object , Object[] )\r\n 在 System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.c__DisplayClass10.b__9(Object instance, Object[] methodParameters)\r\n 在 System. Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, CancellationToken cancelToken)\r\n---从先前引发异常的位置结束堆栈跟踪 ---\r\n 在 System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n 在 System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(任务任务)\r \n 在 System.Web.Http.Controllers.ApiControllerActionInvoker.d__0.MoveNext()\r\n--- 从先前引发异常的位置结束堆栈跟踪 ---\r\n 在 System.Runtime.ExceptionServices。 ExceptionDispatchInfo.Throw()\r\n 在 System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification（任务任务）\r\n 在 System.Web.Http.Controllers.ActionFilterResult.d__2.MoveNext()\r\n---从先前引发异常的位置结束堆栈跟踪 ---\r\n 在 System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n 在 System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(任务任务)\r \n 在 System.Web.Http.Controllers.ExceptionFilterResult.d__0.MoveNext()\r\n--- 结束从先前抛出异常的位置的堆栈跟踪 ---\r\n 在 System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n 在 System.Web.Http.Controllers.ExceptionFilterResult.d__0.MoveNext()\ r\n--- 从先前引发异常的位置结束堆栈跟踪 ---\r\n 在 System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n 在 System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification （任务任务）\r\n 在 System.Web.Http.Dispatcher.HttpControllerDispatcher.d__1.MoveNext()

Answer 1

就像@Raphaël 和@Evk 建议的那样：

public class AndSpecification<T> : Specification<T>
    where T : class
{
    private readonly Specification<T> _left;
    private readonly Specification<T> _right;

    public AndSpecification(Specification<T> left, Specification<T> right)
    {
        _right = right;
        _left = left;
    }

    public override Expression<Func<T, bool>> ToExpression()
    {
        var leftExpression = _left.ToExpression();
        var rightExpression = _right.ToExpression();

        var paramExpr = Expression.Parameter(typeof(T));

        var andExpression = Expression.AndAlso(
            leftExpression.Body, rightExpression.Body);

        andExpression = (BinaryExpression) new ParameterReplacer(paramExpr).Visit(andExpression);

        return Expression.Lambda<Func<T, bool>>(andExpression, paramExpr);
    }
}
public class ParameterReplacer : ExpressionVisitor
{
    private readonly ParameterExpression _parameter;

    protected override Expression VisitParameter(ParameterExpression node)
    {
        return base.VisitParameter(_parameter);
    }

    internal ParameterReplacer(ParameterExpression parameter)
    {
        _parameter = parameter;
    }
}

Answer 2

使用此处的代码https://petemontgomery.wordpress.com/2011/02/10/a-universal-predicatebuilder/，您可以创建正确组合两个表达式的 AndSpecification

public class AndSpecification<T> : Specification<T>
    where T : class
{
    private readonly Specification<T> _left;
    private readonly Specification<T> _right;

    public AndSpecification(Specification<T> left, Specification<T> right)
    {
        _right = right;
        _left = left;
    }

    public override Expression<Func<T, bool>> ToExpression()
    {
        var leftExpression = _left.ToExpression();
        var rightExpression = _right.ToExpression();

        return PredicateBuilder.And(leftExpression, rightExpression);
    }
}

这将修复它们，因此表达式的主体将使用相同的参数，并且应该避免您出错。