c# - How do I fix my platformer collision detection? -

i'm working on game allows 2d sprite jump on platform when lands on platform, 2d sprite not on top of platform, exceeding or same position of platform.

//here code:  texture2d jumper; texture2d tile; vector2 position, velocity; rectangle top; rectangle tile_rectangle; rectangle jumper_rectangle; keyboardstate ks;  bool jump;  //here load content:  protected override void loadcontent() {     jump = true;     position.x = 10;     position.y = 10;      jumper = content.load<texture2d>("character");     tile = content.load<texture2d>("tile");      tile_rectangle = new rectangle(300, 350, tile.width, tile.height);     top = new rectangle(tile_rectangle.x, tile_rectangle.y - 16, tile_rectangle.width, 10); }  //here update:  protected override void update(gametime gametime) {     ks = keyboard.getstate();     position += velocity;      float = 1;      if (ks.iskeydown(keys.up) && jump == false)     {         position.y -= 10f;         velocity.y = -25f;         jump = true;     }      if (jump == true)     {          velocity.y += 2f * i;     }      if (position.y > 400)     {          jump = false;     }      if (jump == false)     {         velocity.y = 0f;     }      boundingbox();      if (ks.iskeydown(keys.right))     {         position.x += 5f;         velocity.x = +0.05f;          if (jumper_rectangle.left > tile_rectangle.right)         {             jump = true;         }     }      if (ks.iskeydown(keys.left))     {         position.x -= 5f;         velocity.x = -0.05f;          if (jumper_rectangle.right < tile_rectangle.left)         {             jump = true;         }     }     jumper_rectangle = new rectangle((int)position.x, (int)position.y, jumper.width, jumper.height);     boundingbox();     base.update(gametime);  }  //here draw:  protected override void draw(gametime gametime) {     graphicsdevice.clear(color.cornflowerblue);      spritebatch.begin();     spritebatch.draw(jumper, jumper_rectangle, color.white);     spritebatch.draw(tile, tile_rectangle, color.white);     spritebatch.end();      base.draw(gametime); }  //here method on landing:  public void boundingbox() {     if (jumper_rectangle.intersects(top))     {         if (jumper_rectangle.bottom > top.top)         {             position.y--;             jump = false;         }     } } 

where did go wrong or there other way?