CODE EXAMPLE FOR CSHARP

how to draw a rectangle in monogame

 Texture2D rect = new Texture2D(graphics.GraphicsDevice, 80, 30);

 Color[] data = new Color[80*30];
 for(int i=0; i < data.Length; i++) data[i] = Color.Chocolate;
 rect.SetData(data);

 Vector2 coor = new Vector2(10, 20);
 spriteBatch.Draw(rect, coor, Color.White);