Question

我正在疯狂地试图理解LINQ中的表达式。非常感谢任何帮助（甚至告诉我，我完全不在这里）。

假设我有三个班级

public class Person
{
    public string Name { get; set;}
    public IEnumerable<PersonLocation> Locations { get; set;}
    public IEnumerable<PersonEducation> Educations { get; set:}
}

public class PersonLocation
{
    public string Name { get; set;}
    public string Floor { get; set;}
    public string Extension { get; set;}
}

public class PersonEducation
{
   public string SchoolName { get; set;}
   public string GraduationYear { get; set;}
}

我正在尝试创建一个接受字符串的方法，例如Locations.Name或Locations.Floor，或者Educations.SchoolName，然后创建一个动态linq查询

IEnumerable<Person> people = GetAllPeople();
GetFilteredResults(people, "Location.Name", "San Francisco");
GetFilteredResults(people, "Location.Floor", "17");
GetFilteredResults(people, "Educations.SchoolName", "Northwestern");

此GetFilteredResults(IEnumerable<Person> people, string ModelProperty, string Value)方法应创建一个大致相当于people.Where(p => p.Locations.Any(pl => pl.Name == Value);

的表达式

如果ModelProperty是一个字符串，那么我有这个工作，即people.Where（p =＆gt; p.Name == Value）如下所示：

string[] modelPropertyParts = ModelProperty.Split('.');
var prop = typeof(Person).GetProperty(modelPropertyParts[0]);
var sourceParam = Expression.Parameter(typeof(Person), "person");
var expression = Expression.Equal(Expression.PropertyOrField(sourceParam, modelPropertyParts[0]), Expression.Constant(option.Name));
var whereSelector = Expression.Lambda<Func<Person, bool>>(orExp, sourceParam);
return people.Where(whereSelector.Compile());

这是我一直在玩的IEnumerable类型，但我只是无法获得内部Any，这似乎是正确的，挂钩到外部Where：

/*i.e. modelPropertyParts[0] = Locations & modelPropertyParts[1] = Name */
string[] modelPropertyParts = ModelProperty.Split('.');

var interiorProperty = prop.PropertyType.GetGenericArguments()[0];
var interiorParameter = Expression.Parameter(interiorProperty, "personlocation");
var interiorField = Expression.PropertyOrField(interiorParameter, modelPropertyParts[1]);
var interiorExpression = Expression.Equal(interiorField, Expression.Constant(Value));
var innerLambda = Expression.Lambda<Func<PersonLocation, bool>>(interiorExpression, interiorParameter);

var outerParameter = Expression.Parameter(typeof(Person), "person");
var outerField = Expression.PropertyOrField(outerParameter, modelPropertyParts[0]);
var outerExpression = ??
var outerLambda == ??

return people.Where(outerLambda.Compile());

Answer 1

问题是System.Linq.Enumerable.Any是一种静态扩展方法。

您的outerExpression必须引用System.Linq.Enumerable.Any(IEnumerable<T>, Func<T, bool>)：

var outerExpression = Expression.Call(
    typeof(System.Linq.Enumerable), 
    "Any", 
    new Type[] { outerField.Type, innerLambda.Type }, 
    outerField, innerLambda);

请查看这些链接以获取更多信息：

LINQ中嵌套的通用Lambda

1 个答案: