Make portion of a Label's Text to be styled bold

The following class illustrates how to do it by overriding OnPaint() in the Label class of WinForms. You can refine it. But what I did was to use the pipe character (|) in a string to tell the OnPaint() method to print text before the | as bold and after it as normal text.

class LabelX : Label
{
    protected override void OnPaint(PaintEventArgs e) {
        Point drawPoint = new Point(0, 0);

        string[] ary = Text.Split(new char[] { '|' });
        if (ary.Length == 2) {
            Font normalFont = this.Font;

            Font boldFont = new Font(normalFont, FontStyle.Bold);

            Size boldSize = TextRenderer.MeasureText(ary[0], boldFont);
            Size normalSize = TextRenderer.MeasureText(ary[1], normalFont);

            Rectangle boldRect = new Rectangle(drawPoint, boldSize);
            Rectangle normalRect = new Rectangle(
                boldRect.Right, boldRect.Top, normalSize.Width, normalSize.Height);

            TextRenderer.DrawText(e.Graphics, ary[0], boldFont, boldRect, ForeColor);
            TextRenderer.DrawText(e.Graphics, ary[1], normalFont, normalRect, ForeColor);
        }
        else {

            TextRenderer.DrawText(e.Graphics, Text, Font, drawPoint, ForeColor);                
        }
    }
}

Here's how to use it:

LabelX x = new LabelX();
Controls.Add(x);
x.Dock = DockStyle.Top;
x.Text = "Hello | World";       

Hello will be printed in bold and world in normal.

WinForms doesn't allow you to do that.

WebForms

Use Literal control, and add a <b> tag around the part of the text you want:

_myLiteral.Text = "Hello <b>big</b> world";

Winforms

Two options:

1. Put two labels side by side (far easier)
2. Subclass Label and do your own custom drawing in the OnPaint() method.

The second choice has been answered already.