Essentials C＃Framework 3.5示例代码中的书错误

Question

I am reading the book Essentials C# 3.0 for .NET framework 3.5 from Mark Michaelis. Since there are more classes involved I was hoping somebody had worked through the book and maybe had the same problem.

The code in chapter 7 fails(page 300). Listing 7.2 shows how to integrate an interface, I've written all of the code like it says in the book. I'm getting the error:

'xxxx.ConsoleListControl.DisplayHeader(string[])': not all code path returns a value.

The code in question is:

    public static void List(string[] headers, Ilistable[] items)
    {
        int[] columnWidths = DisplayHeaders(headers);

        for (int count = 0; count < items.Length; count++)
        {
            string[] values = items[count].ColumnValues;
            DisplayItemsRow(columnWidths, values);
        }
    }

    /// <summary>
    /// Displays the column headers
    /// </summary>
    /// <returns>returns an array of column widths</returns>
    private static int[] DisplayHeaders(string[] headers)
    {

    }

    private static void DisplayItemsRow(int[] columnWidths,string[] values)
    {

    }
}

The string[] headers arefilled with 4 items (FirstName, LastName, Address, Phone). I don't know what is causing this problem, or how to fix it. I see DisplayHeaders has no value, and columnwidths also has no value.

I haven't put all of the code here; there are 5 classes and 1 interface. I thought maybe that would be to much and not be needed. If somebody wants all the code I will be happy to put it here.

Answer 1

Turn the page, or read again. I guess you're supposed to write code in the method, as it has a return type but no return statement.

Edit: alright, downloaded the PDF, the book explicitly says above this code listing:

Consider another example

And in the code it says:

private static int[] DisplayHeaders(string[] headers)
{
    // ...
}

The // ... part indicates something not interesting to the concept being explained is left out for brevity.

The code is shown to explain what an interface can do (in this case printing a list of any kind of object that implements Ilistable ), the static helper methods are irrelevant to this. The code is not meant to be run.

Answer 2

Any method that has a type other than void must return an object of that type. So DisplayHeaders must return an integer array.

private static int[] DisplayHeaders(string[] headers)

private - access modifier; indicates this method can only be called from within the class

static - static modifier; this method does not need an instance to be called

int[] - return type; this is the type of the object that this method will return

DisplayHeaders - method name; this is how you refer to this method

(string[] headers) - parameters; this indicates which arguments you need to pass to the method

We can infer from the method summary that its implementation may look something like this:

    /// <summary>
    /// Displays the column headers
    /// </summary>
    /// <returns>returns an array of column widths</returns>
    private static int[] DisplayHeaders(string[] headers)
    {
        // builds a new int array with the same 
        // number of elements as the string array parameter
        int[] widths = new int[headers.Length];

        for (int i = 0; i < headers.Length; i++)
        {
            Console.WriteLine(headers[i]); // displays each header in the Console
            widths[i] = headers[i].Length; // populates the array with the string sizes
        }

        // the return keyword instructs the program to send the variable 
        // that follows back to the code that called this method
        return widths; 
    }

I would continue reading the chapter. More than likely the author fills in the implementation details of that method later on.

Answer 3

The method DisplayHeaders says it returns an array of integers ( int[] ) but it is not in fact returning anything. There is quite likely code a little bit later on that fills in the method to do something useful, but in order to make the code compile, it needs to return an array. A simple way to do that would be to change it to

private static int[] DisplayHeaders(string[] headers)
{
    return new int[0];
}

This causes it to return an empty array of integers.