History: Ray query with MOGRE

//-- set params --

 Vector3 startPosition = new Vector3(0, 0, 0); 
 Vector3 rayDirection = new Vector3(0, -1, 0);      // here: ray into depth
        
 // OR use position and orientation of a SceneNode
 //SceneNode someNode = mScene.Smgr.GetSceneNode("name_of_node");
 //Vector3 startPosition = someNode._getDerivedPosition;
 //Vector3 rayDirection = someNode._getDerivedOrientation.XAxis;  // in this case: base quaternion direction is along XAxis

 Ray myRay = new Ray(startPosition, rayDirection);
 RaySceneQuery raySQuery = mScene.Smgr.CreateRayQuery(myRay);

 // optionally sort the results
 raySQuery.SetSortByDistance(true);  // second possible parameter: maxResults

//-- calculate --

 Vector3 rayHitPoint;
 Single  rayDistance;
 String  rayHitPointName = "";

 RaySceneQueryResult raySQResult = raySQuery.Execute();
 RaySceneQueryResultEntry raySQREntry;

 for (Int16 i = 0; i < raySQResult.Count; i++)  // for all hits
 {
     raySQREntry = raySQResult[i];
     rayDistance = raySQREntry.distance;
     rayHitPoint = myRay.Origin + (myRay.Direction * rayDistance); // calculate hit point

     // save names of objects (or world fragment) that was hit
     if (raySQREntry.worldFragment != null)
         rayHitPointName = raySQREntry.worldFragment.ToString();
     if (raySQREntry.movable != null)
         rayHitPointName = raySQREntry.movable.Name;

     // Also bounding boxes of terrains can be hit.
     // Skip them to prevent hits on strange places ("just in the air").
     if (rayHitPointName.StartsWith("tile["))  // e.g. "tile[1][1,1]"
         continue;

     // ... do something with the hit ...

     if (raySQREntry.worldFragment != null)
     {
         // ... only do something with hits to terrain
     }

     if (raySQREntry.movable != null)
     {
         // ... only do something with hits to other objects
     }

     // If you just want to find a special hit, you can leave the for() loop.
     // The results will be stored in the variables (rayHitPoint, raySQREntry etc.)
     if (this_is_your_hit)
         break;

 } // for

//-- if nothing was hit --

 if (raySQResult.IsEmpty)
 {
     // ... do something ...

     // e.g. prevent null values
     rayHitPoint = Vector3.ZERO;
     rayDistance = 0;
     rayHitPointName = "no_hitPoint";
 }

(put this enum in a shared class)
[FlagsAttribute] // optional, but it makes clear, that this is a flag enum.
enum QueryMasks : uint //Depends on your needs, but I need a lot of flags and bit shifting needs big numbers (int = 32bit = 32 flags, byte = 8 bit = 8 flags, ...).
{
Ground = 1<<0, //bit shift required (2,4,8,16,32,64,128,256,...) 
Wall = 1<<1
[...]
}

(somewhere in your code, e.g. in CreateScene())

Entity Groundent = (...);
Groundent.QueryFlags = QueryMask.Ground; //assign a custom query flag

(Somewhere in FrameStarted or similar)

static CollisionTools ct = new CollisionTools(MySceneMgr);
//C# does not support static variables in methods (VB.Net does ;) ), make a global var instead
var Result = ct.RaycastFromCamera(RenderWindow, Camera, ScreenPosition, QueryMask.Ground); //search for a specific query flag
if (Result != null) 
{
 //success, this has to be an entity with the Mask "Ground".
}