Convert.ToBoolean and Boolean.Parse don't accept 0 and 1

0 and (not-zero) are not equal to "false" and "true", they're just the representation chosen by C. Other languages use 0 for true and -1 for false, or other schemes entirely. A boolean is not a 0 or a 1, it's a true or a false.

Should it also handle "yes" and "no", "off" and "on", and all of the myriad other things that are analogous to booleans? Where would you draw the line?

What makes booleans special? They are essentially 0 as false, and non-zero as true in my experience...

That is an implementation detail, and isn't at all relevant.

true is a boolean value. false is a boolean value. Anything else is not.

If you want to parse something such that the string "0" evaluates false while anything else evaluates true, you can use:

!mystr.Equals("0");

The shared FormatHelperclass shown below provides a simple solution using two variations of an overloaded method called StringToBoolean.

FormatHelper.StringToBoolean(String value)
FormatHelper.StringToBoolean(String value, Boolean NullOrEmptyDefault)

Both variations provide a case-insentive string match

1) The normal convertion from string to boolean defaulting empty or null strings to false

The following examples will result in a boolean value of false:-

 Boolean myBool = FormatHelper.StringToBoolean("");
 Boolean myBool = FormatHelper.StringToBoolean("0");
 Boolean myBool = FormatHelper.StringToBoolean("false");
 Boolean myBool = FormatHelper.StringToBoolean("False");
 Boolean myBool = FormatHelper.StringToBoolean("no");
 Boolean myBool = FormatHelper.StringToBoolean("off");

All other string values will result in a Boolean value of true such as:-

 Boolean myBool = FormatHelper.StringToBoolean("1");
 Boolean myBool = FormatHelper.StringToBoolean("true");
 Boolean myBool = FormatHelper.StringToBoolean("True");
 Boolean myBool = FormatHelper.StringToBoolean("yes");
 Boolean myBool = FormatHelper.StringToBoolean("xyz blah");

Note: Edit the value of BooleanStringOff in the class below to include more (or less) values for false/off

2) Follows the same rules as 1) above but allows a default value of true to be supplied as the 2nd argument to the conversion.

The default value is used when the String value is empty or null. This is useful if a missing string value needs to signify a true state.

The following code example will return true

 Boolean myBool = FormatHelper.StringToBoolean("",true);

The following code example will return false

 Boolean myBool = FormatHelper.StringToBoolean("false",true);

This is the code for the FormatHelper class

public class FormatHelper
{
    public static Boolean StringToBoolean(String str)
    {
        return StringToBoolean(str, false);
    }

    public static Boolean StringToBoolean(String str, Boolean bDefault)
    {
        String[] BooleanStringOff = { "0", "off", "no" };

        if (String.IsNullOrEmpty(str))
            return bDefault;
        else if(BooleanStringOff.Contains(str,StringComparer.InvariantCultureIgnoreCase))
            return false;

        Boolean result;
        if (!Boolean.TryParse(str, out result))
            result = true;

        return result;
    }
}