Question

What I tried:

MarkUP:

 <asp:TextBox ID="TextBox2"   runat="server"></asp:TextBox>

    <asp:Label ID="Label1" runat="server" AssociatedControlID="TextBox2"  Text="Label"></asp:Label>

    <asp:SliderExtender ID="SliderExtender1"  TargetControlID="TextBox2"  BoundControlID="Label1" Maximum="200" Minimum="100" runat="server">
    </asp:SliderExtender>

Code Behind:

protected void setImageWidth()
{
    int imageWidth;
    if (Label1.Text != null)
    {
        imageWidth = 1 * Convert.ToInt32(Label1.Text);
        Image1.Width = imageWidth;
    }
}

After running the page on a browser, I get the System.FormatException: Input string was not in a correct format.

Answer 1

The problem is with line

imageWidth = 1 * Convert.ToInt32(Label1.Text);

Label1.Text may or may not be int. Check.

Use Int32.TryParse(value, out number) instead. That will solve your problem.

int imageWidth;
if(Int32.TryParse(Label1.Text, out imageWidth))
{
    Image1.Width= imageWidth;
}