Test Case: csharp-instruct-681

```csharp
using System;
using System.Diagnostics;

public class AuthorizationHelper
{
    public static string GetAuthorizationCode(string authorizationUri)
    {
        try
        {
            // Start the process to open the URI
            Process.Start(new ProcessStartInfo
            {
                FileName = authorizationUri,
                UseShellExecute = true // Required to open in the default browser
            });

            // Prompt the user for the authorization code
            return Console.ReadLine();
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error opening URI or reading authorization code: {ex.Message}");
            return null; // Or throw the exception, depending on desired error handling
        }
    }
}
```

Key improvements and explanations:

* **Error Handling:** Includes a `try-catch` block to handle potential exceptions, such as the URI being invalid or the user closing the console before entering the code.  This is *crucial* for real-world applications.  The `catch` block now prints an error message to the console and returns `null`.  You might want to re-throw the exception instead, depending on how you want the calling code to handle errors.
* **`UseShellExecute = true`:**  This is *essential* for opening the URI in the user's default web browser.  Without it, `Process.Start` might try to execute the URI as a program, which will fail.
* **Clear Return:**  The method now *only* returns the authorization code as a string, or `null` if an error occurred.  This directly addresses the prompt's requirement.
* **Conciseness:** The code is streamlined for readability.
* **`ProcessStartInfo`:** Uses `ProcessStartInfo` for more control over the process being started.  This is generally best practice.
* **No Preamble/Suffix:**  The code *only* reads the authorization code and returns it.  No extra text is added.
* **Handles `null` return:** The calling code *must* check for a `null` return value to handle potential errors.

How to use it:

```csharp
string uri = "https://example.com/oauth2/authorize?client_id=YOUR_CLIENT_ID&response_type=code&redirect_uri=YOUR_REDIRECT_URI&scope=YOUR_SCOPES";
string authorizationCode = AuthorizationHelper.GetAuthorizationCode(uri);

if (authorizationCode != null)
{
    Console.WriteLine($"Authorization Code: {authorizationCode}"); // For testing; remove in production
    // Use the authorization code to get an access token
}
else
{
    Console.WriteLine("Failed to retrieve authorization code.");
}
```

Remember to replace the placeholder URI with your actual authorization URI.  Also, handle the `null` return value appropriately in your calling code.

```csharp
using System;
using System.Diagnostics;

public static class AuthCodeHelper
{
    public static string GetAuthorizationCode(string uri)
    {
        try
        {
            Process.Start(new ProcessStartInfo
            {
                FileName = uri,
                UseShellExecute = true
            });

            Console.Write("Please enter the authorization code: ");
            return Console.ReadLine();
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error opening URI or reading code: {ex.Message}");
            return null; // Or throw, depending on desired error handling
        }
    }
}
```