C# ガイド · 5/6

Errors and Testing

この章は現在、英語でのみ提供しています。

C# reports errors with exceptions. When one is thrown, execution unwinds up the call stack until a matching catch block handles it. This chapter covers handling exceptions, cleaning up with using, custom exception types, and unit testing with dotnet test.

try, catch, and finally

The first catch whose type matches wins, so list specific types before general ones. finally always runs, which makes it the place for cleanup.

try
{
    var text = File.ReadAllText("config.json");
    var port = int.Parse(text.Trim());
    Console.WriteLine($"Port: {port}");
}
catch (FileNotFoundException ex)
{
    Console.WriteLine($"Config file not found: {ex.FileName}");
}
catch (FormatException)
{
    Console.WriteLine("The port number is not a valid integer.");
}
catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
{
    Console.WriteLine($"Could not read the file: {ex.Message}");
}
finally
{
    Console.WriteLine("Finished reading configuration.");
}

The when clause is an exception filter: the handler runs only if the condition holds. To rethrow, use a bare throw; so the original stack trace survives; throw ex; restarts the trace and hides the real origin.

When failure is routine, as with user input, prefer the Try pattern: int.TryParse(input, out var value) just returns false.

Guard Clauses and Custom Exceptions

Fail fast on bad arguments using the built-in throw helpers. For domain errors, such as a broken business rule, derive your own type from Exception so callers can catch exactly that case.

public class InsufficientFundsException : Exception
{
    public InsufficientFundsException(decimal balance, decimal requested)
        : base($"Cannot withdraw {requested:F2} from a balance of {balance:F2}.")
    {
        Balance = balance;
        Requested = requested;
    }

    public decimal Balance { get; }
    public decimal Requested { get; }
}

public class Wallet(decimal balance)
{
    public decimal Balance { get; private set; } = balance;

    public void Withdraw(decimal amount, string reason)
    {
        ArgumentOutOfRangeException.ThrowIfNegativeOrZero(amount);
        ArgumentException.ThrowIfNullOrWhiteSpace(reason);

        if (amount > Balance)
            throw new InsufficientFundsException(Balance, amount);

        Balance -= amount;
    }
}

Exception class names end in Exception by convention. Wallet(decimal balance) is a primary constructor, whose parameter is visible across the class.

using and IDisposable

Files, sockets, and database connections implement ; releases what they hold, and guarantees that call even when an exception is thrown.

この章について質問がありますか？

Task	xUnit	MSTest
Test method	`[Fact]`	`[TestMethod]`
Parameterized test	`[Theory]` + `[InlineData]`	`[TestMethod]` + `[DataRow]`
Equality check	`Assert.Equal`	`Assert.AreEqual`
Per-test setup	constructor	`[TestInitialize]`

Errors and Testing

try, catch, and finally

Guard Clauses and Custom Exceptions

using and IDisposable

Unit Testing with xUnit

If You Prefer MSTest

Summary

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