copying one Array value to another array

You can't just project an array like that. You effectively have to loop - although you don't need to do that manually in your own code. LINQ makes it very easy, for example:

RateInfos[] rt = hr.Select(x => x.RateInfo).ToArray();

Or you could use Array.ConvertAll:

RateInfos[] rt = Array.ConvertAll(hr, x => x.RateInfo);

In both of these cases there's still a loop somewhere - it's just not in your code.

If you're quite new to C# and don't understand LINQ, lambda expressions, delegates etc yet, then you could just write the code yourself:

RateInfos[] rt = new RateInfos[hr.Length];
for (int i = 0; i < rt.Length; i++)
{
    rt[i] = hr[i].RateInfo;
}

All of these three will achieve the same result.

• The first approach is probably the most idiomatic in modern C#. It will work with any input type, and you can change from ToArray() to ToList() to get a List<RateInfos> instead of an array, etc.
• The second approach is slightly more efficient than the first and will work with .NET 2.0 (whereas LINQ was introduced in .NET 3.5) - you'll still need a C# 3 compiler or higher though. It will only work as written with arrays, but there's a similar ConvertAll method for List<T>.
• The third approach is the most efficient, but obviously more code as well. It's simpler for a newcomer to understand, but doesn't express what you're trying to achieve as clearly when you know how all the language features work for the first two solutions.