Construct C# Lambda Expression With Reference To List Item

Question

I need to construct a dynamic lambda expression like: m => m.Data[0].Name

I have model classes:

public class GridItem
{
    [Required]
    [Display(Name = "Name")]
    public string Name{ get; set; }

    [Required]
    [Display(Name = "Address")]
    public string Password { get; set; }

    [Display(Name = "Age")]
    public string Age{ get; set; }
}

public class GridModelList
{
    public List<GridItem> Data { get; set; }

    public GridModelList()
    {
        Data = new List<GridItem>();
        Data.Add(new GridItem() {Name = "Name", Address= "Address", Age= 1 });
        Data.Add(new GridItem() {Name = "Name", Address= "Address", Age= 1 });
    }
}

And I am trying to consruct it like so:

ParameterExpression fieldName = Expression.Parameter(typeof(TGridModel), "m");
MemberExpression fieldExpr = Expression.Property(fieldName, "m.Data.getItem(0).Name");
Expression<Func<TGridModel, object>> exp = Expression.Lambda<Func<TGridModel, object>>(fieldExpr, fieldName);

However, my problem is that I don't know how to use '.getItem(0)' properly, as it gives me exception:

Instance property 'Data.get_Item(0).Name' is not defined for type 'GridModelList'

Any help on how to build the needed expression m => m.Data[0].Name would be greatly appreciated!!

Any reason you aren't just using an ordinary lambda e.g. Func<TGridModel, object> lambda = m => m.Data[0].Name; ? — Iridium
– Iridium, Commented Apr 8, 2014 at 12:52
Hi @Iridium, I have different 'GridModels' I have to deal with so I need to construct these expressions dynamically using reflection. The end goal is so I can use MVC HtmlHelper methods like TextboxFor — AmmarCSE
– AmmarCSE, Commented Apr 8, 2014 at 13:08

T.J. Crowder · Accepted Answer · 2015-10-04 07:14:01Z

3

You need use Expression.Property for index property like this

ParameterExpression fieldName = Expression.Parameter(typeof(TGridModel), "m");

var fieldDataExpr = Expression.Property(fieldName, "Data");
var fieldExpr = Expression.Property(fieldDataExpr, "Item", Expression.Constant(0));
var fieldNameExpr = Expression.Property(fieldExpr, "Name");

and then

Expression<Func<TGridModel, object>> exp = Expression.Lambda<Func<TGridModel, object>>(fieldNameExpr, fieldName);

edited Oct 4, 2015 at 7:14

T.J. Crowder

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answered Apr 8, 2014 at 12:51

Grundy

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1 Comment

AmmarCSE Over a year ago

Thanks @Grundy. I am new to lambda expressions and literally spent hours trying to get it right. Thanks again!

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