How to get a variable replaced with a field name in a LINQ? - asp.net

Question

string companyName="ABC";
var query = from q in context.Company where q.CompanyName == companyName select q;
Is there any way to replace the q.CompanyName part of the query with a string variable
so that the field used for filtering be a parametric?
I tried
string str1 = "companySize";
string str2 = "q." + str1;
string companySize = "Mid";
var query = from q in context.Company where str2 == companySize select q;
Didn't work.
Been trying to let the user choose the columns for the query.

Answer 1

Read more about both below option at : Dynamic query with Linq
you can use one of this
Use Dynamic LINQ library
Example for the the blog below
string strWhere = string.Empty;
string strOrderBy = string.Empty;
if (!string.IsNullOrEmpty(txtAddress.Text))
strWhere = "Address.StartsWith(\"" + txtAddress.Text + "\")";
if (!string.IsNullOrEmpty(txtEmpId.Text))
{
if(!string.IsNullOrEmpty(strWhere ))
strWhere = " And ";
strWhere = "Id = " + txtEmpId.Text;
}
if (!string.IsNullOrEmpty(txtDesc.Text))
{
if (!string.IsNullOrEmpty(strWhere))
strWhere = " And ";
strWhere = "Desc.StartsWith(\"" + txtDesc.Text + "\")";
}
if (!string.IsNullOrEmpty(txtName.Text))
{
if (!string.IsNullOrEmpty(strWhere))
strWhere = " And ";
strWhere = "Name.StartsWith(\"" + txtName.Text + "\")";
}
EmployeeDataContext edb = new EmployeeDataContext();
var emp = edb.Employees.Where(strWhere);
Predicate Builder
EXample
var predicate = PredicateBuilder.True<employee>();
if(!string.IsNullOrEmpty(txtAddress.Text))
predicate = predicate.And(e1 => e1.Address.Contains(txtAddress.Text));
if (!string.IsNullOrEmpty(txtEmpId.Text))
predicate = predicate.And(e1 => e1.Id == Convert.ToInt32(txtEmpId.Text));
if (!string.IsNullOrEmpty(txtDesc.Text))
predicate = predicate.And(e1 => e1.Desc.Contains(txtDesc.Text));
if (!string.IsNullOrEmpty(txtName.Text))
predicate = predicate.And(e1 => e1.Name.Contains(txtName.Text));
EmployeeDataContext edb= new EmployeeDataContext();
var emp = edb.Employees.Where(predicate);

Answer 2

If you don't want to use libraries like dynamicLINQ, you can just create the Expression Tree by yourself:
string str1 = "companySize";
string str2 = "q." + str1;
string companySize = "Mid";
var param = Expression.Parameter(typeof(string));
var exp = Expression.Lambda<Func<Company, bool>>(
Expression.Equal(
Expression.Property(param, str1),
Expression.Constant(companySize)),
param);
var query = context.Company.Where(exp);

Answer 3

I think the best way to do this is with built in libraries (and PropertyDescriptor type).
using System.ComponentModel;
void Main()
{
Test test = new Test();
test.CompanyName = "ABC";
object z = TypeDescriptor.GetProperties(test).OfType<PropertyDescriptor>()
.Where(x => x.Name == "CompanyName").Select(x => x.GetValue(test)).FirstOrDefault();
Console.WriteLine(z.ToString());
}
public class Test
{
public string CompanyName { get; set; }
}