Question

When I pass call a Generic Method that return A Func and pass in parameter of the Where, that’s dosen’t work. (System.InvalidOperationException: Internal .NET Framework Data Provider error 1025.)
The error is when I want to get the Role information.
For the Role, I need to perform a Where Clause Expression EX: (p => p.LangID == 1)

This code dosen’t Work

In the repository

public Func<T, bool> GetLmbLang<T>() where T:class,IBaseGenericTxt
    {
        int lang = -1;
        lang = Convert.ToInt32(HttpContext.Current.Session["Language"]);
        return (p => p.LangID == lang);
    }

In the controller

                var ViewModel = _db.Contacts.Where(a=> a.IsActive == true).Select(a => new ContactListViewModel { 
                    ContactID = a.ContactID,
                    ContactName = a.ContactName,
                    Role = a.ContactType.ContactTypeTexts.Where(repGeneric.GetLmbLang<ContactTypeText>()).Select(af => af.Txt).FirstOrDefault(),
                    CompanyType = a.Supplier.SupplierName,
                    Addr = a.Address ,
                    Email = a.ContactEmail,
                    Phone = a.ContactPhone
                }).ToList();
                for (int i = 0; i < ViewModel.Count(); i++)
                {
                    Response.Write(ViewModel.ElementAt(i).ContactID + "<br />");
                }

This code WORK

 int lang = -1;
            lang = Convert.ToInt32(Session["Language"]);
            var ViewModel = _db.Contacts.Where(a=> a.IsActive == true).Select(a => new ContactListViewModel { 
                ContactID = a.ContactID,
                ContactName = a.ContactName,
                Role = a.ContactType.ContactTypeTexts.Where(p => p.LangID == lang).Select(af => af.Txt).FirstOrDefault(),
                CompanyType = a.Supplier.SupplierName,
                Addr = a.Address ,
                Email = a.ContactEmail,
                Phone = a.ContactPhone
            }).ToList();
            for (int i = 0; i < ViewModel.Count(); i++)
            {
                Response.Write(ViewModel.ElementAt(i).ContactID + "<br />");
            }

My ContactListViewModel

public class ContactListViewModel
    {
        public int ContactID { get; set; }
        public string ContactName { get; set; }
        public string Role { get; set; }
        public string Company { get; set; }
        public string CompanyType { get; set; }
        public Address Addr { get; set; }
        public string Email { get; set; }
        public string Phone { get; set; }
    }

My List View

 ..... Inherits="System.Web.Mvc.ViewPage<List<mvcinfosite.ViewModels.ContactListViewModel>>" %>
 <table class="genTable">


    <% for (int i = 0;i < Model.Count; i++) { %>
        <tr>
            <td>
                <%: Html.ActionLink(item.ContactName, "Edit", new { id=item.ContactID }) %>
            </td>

            <td>
                <%: item.Role  %>
            </td>

            <td>
                <%: item.Company %>
            </td>

            <td>
                <%: item.CompanyType  %>
            </td>

            <td>
                <%: GlobalHelper.GetAddress(item.Addr) %>
            </td>
            <td>
                <%: item.Email %>
            </td>
            <td>
                <%: item.Phone %>
            </td>
        </tr>

    <% } %>

    </table>

Answer 1

As naasking points out, you need to use an Expression of a Func instead of a straight Func:

public Expression<Func<T, bool>> GetLmbLang<T>() where T:class,IBaseGenericTxt
{
    int lang = -1;
    lang = Convert.ToInt32(HttpContext.Current.Session["Language"]);
    return (p => p.LangID == lang);
}

Edit

Ah, yes, well the problem is that your function doesn’t actually know what class it’s working with at compile time: it only knows that it’s a class, and it implements IBaseGenericTxt. So when you say p.LangId, that part of the expression is calling IBaseGenericTxt.LangId, and not ContactTypeText.LangId.

You’ll need to build your own expression tree in order to get this to work right. Something like this:

var paramExpr = Expression.Parameter(typeof(T), "p");
return Expression.Lambda<Func<T, bool>>(
    Expression.Equal(
        Expression.Property(paramExpr, "LangId"),
        Expression.Constant(lang)),
    paramExpr);

Edit 2

Two things:

1. Because LINQ to Entities will try to take anything in a query expression and convert it to a SQL statement, you have to be careful not to go calling methods in the middle of your query. You’ll want to call the GetLmbLang method first and store its value in a variable to use in the query.

2. As you point out in your comment, because the ContactTypeTexts property does not implement IQueryable, this gets particularly tricky. You have three options as far as I can tell:

   1. Create your entire select statement as an expression tree. This is very annoying and error-prone.
   2. Use Joe Albari’s LinqKit to “Compile” and “Expand” your query. LinqKit will traverse the expression tree and build a new tree wherein your query Expression is converted to its equivalent Func.
   3. Go back to your data context rather than using the ContactTypeTexts property.

Personally, I would probably go with the last option, like this:

var lambdaLang = repGeneric.GetLmbLang<ContactTypeText>();
var ViewModel = _db.Contacts
    .Where(a=> a.IsActive == true)
    .Select(a => new ContactListViewModel { 
    ContactID = a.ContactID,
    ContactName = a.ContactName,
    Role = _db.ContactTypeTexts
        .Where(ct => ct.ContactType.Contacts.Any(
            c => c.ContactId == a.ContactId)
        .Where(lambdaLang)
        .Select(af => af.Txt).FirstOrDefault(),
    CompanyType = a.Supplier.SupplierName,
    Addr = a.Address ,
    Email = a.ContactEmail,
    Phone = a.ContactPhone
}).ToList();