Question

So I have a query I’m constructing using Entity Framework, LINQ. Depending on the search parameter, and the type of the parameter being searched.

I have created a function like this:

public static IQueryable<T> WhereInt<T>(this IQueryable<T> query, string propertyName, string contains)
        {
            var parameter = Expression.Parameter(typeof(T), "type");
            var propertyExpression = Expression.Property(parameter, propertyName);

            MethodInfo method = typeof(string).GetMethod("First");
            var someValue = Expression.Constant(contains, typeof(string));
            var containsExpression = Expression.Call(propertyExpression, method, someValue);

            return query.Where(Expression.Lambda<Func<T, bool>>(containsExpression, parameter));

        }

Used as such:

WhereInt(DCs, searchfield, search);

searchfield could be userid, username, integer or string, and ideally I want to do:

searchfield.value == search.toInt32()

sort of feature.

I want to also make DateTime types searchable too. Maybe I’ll make a WhereDateTime for that.

I get an error on var containsExpression = Expression.Call(propertyExpression, method, someValue);

This works fine with strings, but fails with the error:

Value cannot be null.
Parameter name: method

Answer 1

There is no .First() method on an integer value. I really don’t know why you thought that would help you to generate an expression like searchfield.value == search.toInt32() (which doesn’t say First anywhere). Use an Equal expression instead.

I think you’re looking for something more along these lines.

public static IQueryable<T> WhereInt<T>(IQueryable<T> query, string propertyName, string value)
{
    var parameter = Expression.Parameter(typeof(T), "type");
    var propertyExpression = Expression.Property(parameter, propertyName);

    var someValue = Expression.Constant(int.Parse(value), typeof(int));
    var containsExpression = Expression.Equal(propertyExpression, someValue);

    return query.Where(Expression.Lambda<Func<T, bool>>(containsExpression, parameter));
}