Question

I have this code copied from C# 4.0 in a nutshell that uses Attribute/Reflection API to control the action: the number of tests to run and show error message.

When I run the code, I got this result.

Method Method1 will be tested; reps=1; msg=; memo=
Test1
Method Method2 will be tested; reps=3; msg=; memo=abc
Test2
Test2
Test2
Method Method3 will be tested; reps=3; msg=Debugging Time!; memo=
Test3
Test3
Test3

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object
  at Hello.Main () [0x00000] in <filename unknown>:0 

Why is this Unhandled Exception?

The source code is as follows :

using System;
using System.Reflection;

[AttributeUsage (AttributeTargets.Method)]
public sealed class TestAttribute : Attribute
{
    public int Repititions;
    public string FailureMessage;
    public string Memo;

    public TestAttribute() : this(1) {}
    public TestAttribute(int repititions) {Repititions = repititions;}
}

class Foo
{
    [Test]
    public void Method1()
    {
        Console.WriteLine("Test1");
    }
    [Test(3, Memo="abc")]
    public void Method2()
    {
        Console.WriteLine("Test2");
    }
    [Test(3, FailureMessage="Debugging Time!")]
    public void Method3()
    {
        Console.WriteLine("Test3");
    }
}

class Hello
{
    static void Main()
    {
        foreach (MethodInfo mi in typeof(Foo).GetMethods())
        {
            TestAttribute att = (TestAttribute) Attribute.GetCustomAttribute(mi, typeof(TestAttribute)); 
            if (att != null)
                Console.WriteLine("Method {0} will be tested; reps={1}; msg={2}; memo={3}", mi.Name, att.Repititions, att.FailureMessage, att.Memo);
                for (int i = 0; i < att.Repititions; i++)
                    try
                    {
                        mi.Invoke(new Foo(), null);
                    }
                    catch (Exception ex)
                    {
                        throw new Exception ("Error: " + att.FailureMessage, ex);
                    }
        }
    }
}

Answer 1

You were missing some curly braces {} in your if (att != null) and foreach. Indentation alone is not enough.

class Hello
{
    static void Main()
    {
        foreach (MethodInfo mi in typeof(Foo).GetMethods())
        {
            TestAttribute att = (TestAttribute) Attribute.GetCustomAttribute(mi, typeof(TestAttribute)); 
            if (att != null)
            {
                Console.WriteLine("Method {0} will be tested; reps={1}; msg={2}; memo={3}", mi.Name, att.Repititions, att.FailureMessage, att.Memo);
                for (int i = 0; i < att.Repititions; i++)
                {
                    try
                    {
                        mi.Invoke(new Foo(), null);
                    }
                    catch (Exception ex)
                    {
                        throw new Exception ("Error: " + att.FailureMessage, ex);
                    }
                }
            }
        }
    }
}