如何在C#整数输入中兼容"null"字符串并避免未处理异常？

Got it, let's break down why your code is throwing that exception and fix it properly.

The Problem with Your Current Code #

When you input "null", Convert.ToInt32(Console.ReadLine()) tries to convert that string to an integer—and since "null" isn't a valid numeric value, it immediately throws a FormatException before your if (age == null) check even runs. That's why you're seeing the unhandled error.

The Fix: Handle Input as a String First #

We need to capture the raw input string first, check if it's "null", and only then attempt to convert it to an integer (safely, to avoid other potential errors). Here's a revised version of your code that works as expected:

int? age = null;
Console.Write("\nInsert age: ");
string userInput = Console.ReadLine();

// Check if the input is exactly "null" (use StringComparison.OrdinalIgnoreCase if you want to accept "Null"/"NULL" too)
if (string.Equals(userInput, "null", StringComparison.Ordinal))
{
    age = null;
}
else
{
    // Use int.TryParse for safe conversion—no exceptions if input is invalid
    if (int.TryParse(userInput, out int parsedAge))
    {
        age = parsedAge;
    }
    else
    {
        // Optional: Handle cases where input is neither "null" nor a valid integer
        Console.WriteLine("Oops, that's not a valid integer or 'null'. Let's default to empty age.");
        age = null;
    }
}

// Output the result
if (age == null)
{
    Console.WriteLine("Your age:");
}
else
{
    Console.WriteLine($"Your age: {age}");
}

Key Improvements: #

• Raw Input Capture: We first read the input as a string, so we can check for "null" before any conversion attempts.
• Safe Conversion: int.TryParse replaces Convert.ToInt32 because it returns a boolean indicating success/failure instead of throwing an exception. This makes your code more robust against typos or invalid inputs.
• Flexibility: If you want to accept case-insensitive "null" inputs (like "Null" or "NULL"), just swap StringComparison.Ordinal with StringComparison.OrdinalIgnoreCase in the string.Equals check.

This code will now:

• Output Your age: when you input "null"
• Output Your age: [number] when you input a valid integer
• Gracefully handle other invalid inputs (with an optional error message) instead of crashing.

内容的提问来源于stack exchange，提问作者James esmenya