C# 要转换IQueryable的表达式<;t>;整数列表<;选择列表项>;
我想创建一个存储库方法,如下所示:C# 要转换IQueryable的表达式<;t>;整数列表<;选择列表项>;,c#,linq,lambda,linq-to-entities,expression-trees,C#,Linq,Lambda,Linq To Entities,Expression Trees,我想创建一个存储库方法,如下所示: public List<SelectListItem> AllAsSelectListItems( Expression<Func<T, string>> valueProperty, Expression<Func<T, string>> textProperty, string selectedValue = "") { // what goes here? I
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!
}
我在使用泛型AllAsSelectListItems()
方法时遇到问题
您可以在下面的代码中看到我迄今为止的尝试。但这并不理想
我使用了硬编码字符串,用T
属性填充SelectListItem
属性。我认为表达式树是解决方案,但我正在努力正确地编码它
另外,分配ID
属性会破坏它,因为它是int
而不是字符串
最后,我还在努力将selectedValue
参数与SelectListItem.Value
属性进行比较
个人类别
public class Person
{
public int ID {get;set;}
public string Name {get;set;}
}
控制器
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);
}
}
存储库
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();
}
}
公共类PersonRepository:存储库
{
//各种专门方法
}
公共类存储库:IRepository,其中T:DBEntity
{
私有ApplicationDbContext db=null;
私有DbSet表=null;
公共存储库()
{
this.db=new ApplicationDbContext();
table=db.Set();
}
公共存储库(ApplicationDbContext数据库)
{
这个.db=db;
table=db.Set();
}
受保护的虚拟IQueryable AllAsQueryable(
参数表达式[]includeExpressions)
{
返回includeExpressions.Aggregate
(表,(当前,表达式)=>current.Include(表达式));
}
公共列表所有选择列表项(
表达式valueProperty,
表达式textProperty,
字符串selectedValue=“”)
{
//临时硬编码值,直到我们了解如何正确使用表达式参数
字符串valuePropertyHardCoded=“Name”;
字符串textPropertyHardCoded=“Name”;
类型currentType=类型(T);
var itemParam=表达式参数(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=表达式.lambda(
选择器,itemParam);
返回AllAsQueryable().Select(lambda.Compile()).ToList();
}
}
你就快到了。要实现的事情很少:
(A) 将传递的valueProperty
和textProperty
表达式绑定到公共参数。由于假设它们表示属性/字段访问器,因此传递的表达式Body
应为MemberExpression
类型,并且可以从MemberExpression.member
属性中提取实际的成员信息
(B) 使用表达式生成所选赋值。Equal
把所有这些放在一起,看起来会像这样
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;
}
我有点困惑,AllAsSelectListItemsSpecifyProperties是如何尝试编写AllAsSelectListItems的?或者这是另一种有效的方法,好吗?对不起,这是复制粘贴时的打字错误。谢谢你指出这一点。它应该是AllAsSelectListItems()
。现在已更新。string selectedValue=”“
参数的用途是什么?与每个SelectListItem.Value
进行比较。如果它们相同,我们将SelectListItem.Selected
标记为true
。非常感谢!!现在我明白了,而且有了你的解释,这是有道理的,但我自己永远也不会想到这个问题。如果我想将int
属性作为valueProperty
参数传递,我该怎么做呢?我试图用Expression
和Expression
创建重载,但我不知道如何编辑绑定部分。@martinhansennox让我想一想。问题是它需要转换成string
我想是吧?我想是的。我正在尝试各种方法,但我不确定我应该在代码中尝试将转换放在哪里。如果这很痛苦,我可以单独提问。我很高兴它现在能在弦上工作:)
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();
}
}
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;
}
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;
}