在实体框架中对通用类型进行文本搜索

Question

I'm trying to implement a text search for database tables. I've a generic repository and don't really want to have to create derived ones for every model I might want to expose because there are quite a few in the database.

So the code I'm having difficulty with is as follows:

var props = typeof(T).GetProperties()
    .Where(p => p.PropertyType == typeof(string));

IEnumerable<T> searched = null;
if (!string.IsNullOrWhiteSpace(searchTerm))
    searched = sorted.Where(c => props
        .Select(p => (string)p.GetValue(c, null))
        .Select(v => v.Contains(searchTerm))
        .Contains(true));

I'm feeding this a collection of PropertyInfo's obtained through a little reflection. Possibly not a high performance idea but I've yet to think of a better way. So these might be all properties of type string (searching all strings in the table) or it might be pulling certain properties in the model that have a custom Searchable attribute.

The runtime exception I'm getting is:

NotSupportedException : Unable to create a constant value of type 'System.Reflection.PropertyInfo'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.

I can see that I'm using reflection but not quite sure what exactly is causing the exception here. If someone could point this out then that would be much appreciated but if someone could suggest a better way to do this then that would be amazing. Thanks in advance!

Answer 1

The problem is that when the LINQ query is executed, it's trying to construct a SQL query to perform on the database. The exception message indicates that only primitive types can be used in the LINQ query because those are the only types that can be converted successfully into a SQL query.

Hopefully solving your problem, you just need to ensure that the database SQL query executes before extending the LINQ query using non-primitive types.

I'm guessing the sorted variable in your code snippet is a LINQ query, so call sorted.AsEnumerable() to execute the SQL query on the database and then you can perform the search functionality.

searched = sorted.AsEnumerable()
                 .Where(c => props.Select(p => (string)p.GetValue(c, null))
                                  .Any(v => v.Contains(searchTerm)));