尚不清楚您所说的大于startHour是什么意思...但是

It's not at all clear what you mean by "is greater than startHour"... but taking

TimeSpan startHour = new TimeSpan(12, 25, 43);
if (endHour.TimeOfDay > startHour)
{
    ...
}

...的工作非常简单。

... works pretty simply.

总而言之，请添加参数检查以确保您未为startHour指定值< 0或> 23小时，但这很容易。

By all means add argument checking to make sure that you don't specify a value for startHour which is < 0 or > 23 hours, but that's all pretty easy.

.NET的日期和时间API与Joda Time相比是相当原始的（即使在3.5中）-但在这种特殊情况下，我认为还不错。

.NET's date and time API is quite primitive (even in 3.5) compared with, say, Joda Time - but in this particular case I think it's not too bad.