Build dynamic LINQ queries from a string - Use Reflection? - entity-framework

Question

I have some word templates(maybe thousands). Each template has merge fields which will be filled from database. I don`t like writing separate code for every template and then build the application and deploy it whenever a template is changed or a field on the template is added!
Instead, I'm trying to define all merge fields in a separate xml file and for each field I want to write the "query" which will be called when needed. EX:
mergefield1 will call query "Case.Parties.FirstOrDefault.NameEn"
mergefield2 will call query "Case.CaseNumber"
mergefield3 will call query "Case.Documents.FirstOrDefault.DocumentContent.DocumentType"
Etc,
So, for a particular template I scan its merge fields, and for each merge field I take it`s "query definition" and make that request to database using EntityFramework and LINQ. Ex. it works for these queries: "TimeSlots.FirstOrDefault.StartDateTime" or
"Case.CaseNumber"
This will be an engine which will generate word documents and fill it with merge fields from xml. In addition, it will work for any new template or new merge field.
Now, I have worked a version using reflection.
public string GetColumnValueByObjectByName(Expression<Func<TEntity, bool>> filter = null, string objectName = "", string dllName = "", string objectID = "", string propertyName = "")
{
string objectDllName = objectName + ", " + dllName;
Type type = Type.GetType(objectDllName);
Guid oID = new Guid(objectID);
dynamic Entity = context.Set(type).Find(oID); // get Object by Type and ObjectID
string value = ""; //the value which will be filled with data from database
IEnumerable<string> linqMethods = typeof(System.Linq.Enumerable).GetMethods(BindingFlags.Static | BindingFlags.Public).Select(s => s.Name).ToList(); //get all linq methods and save them as list of strings
if (propertyName.Contains('.'))
{
string[] properies = propertyName.Split('.');
dynamic object1 = Entity;
IEnumerable<dynamic> Child = new List<dynamic>();
for (int i = 0; i < properies.Length; i++)
{
if (i < properies.Length - 1 && linqMethods.Contains(properies[i + 1]))
{
Child = type.GetProperty(properies[i]).GetValue(object1, null);
}
else if (linqMethods.Contains(properies[i]))
{
object1 = Child.Cast<object>().FirstOrDefault(); //for now works only with FirstOrDefault - Later it will be changed to work with ToList or other linq methods
type = object1.GetType();
}
else
{
if (linqMethods.Contains(properies[i]))
{
object1 = type.GetProperty(properies[i + 1]).GetValue(object1, null);
}
else
{
object1 = type.GetProperty(properies[i]).GetValue(object1, null);
}
type = object1.GetType();
}
}
value = object1.ToString(); //.StartDateTime.ToString();
}
return value;
}
I`m not sure if this is the best approach. Does anyone have a better suggestion, or maybe someone has already done something like this?
To shorten it: The idea is to make generic linq queries to database from a string like: "Case.Parties.FirstOrDefault.NameEn".

Answer 1

Your approach is very good. I have no doubt that it already works.
Another approach is using Expression Tree like #Egorikas have suggested.
Disclaimer: I'm the owner of the project Eval-Expression.NET
In short, this library allows you to evaluate almost any C# code at runtime (What you exactly want to do).
I would suggest you use my library instead. To keep the code:
More readable
Easier to support
Add some flexibility
Example
public string GetColumnValueByObjectByName(Expression<Func<TEntity, bool>> filter = null, string objectName = "", string dllName = "", string objectID = "", string propertyName = "")
{
string objectDllName = objectName + ", " + dllName;
Type type = Type.GetType(objectDllName);
Guid oID = new Guid(objectID);
object Entity = context.Set(type).Find(oID); // get Object by Type and ObjectID
var value = Eval.Execute("x." + propertyName, new { x = entity });
return value.ToString();
}
The library also allow you to use dynamic string with IQueryable
Wiki: LINQ-Dynamic