Querying Entity with LINQ using Dyanmic Field Name

Question

I have created a dynamic search screen in ASP.NET MVC. I retrieved the field names from the entity through reflection so that I could allow the user to choose which fields

Answer 1

After a lot more trial and error and searching I accidentally found another SO post that covers the same issue:

InvalidOperationException: No method 'Where' on type 'System.Linq.Queryable' is compatible with the supplied arguments

Here is my modified code that works:

        IQueryable query = entities.People;
        Type[] exprArgTypes = { query.ElementType };

        string propToWhere = "FirstName";            

        ParameterExpression p = Expression.Parameter(typeof(People), "p");
        MemberExpression member = Expression.PropertyOrField(p, propToWhere);
        LambdaExpression lambda = Expression.Lambda<Func<People, bool>>(Expression.Equal(member, Expression.Constant("Scott")), p);                            

        MethodCallExpression methodCall = Expression.Call(typeof(Queryable), "Where", exprArgTypes, query.Expression, lambda);

        IQueryable q = query.Provider.CreateQuery(methodCall);

With some hopefully pretty easy modifications, I should be able to get this to work with any type.

Thanks again for your answers Ani & John Bowen

Answer 2

  public List<People> SearchPeople(Dictionary<string, string> fieldValueDictionary)
        {
            return !fieldValueDictionary.Any()
                   ? entities.People 
                   : entities.People.Where(p => fieldValueDictionary.All(kvp => PropertyStringEquals(p, kvp.Key, kvp.Value)))
                                    .ToList();
        }

  private bool PropertyStringEquals(object obj, string propertyName, string comparison)
        {
            var val = obj.GetType().GetProperty(propertyName).GetValue(obj, null);
            return val == null ? comparison == null : val.ToString() == comparison; ;
        }

Answer 3

Have you tried getting the value from PropertyInfo?

entities.People.Where(p => (p.GetType().GetProperty(key).GetValue(p, null) as string) == fieldValueDictionary[key])