t3dpoint __fastcall TModelGroupHandler::GetPositionFromSpaceDepth(int X, int Y, HWND hwnd)
{
glClear(GL_COLOR_BUFFER_BIT);
glClear(GL_DEPTH_BUFFER_BIT);
  glViewport(0, 0, int( DISPLAY_WIDTH), int(DISPLAY_HEIGHT));
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  gluPerspectiveA(90.0, DISPLAY_WIDTH/DISPLAY_HEIGHT,0.10f, 1000.0f * 1000.0f * 1000.0f);
  glMatrixMode(GL_MODELVIEW);

glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
glPushMatrix();
glLoadIdentity();

gluLookAt(base->eyepos.x,base->eyepos.y,base->eyepos.z,
base->eyepointingpos.x,base->eyepointingpos.y,base->eyepointingpos.z,base->upvector.x,
base->upvector.y,base->upvector.z);



base->DrawDepthBillboard(last_depth); //default 100 meters away


TPoint mouse;
GetCursorPos(&mouse);
ScreenToClient(hwnd, &mouse);
t3dpoint depth_result;
depth_result = base->Reproduce_Mouse_coordinates(mouse.x,mouse.y);


glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);

glPopMatrix();
return depth_result;
}

t3dpoint __fastcall ModelGroupEditor::Reproduce_Mouse_coordinates(int x, int y)
{
t3dpoint res;
glGetDoublev( GL_PROJECTION_MATRIX, mprojection );
glGetDoublev( GL_MODELVIEW_MATRIX, mmodelview );
glGetIntegerv( GL_VIEWPORT, mviewport );

				  int wy = mviewport[3] -y;
	  double nx,ny,nz;      //  double fx,fy,fz;
  double	winX = double(x);
   double	winY = double(mviewport[3]) - double(y);
double winZ;
			  // glReadBuffer(GL_DEPTH_BUFFER_BIT);
float * pdata = new float;
float pk;



	glReadPixels(x,wy,1,1,GL_DEPTH_COMPONENT,GL_FLOAT,pdata);
	pk = (*pdata);
delete pdata;	
winZ = double(pk);


gluUnProject(winX, winY, winZ, mmodelview, mprojection, mviewport, &nx, &ny, &nz);

res.x = float(nx);
res.y = float(ny);
res.z = float(nz);

 return res;
}

if your code looks like this and you still have no results check your drawing code / or you just have float / double problem anyway this code works in C++

i gave you working code just port it to c#, these exe files don't work

lets see your code:

  void ikt_viewPort3d_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
        {
            unsafe
            {
                
                float winX, winY, winZ;
                ilkTikYapildi = true;
                Int32[] viewPort = new Int32[4];
                GL.glGetIntegerv(GL.GL_VIEWPORT, viewPort);
                double[] modelView = new double[16];
                GL.glGetDoublev(GL.GL_MODELVIEW_MATRIX, modelView);
                double[] projection = new double[16];
                GL.glGetDoublev(GL.GL_PROJECTION_MATRIX, projection);
                winX = eXPre = (float)e.X;
                winY = eYPre = (float)e.Y;
                winY = viewPort[3] - winY;
                byte[] byt = new byte[4];
                float[] flt=new float[4];
                uint[] pix=new uint[1];
                GL.glEnable(GL.GL_DEPTH_TEST);
                GL.glPixelStorei(GL.GL_UNPACK_ALIGNMENT, 1);// and why are you using it
                GL.glPixelStorei(GL.GL_PACK_ALIGNMENT, 1);// and why are you using it
                GL.glFinish();// and why are you using it
                GL.glReadPixels((int)winX, (int)winY, 1, 1, GL.GL_DEPTH_COMPONENT, GL.GL_FLOAT, byt);//wtf man
                winZ = System.BitConverter.ToSingle(byt, 0);//wtf man
   
             uint[] den1 = new uint[100];<-- since when float result is uint[100]

                GL.glReadPixels((int)winX, (int)winY, 1, 1, GL.GL_DEPTH_COMPONENT, GL.GL_FLOAT, den1);//here its okay but should be integers without cast
                winZ = den1[0];//no comments
          
     
                //now see this code below and compare it  with mine
                double objX, objY, objZ;
                GL.gluUnProject(winX, winY, aktifZoomDegeri, modelView, projection, viewPort, out objX, out objY, out objZ);
              why the f*** are you using here floats????????? and wtf is aktifzoomdegeri dont even tell me i dont want to know
            }

        }
    }

i told you to make it like mine but you use floats instead integers, floats instead doubles and aktifzoomdegeri is what? should be winZ (in your code it should be den1 cast to double since den1 should be float (make new variable and cast to it), i am tired of telling you this over and over and over again.