C# Linq Query on database using field name as a variable

Question

Here is my code. Can anyone help me refactor this piece of code. i'm trying to use field name as variable so I wouldn't have to use switch case. Something like from dr in db.Users where dr.FieldName.Contains(value).

public static List<User> GetUser(string value, string fieldName)
{
    var db = new CCPTDataContext();
    var users = new List<User>();

    switch (fieldName)
    {
        case "FirstName":
            users = (from dr in db.Users where dr.FirstName.Contains(value)
                     select new User
                     {
                         Username = dr.Username,
                         FirstName = dr.FirstName,
                         LastName = dr.LastName
                     }).ToList();

            break;
        case "LastName":
            users = (from dr in db.Users where dr.LastName.Contains(value)
                     select new User
                     {
                         Username = dr.Username,
                         FirstName = dr.FirstName,
                         LastName = dr.LastName
                     }).ToList();
            break;
    }
    return users;
}

Take a look at [this similar question](http://stackoverflow.com/questions/3682835/if-condition-in-linq-where-clause) — Wonko the Sane, Jan 16 '14 at 18:12

score 4 · Accepted Answer · answered Jan 16 '14 at 18:16

4

You have to create the Expression Tree manually using classes from System.Linq.Expression namespace:

public static Expression<Func<TSource, bool>> GetPropertyContainsValueExpression<TSource>(string propertyName, string value)
{
    var param = Expression.Parameter(typeof(TSource), "x");
    var prop = Expression.Property(param, propertyName);
    var valueExp = Expression.Constant(value, typeof(string));
    var contains = Expression.Call(prop, "Contains", null, valueExp);
    return Expression.Lambda<Func<TSource, bool>>(contains, param);
}

then you can use it like that:

public static List<User> GetUser(string value, string fieldName)
{
    var db = new CCPTDataContext();

    return db.Users.Where(GetPropertyContainsValueExpression<User>(fieldName, value)).ToList();
}

answered Jan 16 '14 at 18:16

MarcinJuraszek

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Some providers (I think EF 5) might not correctly parameterize the query in the above snippet. In order to get the parameterized queries, what I do is wrap the value in a `Tuple` and get the `Item1` property: `var container = new Tuple(value); var containerExpression = Expression.Constant(container, typeof(Tuple)); var valueExp = Expression.Property(containerExpression, "Item1");` – felipe Jan 16 '14 at 19:25
@Kosar you're welcome! You should consider marking the answer as accepted if it solved your problem. That's how SO works! – MarcinJuraszek Jan 16 '14 at 21:53
@Marcin, I tried to mark as accepted, couldn't find any link, button or something like that for it. Can you guide me how to do that. – Ali Jan 21 '14 at 05:23
@Kosar Right below post score the should be gray mark you can click. Here is a great question about that: http://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work – MarcinJuraszek Jan 21 '14 at 05:30

score 0 · Answer 2 · answered Jan 16 '14 at 18:13

This should work:

users = (from dr in db.Users 
where (fieldName.Equals("FirstName") && dr.FirstName.Contains(value))
      || (fieldName.Equals("LastName") && dr.LastName.Contains(value))
                 select new User
                 {
                     Username = dr.Username,
                     FirstName = dr.FirstName,
                     LastName = dr.LastName
                 }).ToList();

C# Linq Query on database using field name as a variable

2 Answers2