[英]Expression to convert IQueryable<t> int List<SelectListItem>
I would like to create a repository method like this: 我想创建一个这样的存储库方法:
public List<SelectListItem> AllAsSelectListItems(
Expression<Func<T, string>> valueProperty,
Expression<Func<T, string>> textProperty,
string selectedValue = "")
{
// what goes here? I am having serious trouble with this bit!
}
That will enable me to call it like this: 这将使我可以这样称呼它:
List<SelectListItem> selectListItems = PersonRepository.AllAsSelectListItems(
m => m.ID,
m => m.Name,
selectedIDAsString
);
And, with the selectedValue
parameter being "1", it should produce a result like this: 并且,在selectedValue
参数为“ 1”的情况下,它应产生如下结果:
List<SelectListItem>(){
{Value: "1", Text: "Ted", Selected: true},
{Value: "2", Text: "Sam", Selected: false},
{Value: "3", Text: "Tracy", Selected: false}
};
I am having trouble with the generic AllAsSelectListItems()
method. 我在使用通用AllAsSelectListItems()
方法时遇到麻烦。
You can see my attempt so far in the code below. 您可以在下面的代码中看到我到目前为止的尝试。 But it's not ideal. 但这不是理想的。
I have resorted to hard-coded strings to populate SelectListItem
properties with T
properties. 我已使用硬编码的字符串来用T
属性填充SelectListItem
属性。 I think an expression tree is the solution, but I'm struggling to code it correctly. 我认为表达式树是解决方案,但是我正在努力对其进行正确编码。
Also, assigning the ID
property breaks it, because it's an int
not a string
. 另外,分配ID
属性会破坏它,因为它是一个int
而不是string
。
Finally, I am also struggling to compare the selectedValue
parameter to the SelectListItem.Value
property. 最后,我还在努力将selectedValue
参数与SelectListItem.Value
属性进行比较。
Person Class 人类
public class Person
{
public int ID {get;set;}
public string Name {get;set;}
}
Controller 控制者
public class PersonController : Controller
{
public IPersonRepository Repository {get;set;}
public PersonController(IPersonRepository repository)
{
Repository = repository;
}
public ActionResult SelectPerson(int selectedID)
{
string selectedIDAsString = selectedID.ToString();
var selectListItems = Repository.AllAsSelectListItems(
m => m.ID,
m => m.Name,
selectedIDAsString
);
return View(selectListItems);
}
}
Repository 资料库
public class PersonRepository : Repository
{
// various specialised methods
}
public class Repository<T> : IRepository<T> where T : DBEntity
{
private ApplicationDbContext db = null;
private DbSet<T> table = null;
public RepositoryBase()
{
this.db = new ApplicationDbContext();
table = db.Set<T>();
}
public RepositoryBase(ApplicationDbContext db)
{
this.db = db;
table = db.Set<T>();
}
protected virtual IQueryable<T> AllAsQueryable(
params Expression<Func<T, object>>[] includeExpressions)
{
return includeExpressions.Aggregate<Expression<Func<T, object>>, IQueryable<T>>
(table, (current, expression) => current.Include(expression));
}
public List<SelectListItem> AllAsSelectListItems(
Expression<Func<T, string>> valueProperty,
Expression<Func<T, string>> textProperty,
string selectedValue = "")
{
// temp hard coded values until we learn how to use the expression parameters properly
string valuePropertyHardCoded = "Name";
string textPropertyHardCoded = "Name";
Type currentType = typeof(T);
var itemParam = Expression.Parameter(currentType, "x");
var valueMember = Expression.PropertyOrField(itemParam, valuePropertyHardCoded);
var textMember = Expression.PropertyOrField(itemParam, textPropertyHardCoded);
var selector = Expression.MemberInit(Expression.New(typeof(SelectListItem)),
Expression.Bind(typeof(SelectListItem).GetMember("Value").Single(), valueMember),
Expression.Bind(typeof(SelectListItem).GetMember("Text").Single(), textMember)
);
var lambda = Expression.Lambda<Func<T, SelectListItem>>(
selector, itemParam);
return AllAsQueryable().Select(lambda.Compile()).ToList();
}
}
You are almost there. 你快到了 There are few things to be realized: 有几件事情要实现:
(A) Binding the passed valueProperty
and textProperty
expressions to a common parameter. (A)将传递的valueProperty
和textProperty
表达式绑定到一个公共参数。 Since the assumption is that they represent a property/field accessor, the passed expression Body
should be of type MemberExpression
and the actual member info can be extracted from MemberExpression.Member
property. 由于假定它们代表属性/字段访问器,所以传递的表达式Body
应该为MemberExpression
类型,并且可以从MemberExpression.Member
属性提取实际的成员信息。
(B) Generating the Selected
assignment by using Expression.Equal
(B)通过使用Expression.Equal
生成Selected
分配
Putting it all together, it will look something like this 放在一起,看起来像这样
public List<SelectListItem> AllAsSelectListItems(
Expression<Func<T, string>> valueProperty,
Expression<Func<T, string>> textProperty,
string selectedValue = "")
{
if (valueProperty == null) throw new ArgumentNullException("valueProperty");
if (textProperty == null) throw new ArgumentNullException("textProperty");
if (!(valueProperty.Body is MemberExpression)) throw new ArgumentException("Must be a field or property.", "valueProperty");
if (!(textProperty.Body is MemberExpression)) throw new ArgumentException("Must be a field or property.", "textProperty");
var item = Expression.Parameter(typeof(T), "x");
var valueMember = Expression.MakeMemberAccess(item, ((MemberExpression)valueProperty.Body).Member);
var textMember = Expression.MakeMemberAccess(item, ((MemberExpression)textProperty.Body).Member);
var targetType = typeof(SelectListItem);
var bindings = new List<MemberBinding>
{
Expression.Bind(targetType.GetProperty("Value"), valueMember),
Expression.Bind(targetType.GetProperty("Text"), textMember)
};
if (!string.IsNullOrEmpty(selectedValue))
bindings.Add(Expression.Bind(targetType.GetProperty("Selected"), Expression.Equal(valueMember, Expression.Constant(selectedValue))));
var selector = Expression.Lambda<Func<T, SelectListItem>>(
Expression.MemberInit(Expression.New(targetType), bindings), item);
var query = AllAsQueryable().Select(selector);
var result = query.ToList();
return result;
}
Update: Unfortunately SelectListItem.Value
is of type string
, and most of the time the source property (usually some sort of Id) is not a string
. 更新:不幸的是SelectListItem.Value
是string
类型的,并且在大多数情况下source属性(通常是某种Id)不是string
。 So let rename valueProperty
to valueSelector
and allow passing something like x => x.Id.ToString()
. 因此,让我们将valueProperty
重命名为valueSelector
并允许传递x => x.Id.ToString()
。 While we cannot easily rebind the passed expression, but we can easily use it unchanged and instead of creating a new parameter, just reuse the parameter of that expression. 虽然我们不能轻易地重新绑定所传递的表达式,但是我们可以轻松地不加更改地使用它,而无需创建新参数,而只需重用该表达式的参数即可。
The modified method now would be something like this 现在修改后的方法将是这样的
public List<SelectListItem> AllAsSelectListItems(
Expression<Func<T, string>> valueSelector,
Expression<Func<T, string>> textProperty,
string selectedValue = "")
{
if (valueSelector == null) throw new ArgumentNullException("valueSelector");
if (textProperty == null) throw new ArgumentNullException("textProperty");
if (!(textProperty.Body is MemberExpression)) throw new ArgumentException("Must be a field or property.", "textProperty");
var item = valueSelector.Parameters[0];
var itemValue = valueSelector.Body;
var itemText = Expression.MakeMemberAccess(item, ((MemberExpression)textProperty.Body).Member);
var targetType = typeof(SelectListItem);
var bindings = new List<MemberBinding>
{
Expression.Bind(targetType.GetProperty("Value"), itemValue),
Expression.Bind(targetType.GetProperty("Text"), itemText)
};
if (!string.IsNullOrEmpty(selectedValue))
bindings.Add(Expression.Bind(targetType.GetProperty("Selected"), Expression.Equal(itemValue, Expression.Constant(selectedValue))));
var selector = Expression.Lambda<Func<T, SelectListItem>>(Expression.MemberInit(Expression.New(targetType), bindings), item);
var query = AllAsQueryable().Select(selector);
var result = query.ToList();
return result;
}
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