Rounding decimals to X positions

Another little extension method. This one allows easy rounding to a specific number of decimals:

double d = 0.66782423;
string s = d.ToString(3);  // s = “0.668”

This is done by the following method:

public static class DoubleExtension

{

    public static string ToString( this double value, int decimals )

    {

        StringBuilder format = new StringBuilder( "0" );

        if ( decimals > 0 )

        {

            format.Append( "." );

        }

        for ( int i = 0; i < decimals; i++ )

        {

            format.Append( "0" );

        }

        return value.ToString( format.ToString() );

    }

}