asp-mvc-rarzor存储lambda表达式以列出并重用它

Question

我使用的是asp.mvc 4。假设我有一个名为Person的带有字段的模型

public class Person
{
    public int ID { get; set; }
    public string FirstName { get; set; }
    public string SecondName { get; set; }
    public DateTime DateOfBirth { get; set; }
    public DateTime DateOfWorkstart { get; set; }
    public int NumberOfChildren { get; set; }
    public int DepartmentID { get; set; }
    public virtual Department Department { get; set; }
}
public class Department
{
    public int ID { get; set; }
    public int NameOfDepartment { get; set; }
}

在我自动生成的剃刀编辑视图中，字段显示如下（为了清晰起见，我在这篇文章中只包含了重要的行）

@Html.DisplayFor(modelItem => item.FirstName)
@Html.DisplayFor(modelItem => item.SecondName)

现在我想把linqlambda表达式存储在一个列表中，稍后再使用，我不知道如何做到这一点，我需要这样的东西：

@{
    string itemsToShow = "namepart"; // this could also be "otherpart"
    List <Expression<>> list = new List();
    if (itemsToShow.equals("namepart")
    {
        list.add(modelItem => item.FirstName);
        list.add(modelItem => item.SecondName);
    }
    else
    {
        list.add(modelItem => item.DateOfBirth);
        list.add(modelItem => item.DateOfWorkStart);
        list.add(modelItem => item.NumberOfChildren);
    }
}

最后，我想使用像这个一样生成的列表

@foreach (var lambda in list)
{
    @Html.DisplayFor(lambda)
}

Answer 1

我会为此编写一个自定义帮助程序：

public static class HtmlExtensions
{
    public static IHtmlString MyHelper(this HtmlHelper<MyViewModel> html, string itemsToShow)
    {
        var sb = new StringBuilder();
        if (itemsToShow == "namepart")
        {
            sb.Append(html.DisplayFor(x => x.FirstName));
            sb.Append(html.DisplayFor(x => x.SecondName));
        }
        else
        {
            sb.Append(html.DisplayFor(x => x.DateOfBirth));
            sb.Append(html.DisplayFor(x => x.DateOfWorkStart));
            sb.Append(html.DisplayFor(x => x.NumberOfChildren));
        }
        return new HtmlString(sb.ToString());
    }
}

然后简单地进入视图：

@Html.MyHelper("namepart")

如果你想渲染另一部分：

@Html.MyHelper("otherpart")

作为替代方案，只需将此内容放入2个不同的局部视图中，然后：

@if (itemsToShow == "namepart")
{
    @Html.Partial("_NamePart", Model)
}
else
{
    @Html.Partial("_OtherPart", Model)
}

Answer 2

我的大脑有了一些小进步。谢谢你的建议。正如@darin-dimitrov所说，秘密是存储一个表达式树。我更新了我的第一篇文章，并添加了一个相关的表格。仅当模型在编辑视图中提取了"数据库表中的1单行"时，此示例才有效；

// first a small helper, which creates the member and checks nullable fields
public static Expression getExpressionPart(ParameterExpression param,
  String s1, String s2)
{
    Expression member = null;
    if (s2 == null)
    {
        member = Expression.Property(param, s1);
    }
    else
    {
        // the second string is to deal with foreign keys/related data
        member = Expression.PropertyOrField(
          Expression.PropertyOrField(param, s1), s2
        );
    }
    Type typeIfNullable = Nullable.GetUnderlyingType(member.Type);
    if (typeIfNullable != null)
    {
        member = Expression.Call(member, "GetValueOrDefault", Type.EmptyTypes);
    }
}

现在创建列表和表达式

List<Expression<Func<Person, object>>> list =
  new List<Expression<Func<Person, object>>>();
ParameterExpression param = Expression.Parameter(typeof(Person), "p");
// maps to expression p => p.FirstName
Expression member = getExpressionPart(param, "Firstname", null);
list.Add(Expression.Lambda<Func<Person, object>>(member, param));
// maps to expression p => p.Department.NameOfDepartment
member = getExpressionPart(param, "Department", "NameOfDepartment");
list.Add(Expression.Lambda<Func<Person, object>>(member, param));

现在它起作用了！

@foreach (var lambda in list)
{
    @Html.DisplayNameFor(lambda)
    @Html.DisplayFor(lambda)
}

Answer 3

你试过这样存储lambda吗：

Func<Person,bool> personExpression = (u => u.FirstName == firstname);
@Html.DisplayFor(personExpression)

对于两种输入类型，您的代码看起来像这样：

Func<Person,Ticket,bool> personExpression =
        ((u,t) => u.FirstName == firstname && u.SecondName == t.SecondName);

Answer 4

要在剃刀视图中循环遍历模型属性，应使用ViewData.ModelMetadata.Properties，如本答案所示。例如：

@* Loop through properties. *@
@foreach (var property in ViewData.ModelMetadata.Properties)
{
    @Html.Display(property.PropertyName)
}