asp mvc razor storing lambda expression to list and reuse it

Question

I am using asp.mvc 4. Assumend I have a Model called Person with the fields

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; }
}

In my automatically generated razor-edit-view fields are shown like this (For clearness, I only included important lines in this post)

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

now I would like to store the linq-lambda expressions in a list an use it later, I don't know how to do that, I need something like this:

@{
    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);
    }
}

and finally I would like to use the generated list like this

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

Darin Dimitrov · Answer 1 · 2013-02-06T09:42:20.687

I'd write a custom helper for this:

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());
    }
}

and then inside the view simply:

@Html.MyHelper("namepart")

and if you want to render the other part:

@Html.MyHelper("otherpart")

As an alternative simply put this content into 2 different partial views and then:

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

Thanks for this advice. Yes that seems nice, but my main-goal is to understand how to store lambda-expressions to lists and use it later; — alpine_anil, Feb 06 '13 at 12:53
You cannot store it in lists because you do not know the exact type of the lambda expression. You could have used `Expression>` but this won't work with the ASP.NET MVC helpers such as `DisplayFor` if you attempt to use it with a value type property on your model (such as DateTime or integer) — Darin Dimitrov, Feb 06 '13 at 13:25

score 1 · Answer 2 · edited Dec 31 '16 at 14:49

There is small progress in my brain. Thanks for your advices. As @darin-dimitrov said, the secret is to store an Expression-Tree. I updated my first post and added a related table. This example works only, when the model has fetched "1 single row from database-table" for example in an edit view;

// 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);
    }
}

now create the list and the expressions

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));

and now it works!

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

score 0 · Answer 3 · edited Jul 04 '17 at 07:49

0

Have you tried to store the lambda like this:

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

@Html.DisplayFor(personExpression)

And for 2 input types your code would look something like this:

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

edited Jul 04 '17 at 07:49

Romano Zumbé

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answered Feb 06 '13 at 09:13

Smartis has left SO again

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Thanks, but this doesn't work. this results in compiling errors. Did you have success with this approach? – alpine_anil Feb 06 '13 at 09:34
Yeah, it works fine in Console-Solutions. I think Razor didn't support it, or? – Smartis has left SO again Feb 06 '13 at 09:36

score 0 · Answer 4 · answered Jul 18 '17 at 20:05

To loop through model properties in a razor view you should use ViewData.ModelMetadata.Properties as in this answer. For example:

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

asp mvc razor storing lambda expression to list and reuse it

4 Answers4