Performance issue with Reflection - asp.net-mvc

Question

I am trying to make a Dynamic search for EF Core. I made the whole thing in a loop like this:
foreach (var i in vm.SearchProperties)
{
if (result == null)
result = db.MyTable.Where(x => x.GetType().GetProperty(i).GetValue(x, null).ToString().ToLower().StartsWith("MySearchString"));
else
result = result.Where(x => x.GetType().GetProperty(i).GetValue(x,null).ToString().ToLower().StartsWith(i.Suchfeld.ToLower(“mySearchString”)));
}
Before I added the reflection part it runs pretty fast. As soon I added the Reflection to it, it got slowed down by a factor of 1000. Any ideas how I get it speeded up or around the reflection part.

Answer 1

Your expression is too complex to be interpreted by the LINQ to SQL converter, so it is being compiled and executed on every item in your table, so it's no surprise it executes exceedingly slow.
You need to build an expression tree based on which properties you want to search, then construct an Expression<Func<MyType, bool>> to pass to your Where(...) method. That way the LINQ to SQL converter recognizes it.
Try this:
ParameterExpression param = Expression.Parameter(typeof(MyType));
MethodInfo stringStartsWith = typeof(string).GetMethods().First(m => m.Name == "StartsWith" && m.GetParameters().Length == 1);
PropertyInfo firstProp = typeof(MyType).GetProperty(vm.SearchProperties.First());
MemberExpression firstMembAccess = Expression.Property(param, firstProp);
MethodCallExpression firstStartsWithExpr = Expression.Call(firstMembAccess, stringStartsWith, Expression.Constant(mySearchString));
Expression current = firstStartsWithExpr;
foreach (string s in vm.SearchProperties.Skip(1))
{
PropertyInfo prop = typeof(MyType).GetProperty(s);
MemberExpression membAccess = Expression.Property(param, prop);
MethodCallExpression startsWithExpr = Expression.Call(membAccess, stringStartsWith, Expression.Constant(mySearchString));
current = Expression.OrElse(current, startsWithExpr);
}
Expression<Func<MyType, bool>> mySearchExpression = Expression.Lambda<Func<MyType, bool>>(current, param);
result = db.MyTable.Where(mySearchExpression);
Note: MyType refers to whatever your entity type is.

Answer 2

As people already answered, reflection cannot be used in IQueryable.
Data are loaded in the memory and the Where clause is performed.
It's similar as if you have done the following code (adding ToList() before the where clause):
foreach (var i in vm.SearchProperties)
{
if (result == null)
result = db.MyTable.ToList().Where(x => x.GetType().GetProperty(i).GetValue(x, null).ToString().ToLower().StartsWith("MySearchString"));
else
result = result.ToList().Where(x => x.GetType().GetProperty(i).GetValue(x,null).ToString().ToLower().StartsWith(i.Suchfeld.ToLower(“mySearchString”)));
}
#Mr Anderson is right. You need to use expression tree instead of reflection.
However, using expression tree may be sometimes to much complex to build.
Disclaimer: I'm the owner of the project Eval-Expression.NET
This project allows you to evaluate and execute dynamic expression at runtime.
In short, you can pass a dynamic string to the where clause.
Wiki: Eval-Expression.NET - LINQ Dynamic
foreach (var i in vm.SearchProperties)
{
if (result == null)
result = db.MyTable.Where(x => "x." + i + ".ToLower().StartsWith('MySearchString')");
else
result = result.Where(x => "x." + i + ".ToLower().StartsWith('MySearchString')");
}