Exceptions and Defensive Coding

Throw and Catch

A method can throw an exception and a caller can catch it.

throw `throw` reports a problem that the caller should handle.

Throw and Catch

age
ThrowCatch.cs
Replay: real traced execution (multi-file project)
using System;

class Program
{
    static string AgeGroup(int age)
    {
        if (age < 0)
        {
            throw new ArgumentException("age");
        }

        if (age < 18)
        {
            return "minor";
        }

        return "adult";
    }

    static void Main()
    {
        int age = 16;
        string group = "unknown";

        try
        {
            group = AgeGroup(age);
        }
        catch (ArgumentException)
        {
            group = "invalid";
        }

        Console.WriteLine($"age={age}");
        Console.WriteLine($"group={group}");
    }
}

using System;

class Program
{
    static string AgeGroup(int age)
    {
        if (age < 0)
        {
            throw new ArgumentException("age");
        }

        if (age < 18)
        {
            return "minor";
        }

        return "adult";
    }

    static void Main()
    {
        int age = -1;
        string group = "unknown";

        try
        {
            group = AgeGroup(age);
        }
        catch (ArgumentException)
        {
            group = "invalid";
        }

        Console.WriteLine($"age={age}");
        Console.WriteLine($"group={group}");
    }
}

using System;

class Program
{
    static string AgeGroup(int age)
    {
        if (age < 0)
        {
            throw new ArgumentException("age");
        }

        if (age < 18)
        {
            return "minor";
        }

        return "adult";
    }

    static void Main()
    {
        int age = 21;
        string group = "unknown";

        try
        {
            group = AgeGroup(age);
        }
        catch (ArgumentException)
        {
            group = "invalid";
        }

        Console.WriteLine($"age={age}");
        Console.WriteLine($"group={group}");
    }
}

  1. age ← 16, group ← unknown

    20static void Main()21{22    int age→ 16 = 16; //@age=-1, 2123    string group→ unknown = "unknown";

  2. try

    25try26{27    group = AgeGroup(age16);28}

  3. static string AgeGroup(int age)

    4{5    static string AgeGroup(int age16)6    {7        if (age < 0)

  4. if (age < 18)

    12if (age16 < 18)13{14    return "minor";15}

  5. group ← minor

    26{27    group→ minor = AgeGroup(age16);28}

  6. Console.WriteLine($"age={age}");

    34    Console.WriteLine($"age={age16}");35    Console.WriteLine($"group={groupminor}");36}
    outputage=16
group=minor

  1. age ← -1, group ← unknown

    20static void Main()21{22    int age→ -1 = -1;23    string group→ unknown = "unknown";

  2. try

    25try26{27    group = AgeGroup(age-1);28}

  3. static string AgeGroup(int age)

    4{5    static string AgeGroup(int age-1)6    {7        if (age < 0)

  4. if (age < 0)

    6{7    if (age-1 < 0)8    {9        throw new ArgumentException("age");10    }

  5. group ← invalid

    28}29catch (ArgumentException)30{31    group→ invalid = "invalid";32}

  6. Console.WriteLine($"age={age}");

    34    Console.WriteLine($"age={age-1}");35    Console.WriteLine($"group={groupinvalid}");36}
    outputage=-1
group=invalid

  1. age ← 21, group ← unknown

    20static void Main()21{22    int age→ 21 = 21;23    string group→ unknown = "unknown";

  2. try

    25try26{27    group = AgeGroup(age21);28}

  3. static string AgeGroup(int age)

    4{5    static string AgeGroup(int age21)6    {7        if (age < 0)8        {9            throw new ArgumentException("age");10        }1112        if (age < 18)13        {14            return "minor";15        }1617        return "adult";18    }

  4. group ← adult

    26{27    group→ adult = AgeGroup(age21);28}

  5. Console.WriteLine($"age={age}");

    34    Console.WriteLine($"age={age21}");35    Console.WriteLine($"group={groupadult}");36}
    outputage=21
group=adult