LINQ 参数"x"不在作用域内

Question

在我关于制定IQueryable来查询递归数据库表的问题之后，我尝试了给出的答案，并更接近我的问题的解决方案。我已经重组了提供的答案，因此对我来说更容易理解正在发生的事情。但是，无论我尝试什么，我都会不断收到错误消息"the parameter 'x' is not in scope"在线return entities.Single();。如何确保参数"x"在 IN 范围内？我尝试过在BuildParentPropertiesExpression方法中制作ParameterExpression，我尝试每次都用不同的名称将其放入for循环中。一切都无济于事。

在此处阅读背景故事

public Entity Single(string path)
{
    if (path[0] == '/')
        path = path.Substring(1); // only absolute paths for now
    List<string> pathParts = path.Split('/').ToList();
    pathParts.Reverse();
    var entities =
        from entity in dataContext.Entities
        select entity;
    // Build up a template expression that will be used to create the real expressions with.
    Expression<Func<Entity, bool>> templateExpression = (x => x.Code == "dummy");
    BinaryExpression equals = (BinaryExpression)templateExpression.Body;
    MemberExpression property = (MemberExpression)equals.Left;
    ParameterExpression entityParameter = Expression.Parameter(typeof(Entity), "x");
    for (int index = 0; index < pathParts.Count; index++)
    {
        string pathPart = pathParts[index];
        Expression parentPropertyExpression = BuildParentPropertiesExpression(index, entityParameter);
        MemberExpression left = Expression.Property(
            parentPropertyExpression,
            (PropertyInfo)property.Member
        );
        ConstantExpression right = Expression.Constant(pathPart);
        BinaryExpression equalExpression = Expression.Equal(
            left, 
            right, 
            equals.IsLiftedToNull, 
            equals.Method
        );
        var entityFilterExpression = Expression.Lambda<Func<Entity, bool>>(
            equalExpression, 
            templateExpression.Parameters
        );
        entities = entities.Where<Entity>(entityFilterExpression);
    }
    return entities.Single();
}
private Expression BuildParentPropertiesExpression(int numberOfParents, ParameterExpression entityParameter)
{
    if (numberOfParents == 0)
        return entityParameter;
    var getParentMethod = typeof(Entity).GetProperty("Entity1").GetGetMethod();
    var property = Expression.Property(entityParameter, getParentMethod);
    for (int count = 2; count <= numberOfParents; count++)
        property = Expression.Property(property, getParentMethod);
    return property;
}

Answer 1

你与原始实现非常接近

public Entity Single(string path) {
    string[] pathParts = path.Split('/');
    string code = pathParts[pathParts.Length -1];
    if (pathParts.Length == 1)
            return dataContext.Entities.Single(e => e.Code == code && e.ParentID == 0);
    IQueryable<Entity> entities = dataContext.Entities.Where(e => e.Code == code);
    for (int i = pathParts.Length - 2; i >= 0; i--) {
        string parentCode = pathParts[i];
        entities = entities.Where(e => e.Entity1.Code == parentCode).Select(e => e.Entity1); //This now gets the parent entity
    }
    return entities.Single();
}