How can I cast an object to string and use it in a Linq query using Reflection

Question

There's the problem I made an Extension method to IQueryable and I want to get property name and value to do a select on it the thing is that I am using Expression tree and try to search rows by the use of Contain() method which is needs a parameter of type string to do the job. my code is doing well while I do the search over string properties but I want this method to do the search on other types like int, decimal, datetime and even booleans so I need to get the property cast it to string and search for the matching values.

what I tried so far is:

        private static readonly MethodInfo _tostring = typeof(Object).GetMethod("ToString") ?? throw new Exception("Cannot create Method");

        public static IQueryable<T> Search<T>(this IQueryable<T> items, string propertyName, string filterValue)
        {
            MethodInfo _compare =
                (((Expression<Func<string, bool>>)(s => s.Contains("aa"))).Body as MethodCallExpression ?? throw new Exception("Cannot Create Method"))
                .Method;
            var property = typeof(T).GetProperty(propertyName) ?? throw new Exception("Couldn't Get the property");
            var row = Expression.Parameter(typeof(T), "row");

            
            Expression prop = Expression.Property(row, property);
            // now making sure if the type is string
            if (property.PropertyType != typeof(string))
            {
                //Here I want to cast it to string but the problem is exactly here anything I try feels like a dead end
                prop = Expression.Call(prop, _tostring);
            }

            var func =
                Expression.Lambda<Func<T, bool>>
                (
                    Expression.Call
                    (
                        prop,
                        _compare,
                        Expression.Constant(filterValue)
                    ),
                    row
                );

            return items.Where(func);
        }

Add details which ORM do you use, version and sample of usage. I have answered a tons of such questions and it is easy to correct of your Expression Tree. — Svyatoslav Danyliv
– Svyatoslav Danyliv, Commented Nov 29, 2022 at 8:19
Your problem is you are attempting to call Object.ToString instead of the correct {PropertyType}.ToString that matches the property's type. You need to find the ToString to call dynamically based on the type of the property e.g. var _tostring = property.PropertyType.GetMethod("ToString"). — NetMage
– NetMage, Commented Nov 29, 2022 at 20:34
I'm using EF core 7 by the way I got help from some other guy. lack of experience in Expression trees. I updated the code with the correct solution. — Ashkan Khalaj
– Ashkan Khalaj, Commented Dec 1, 2022 at 8:58
@NetMage There's nothing wrong with that call. I should have used polymorphism there was a small mistake now it's completely functional. — Ashkan Khalaj
– Ashkan Khalaj, Commented Dec 1, 2022 at 9:04

Charles Han · Accepted Answer · 2022-11-30 02:49:09Z

1

I tried and tested it with some dummy objects; it seems to work.

I refined my answer to use ToString

var prop = Expression.Property(row, property);

Expression expression = prop;

if (prop.Type != typeof(string))
{
   expression = Expression.Call(prop, "ToString", Type.EmptyTypes);
}

var func = Expression.Lambda<Func<T, bool>>
(
     Expression.Call
     (
       expression,
       _compare,
       Expression.Constant(filterValue)
     ),
     row
);

The above expression tree will be compiled to,

If the property type is not a string,

.Where(s => s.property.ToString().Contains("filter")))

If it's a string type,

.Where(s => s.property.Contains("filter")))

edited Nov 30, 2022 at 2:49

answered Nov 29, 2022 at 2:39

Charles Han

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6 Comments

Ashkan Khalaj Over a year ago

Unfortunately it's not working it throws runtime exception rather than compile time exception.The LINQ expression 'DbSet<Product>()\r\n .Where(f => ((object)f.Name).ToString().Contains(\"Pro\"))' could not be translated. Additional information: Translation of method 'object.ToString' failed.

Charles Han Over a year ago

I see, you are using this for EF queries. This only works for POCO. I will take another look. The error says the LINQ cannot be translated to SQL query.

Charles Han Over a year ago

I did some search, and it turns out that if you want to use LINQ to SQL, EF does not support toString() see github.com/dotnet/efcore/issues/6264

NetMage Over a year ago

@CharlesHan Actually that issue says it was fixed in EF Core 2

Charles Han Over a year ago

Did you try EF Core 7? you are right that it seems to support ToString() learn.microsoft.com/en-us/ef/core/providers/sql-server/…

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