update.cs

protected override void Draw(GameTime gameTime)
{
    GraphicsDevice.Clear(Color.CornflowerBlue);

    if (_testRenderTarget)
    {
        _spriteBatch.Begin(SpriteSortMode.Immediate);
        GraphicsDevice.SetRenderTarget(_renderTarget);
        _spriteBatch.Draw(_guiTexture, new Rectangle(0, 0, 64, 64), new Rectangle(0, 0, 64, 64), Color.White);
        _spriteBatch.End();

        _spriteBatch.Begin();
        GraphicsDevice.SetRenderTarget(null);
        _spriteBatch.Draw(_renderTarget, Vector2.Zero, Color.White);
        _spriteBatch.End();
    }
    else
    {
        _spriteBatch.Begin();
        GraphicsDevice.SetRenderTarget(null);
        _spriteBatch.Draw(_guiTexture, new Rectangle(0, 0, 64, 64), new Rectangle(0, 0, 64, 64), Color.White);
        _spriteBatch.End();
    }


    base.Draw(gameTime);
}