How to pass a user-input string as a field name to access a field within an object?

Question

I'm doing a little bit of search from a database using linq. I have multiple column names like country, name, phone number...

Now I've created a dropdownlist and pass the user selected data as a parameter "searchedField" to my controller method. Now if I take the input of a "country", I expect the code to be

    entries = entries.Where(s => s.country.Contains(searchString));

If user selected "name"

    entries = entries.Where(s => s.name.Contains(searchString));

Excuse me for this rather unreasonable example, since I can always just copy lines and make cases, but I wonder if there is a way to utilize things like reflection to convert string to "code" to access a field?

    String searchedField = "name"

    ...

    entries = entries.Where(s => s.searchedField.Contains(searchString));

This is my first question here, thanks!

You'll need reflection for this, and specifically GetMethod(): msdn.microsoft.com/en-us/library/6hy0h0z1(v=vs.110).aspx (you'll also need GetField and GetProperty, if memory serves me well) — Nick Udell
– Nick Udell, Commented Jun 26, 2014 at 9:19
possible duplicate of Linq dynamic queries for user search screens — Gert Arnold
– Gert Arnold, Commented Jun 29, 2014 at 9:02

Maarten · Accepted Answer · 2014-06-26 09:19:52Z

3

You can use Dynamic Linq.

entries = entries
    .Where(
        string.Format(
            "{0} = '{1}'",
            searchedField,
            searchString
        )
    );

Note: depending on the type of field you'll need to add quotes, or not.

answered Jun 26, 2014 at 9:19

Maarten

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Sean · Accepted Answer · 2014-06-26 09:22:56Z

1

You can do a reflection lookup (note: I've omitted error checking):

string GetPropertyAsString(object obj, string propertyName)
{
  var propertyInfo - obj.GetType().GetProperty(propertyName);
  return propertyInfo.GetValue(obj).ToString();
}

and then say

entries = entries.Where(s => GetPropertyAsString(s, searchedField).Contains(searchString));

answered Jun 26, 2014 at 9:22

Sean

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Comments

Raphaël Althaus · Accepted Answer · 2014-06-26 09:24:26Z

You can build an expression tree for this (which will work with Linq to entities).

public static class QueryableExtensions {
   public static IQueryable<T> Filter<T>(this IQueryable<T> queryable, string propertyName, string searchValue)
        {
            var type = typeof (T);
           //this will be the left part of the lambda
            var parameter = Expression.Parameter(type, "s");
            //s.country for example
            var property = Expression.Property(parameter, propertyName);
            //string.Contains method
            var containsMethod = typeof (string).GetMethod("Contains", new[] {typeof (string)});
            //s.country.Contains(<yoursearchvalue>)
            var expression = Expression.Call(property, containsMethod, Expression.Constant(searchValue));
            //s => s.country.Contains(<yoursearchvalue>)
            var lambda = Expression.Lambda<Func<T, bool>>(expression, parameter);
            //filter your queryable with predicate
            return queryable.Where(lambda);
        }
}

usage

var fieldtosearch = "country";
entries = entries.Filter(fieldtosearch , searchString);

Rex · Accepted Answer · 2014-06-26 09:29:12Z

In general, other than using dynamic linq queries, you could also use Expression to build a generic method to construct a property Getter and use it in your linq query, a quick sampel:

    public static Func<TObject, TProperty> GetPropGetter<TObject, TProperty>(string propertyName)
    {
        ParameterExpression paramExpression = Expression.Parameter(typeof(TObject), "value");
        Expression propertyGetterExpression = Expression.Property(paramExpression, propertyName);
        Func<TObject, TProperty> result =
            Expression.Lambda<Func<TObject, TProperty>>(propertyGetterExpression, paramExpression).Compile();
        return result;
    }

to use it:

        var getter = GetPropGetter<YourEntity, string>("Name");
        var found = entites.Where(m => getter(m).Contains("someInput"));

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