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用于转换IQueryable<t>int List<SelectListItem>的表达式

expression-trees linq-to-entities lambda linq c#

Martin Hansen Lennox · 技术社区 · 9 年前

我想创建这样一个存储库方法:

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!
}

这将使我能够这样称呼它:

List<SelectListItem> selectListItems = PersonRepository.AllAsSelectListItems(
    m => m.ID,
    m => m.Name,
    selectedIDAsString
);

而且,随着 selectedValue 参数为“1”时,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}
};

我对通用型有问题 AllAsSelectListItems() 方法

您可以在下面的代码中看到我的尝试。但这并不理想。

我使用硬编码字符串来填充 SelectListItem 属性与 T 属性。我认为表达式树是解决方案,但我很难正确编码。

此外,分配 ID 属性会破坏它,因为它是 int 不是 string .

最后,我也在努力比较 选定的值 参数 SelectListItem.Value 所有物

人员类别

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();
    }
}

1 回复 | 直到 9 年前

Ivan Stoev 9 年前

你快到了。要实现的事情很少:

(A) 绑定传递的 valueProperty 和 textProperty 表达式转换为公共参数。由于假设它们表示属性/字段访问器,因此传递的表达式 Body 应为类型 MemberExpression 并且可以从中提取实际成员信息 MemberExpression.Member 所有物

(B) 生成 Selected 使用赋值 Expression.Equal

把它们放在一起,看起来会是这样的

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;
}

更新: 不幸地 SelectListItem.Value 属于类型 string ,并且大多数时候源属性(通常是某种Id)不是 一串 。所以让我们重命名 value属性 到 valueSelector 允许传递类似 x => x.Id.ToString() 虽然我们无法轻松地重新绑定传递的表达式,但我们可以轻松地使用它,而不是创建新的参数,只需重用该表达式的参数即可。

修改后的方法如下

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;
}