4

Context : Since we are developing in C# MVC3, we wanted to have some classes designed to handle the tables on a web page. (Pagination / search / etc...).

So we finally found that it could be the best to have the following classes :

The table object that will hold all other object and knows the current page / current search etc... (misc informations)

public class Table<T> where T : IPrivateObject
{
    ...

    public ICollection<Column<T>> Columns { get; set; }
    public ICollection<Row<T>> Rows { get; set; }
    public ICollection<RowMenu<T>> Menus { get; set; }

    public ICollection<T> Items { get; set; }

    public Table(
        ICollection<T> inputItems,
        ICollection<Column<T>> columns, 
        ICollection<RowMenuItem<T>> rowMenuItems,
        ...)
        {
            ...
            this.Columns = columns;
        }

The column object that knows which property should be displayed and and a header value

public class Column<T> where T : IPrivateObject
{
    public string Value { get; set; }
    public Expression<Func<T, object>> Property { get; set; }

    public Column(Expression<Func<T, object>> property, string value)
    {
        this.Property = property;
        this.Value = value;
    }
}

The other classes are not really interesting so i won't post them here.

In the controller, we use these classes like that :

public ActionResult Index(string search = null, string sort = null, int order = 1, int take = 10, int page = 1)
    {
        ICollection<Person> people = prismaManager.PersonManager.Search(search);
        ICollection<Column<Person>> columns= new List<Column<Person>>();
        columns.Add(new Column<Person>(Person => Person, "Person"));
        columns.Add(new Column<Person>(Person => Person.LastMembershipApproval, "Last Membership approval"));
        Table<Person> table = people.ToTable(columns);
    }

We are now writing a helper that will display the table correctly. It works well for the header but we face a problem with the Expressions when we want to use the @Html.DisplayFor() helper.

This is what we currently have for the content :

private static string TableRows<T>(HtmlHelper<Table<T>> helper, Table<T> table) where T : IPrivateObject
    {
        StringBuilder sb = new StringBuilder();
        foreach (var item in table.Items)
        {
            sb.AppendLine("<tr>");
            foreach (var column in table.Columns)
            {
                sb.AppendLine("<td>");
                sb.AppendLine(helper.DisplayFor(obj => ??? ).ToString()); // How should I use the Expression that is stored in the column but for the current element ?
                sb.AppendLine("</td>");
            }
            sb.AppendLine("</tr>");
        }
        return sb.ToString();
    }

For this to work, we should set the value of the "Person" parameter from the Expression stored in the column to the current item.

new Column<Person>(Person => Person, "Person"));

How are we supposed to do that ? Should we (if it is possible) modify the expression to set the value ? Should we recreate a new Expression using the old one as a basic expression ?

I've been searching for 3 days now and I can't find any answers.

Thanks for your help.

UPDATE :

The problem is (as @Groo & @Darin Dimitrov said) that the Helper is of type HtmlHelper> and not HtmlHelper. Any idea how I could get an HtmlHelper from a HtmlHelper> ?

UPDATE :

Person class is as following :

public class Person : IPrivateObject
{
    public int Id { get; set; }
    public int? AddrId { get; set; }

    [DisplayName("First Name")]
    [StringLength(100)]
    [Required]
    public string FirstName { get; set; }

    [DisplayName("Last Name")]
    [StringLength(100)]
    [Required]
    public string LastName { get; set; }

    [DisplayName("Initials")]
    [StringLength(6)]
    public string Initials { get; set; }

    [DisplayName("Last membership approval")]
    public Nullable<DateTime> LastMembershipApproval { get; set; }

    [DisplayName("Full name")]
    public string FullName
    {
        get
        {
            return FirstName + " " + LastName;
        }
    }
    public override string ToString()
    {
        return FullName;
    }
}
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3 Answers 3

Reset to default
4

Here's how you could proceed. Start by writing a custom view data container implementation which could be as simple as:

public class ViewDataContainer : IViewDataContainer
{
    public ViewDataContainer(ViewDataDictionary viewData)
    {
        ViewData = viewData;
    }

    public ViewDataDictionary ViewData { get; set; }
}

and then just instantiate a HtmlHelper<T> which is what you need:

private static string TableRows<T>(HtmlHelper<Table<T>> helper, Table<T> table) where T : IPrivateObject
{
    var sb = new StringBuilder();
    sb.AppendLine("<table>");
    foreach (var item in table.Items)
    {
        sb.AppendLine("<tr>");
        foreach (var column in table.Columns)
        {
            var viewData = new ViewDataDictionary<T>(item);
            var viewContext = new ViewContext(
                helper.ViewContext.Controller.ControllerContext,
                helper.ViewContext.View,
                new ViewDataDictionary<T>(item),
                helper.ViewContext.Controller.TempData,
                helper.ViewContext.Writer
            );
            var viewDataContainer = new ViewDataContainer(viewData);
            var itemHelper = new HtmlHelper<T>(viewContext, viewDataContainer);

            sb.AppendLine("<td>");
            sb.AppendLine(itemHelper.DisplayFor(column.Property));
            sb.AppendLine("</td>");
        }
        sb.AppendLine("</tr>");
    }
    sb.AppendLine("</table>");
    return sb.ToString();    
}

UPDATE:

The previous example doesn't handle value types because the expression in the column is of type Expression<Func<T, object>> and when you are pointing to a value type property the value will be boxed and ASP.NET MVC doesn't allow such expressions to be used with the template helpers. To remedy this problem one possibility is to test whether the value was boxed and extract the actual type:

sb.AppendLine("<td>");
var unary = column.Property.Body as UnaryExpression;
if (unary != null && unary.NodeType == ExpressionType.Convert)
{
    var lambda = Expression.Lambda(unary.Operand, column.Property.Parameters[0]);
    sb.AppendLine(itemHelper.Display(ExpressionHelper.GetExpressionText(lambda)).ToHtmlString());
}
else
{
    sb.AppendLine(itemHelper.DisplayFor(column.Property).ToHtmlString());
}
sb.AppendLine("</td>");
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14 Comments

Thanks for your help. It works well for the Person => Person expression. But It doesn't for properties on the Person... I sill get the exception : "Templates can be used only with field access, property access, single-dimension array index, or single-parameter custom indexer expressions." when comes the turn to the expression "Person => Person.LastMembershipApproval" to be evaluated... Any idea why ?
@Whoami, weird, it worked when I tested it. If you send me a sample project illustrating the problem I might be able to help you.
I will add the Person class so you can check. UPDATE Class added.
,I'll do that. Will send that in a few minutes.
@Whoami, you could make the Column class take 2 generic arguments: Column<T, TValue> but as I said this might have impact on other parts of your design such as the Table class. Another possibility is to test whether column.Property is being boxed by trying to cast the body of the expression to a UnaryExpression and then testing if its node type equals ExpressionType.Convert. In this case you could simply use the Operand of this unary expression to get the actual lambda and build a new expression that you would pass to the Display helper...
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2

There are several things you should change.

  1. The thing that first surprised me is that your table both has a list of Columns and Rows. You should change the design to something like: a Table has a list of Rows, and each Row has a list of Columns (or vice versa).

    But this remark is less relevant. I guess a "Column" is something like a "Column definition" and contains no data, but in that case I see no point in having ICollection<Row<T>> instead of just ICollection<T>.

  2. Next, you probably want to store a delegate, like Func<T, object>, instead of an Expression<Func<T, object>>.

  3. Property should at least have private setters (or, even better, readonly backing fields). This is not something that you would want other parts of your code to change.

  4. Naming is very confusing IMHO. I would choose better property names. If I got you right, Value and Property should actually be called HeaderName and GetValue, respectively.

Having said all that, I would change Column to something like this:

public class Column<T> where T : IPrivateObject
{
    private readonly string _name;
    private readonly Func<T, object> _valueGetter;

    /// <summary>
    /// Gets the column name.
    /// </summary>
    public string Name
    {
        get { return _name; }
    }
            
    /// <summary>
    /// Gets the value of this column from the
    /// specified object.
    /// </summary>
    /// <param name="obj">The object.</param>
    /// <returns></returns>
    public object GetValueFrom(T obj)
    {
        return _valueGetter(obj);
    }

    public Column(string columnName, Func<T, object> valueGetter)
    {
        _name = columnName;
        _valueGetter = valueGetter;
    }
}

And then simply use this in your loop:

sb.AppendLine(column.GetValueFrom(item).ToString());
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8 Comments

Thanks for your quick reply ! But for the 1. I do not agree since the table is driven by the columns. The row is only here if there is something special to a row (e.g. Row with personId % 2 == 0 should be displayed in red or whatever). For the rest, I agree and I appreciate your reply. Thanks
@Whoami: I've updated the answer. I believe the last line is what you were asking.
Yeah that's what I was asking. I'll try that way and check if it can be used with the DisplayFor helper.
@Groo, because it won't compile. The DisplayFor expects Expression<Func<Table<T>, TValue>> whereas column.Property is Expression<Func<T, object>>.
@Whoami: Hm... Ok, I must admin haven't used HtmlHelper that much, so I got a bit confused. Here is the problem: if you are using HtmlHelper<Table<T>>, this means your model is of type Table<T>, which in turn means that your expression must be of type Expression<Table<T>, Tvalue>. In other words, it requires an expression tree which will get the required property out of your Table<T>, instead of Column<T>. I will rethink this and update when I sort it out.
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1

You need to compile the expression using expression.Compile() (you have the property expression in your column.Property). This will give you a delegate. You can pass the object there and get the value. You also will need to pass the person, or the T, to the helper method.

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4 Comments

Thanks for your reply but this doesn't work. It can't be use with the DisplayFor template since i get the following exception : Templates can be used only with field access, property access, single-dimension array index, or single-parameter custom indexer expressions.
Any idea how I could cast / instantiate a new HtmlHelper of type T so I can use it for the DisplayFor ?
oh! so you want to use it in a binding? you need the property name from the lambda expression?
so, you can't do this: sb.AppendLine(helper.DisplayFor(column.Property).ToString()); ?

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