round off a float number

Check this code

double f =8.25;
            double dresult;

            double dDecim = f - (int)f;
            if (dDecim > 0.5)
                dresult = Math.Round(f, 0);
            else
                dresult = Math.Round(f, 1);

I don't think you really need rounding. It is rarely needed and can be dangerous. This is a typical situation.
Much more likely that you need a rounded string presentation of a number to show in UI, and this is completely different thing, which is really needed very often.

So, you need to use

string.Format<code> with appropriate format string.<br />
See:<br />
<a href="http://msdn.microsoft.com/en-us/library/system.string.format.aspx">http://msdn.microsoft.com/en-us/library/system.string.format.aspx</a>[<a href="http://msdn.microsoft.com/en-us/library/system.string.format.aspx" target="_blank" title="New Window">^</a>],<br />
<a href="http://msdn.microsoft.com/en-us/library/dwhawy9k.aspx">http://msdn.microsoft.com/en-us/library/dwhawy9k.aspx</a>[<a href="http://msdn.microsoft.com/en-us/library/dwhawy9k.aspx" target="_blank" title="New Window">^</a>],<br />
<a href="http://msdn.microsoft.com/en-us/library/0c899ak8.aspx">http://msdn.microsoft.com/en-us/library/0c899ak8.aspx</a>[<a href="http://msdn.microsoft.com/en-us/library/0c899ak8.aspx" target="_blank" title="New Window">^</a>].<br />
<br />
<dd>—SA</dd>