C# using statement more understanding

Question

Can someone help me to understand following code

public Font MyFont { get; set; }

void AssignFont()
{
    using (Font f = new Font("Shyam",2))
    {
        this.MyFont = f;
    }
}

Is it valid to assign a disposing object to MyFont property?

Answer 1

Whilst it may be "valid to assign a disposed object to MyFont property", the object may no longer be useful because it may have released managed and/or unmanaged resources. Objects that are instantiated in the opening of a using statement indicate that the object's underlying class realises the IDisposable interface. This should be treated as a warning sign that when the object is disposed you should really stop interacting with it including keeping a reference to it.

In the case of a font, the font's underlying resources were disposed when you exited the using block:

using (Font f = new Font("Shyam",2))
{
    this.MyFont = f;
}

This is easily proven if you attempt to use the font in any drawing operation.

This code will fail with a System.ArgumentException because the font is disposed:

public partial class UserControl1 : UserControl
{
    public UserControl1()
    {
        InitializeComponent();
    }

    private void UserControl1_Load(object sender, EventArgs e)
    {
        if (DesignMode)
        {
            return;
        }

        AssignFont();
    }

    #region Overrides of Control

    /// <summary>
    /// Raises the <see cref="E:System.Windows.Forms.Control.Paint"/> event.
    /// </summary>
    /// <param name="e">A <see cref="T:System.Windows.Forms.PaintEventArgs"/> that contains the event data. </param>
    protected override void OnPaint(PaintEventArgs e)
    {
        try
        {
            var g = e.Graphics;

            g.FillRectangle(Brushes.White, e.ClipRectangle);

            g.DrawString("Hi there", MyFont, Brushes.Black, 0, 0); // <--- this will fail
        }
        catch (Exception ex)
        {
            Trace.TraceError(ex.Message);
        }
    }

    #endregion

    void AssignFont()
    {
        using (Font f = new Font("Shyam", 2))
        {
            this.MyFont = f;
        } // <---- MyFont now points to a disposed object
    }

    public Font MyFont { get; set; }
}

The problem with your code is that you are allocating something in a using block and keeping a reference to it elsewhere. In your scenario because you want to use the font elsewhere it makes no sense to have a using block.

Bad:

using (Font f = new Font("Shyam",2))
{
    this.MyFont = f;
}

Better:

this.MyFont = new Font("Shyam",2)

Fonts I suspect make use of native fonts hence the resource.