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<h3>ExpectedExceptionAttribute (NUnit 2.0 plus Updates)</h3>
<p>This is the way to specify that the execution of a test will throw an
exception. This attribute has a number of positional and
named parameters, which we will discuss in separate sections
according to the purpose they serve.</p>
<h4>Specifying the Expected Exception Type</h4>
<p>The original attribute, introduced with NUnit 2.0 took a single
argument giving the exact type of the expected exception. For example...</p>
<div class="code"><pre>[ExpectedException( typeof( ArgumentException ) )]
public void TestMethod()
{
...</pre></div>
<p>Beginning with NUnit 2.2.4, it became possible to specify the type
of exception as a string, avoiding the need for a reference to the
defining assembly...</p>
<div class="code"><pre>[ExpectedException( "System.ArgumentException" ) )]
public void TestMethod()
{
...</pre></div>
<p>The above two examples function identically: the test only succeeds if a
System.Argument exception is thrown.</p>
<h4>Specifying the Expected Message</h4>
<p>NUnit 2.1 introduced a constructor with a second argument, specifying the
exact text of the message property of the exception. After NUnit 2.2.4, the
same extension was made to the constructor taking a string argument. With
NUnit 2.4, these arguments are marked obsolete, and a named parameter
is provided instead...</p>
<div class="code" style="width: 44em"><pre>// Obsolete form:
[ExpectedException( typeof( ArgumentException ), "expected message" )]
[ExpectedException( "System.ArgumentException", "expected message" )]
// Prefered form:
[ExpectedException( typeof( ArgumentException ), ExpectedMessage="expected message" )]
[ExpectedException( "System.ArgumentException", ExpectedMessage="expected message" )]</pre></div>
<p>With NUnit 2.4, it is possible to specify additional tests on the
exception message, beyond a simple exact match. This is done using the
MatchType named parameter, whose argument is an enumeration, defined as
follows:</p>
<div class="code">
<pre>public enum MessageMatch
{
/// Expect an exact match
Exact,
/// Expect a message containing the parameter string
Contains,
/// Match the regular expression provided as a parameter
Regex,
/// Expect a message starting with the parameter string
StartsWith
}</pre></div>
<p>The following example is for a test that passes only if an ArgumentException
with a message containing "unspecified" is received.</p>
<div class="code" style="width: 57em">
<pre>[ExpectedException( typeof( ArgumentException), ExpectedMessage="unspecified", MatchType=MessageMatch.Contains )]
public void TestMethod()
{
...</pre></div>
<p>If MatchType is not specified, an exact match is used as before.</p>
<h4>Specifying a Custom Error Message</h4>
<p>With NUnit 2.4, it is possible to specify a custom message to be
displayed if the ExpectedException attribute is not satisfied. This
is done through the UserMessage named parameter...</p>
<div class="code" style="width: 41em">
<pre>[ExpectedException( typeof( ArgumentException ), UserMessage="Custom message" )]
public void TestMethod()
{
...</pre>
</div>
<h4>Handling the Exception in Code</h4>
<p>If the processing required for an exception is too complex to express
in the attribute declaration, the normal practice is to process it in the
test code using a try/catch block. As an alternative, NUnit 2.4 allows
designating a method that will be called to process the exception. This
is particularly useful when multiple exceptions need to be processed
in the same way.</p>
<p>An common exception handler may be designated by implementing the
<b>IExpectExceptionInterface</b>, which is defined as follows...</p>
<div class="code">
<pre>public interface IExpectException
{
void HandleException( System.Exception ex );
}</pre>
</div>
<p>The exception handler is only called for methods marked with
the <b>ExpectedException</b> attribute. If all checks - including
the type of the exception - are to be performed in code, the
attribute may be specified without any arguments in order to
indicate that an exception is expected.</p>
<p>An handler may be designated for a particular method
using the <b>Handler</b> named parameter.</p>
<div class="code"><pre>[ExpectedException( Handler="HandlerMethod" )]
public void TestMethod()
{
...
}
public void HandlerMethod( System.Exception ex )
{
...
}</pre></div>
<p>This technique may be
used without implementing <b>IExpectException</b> or in
combination with it. In the latter case, the designated handler
applies to any method that specifies it, while the normal
exception handler applies to any other methods that specify
an <b>ExpectedException</b>.</p>
<p>However it is specified, the handler method should examine the exception and
<b>Assert</b> on whatever properties are relevant. Any resulting failure message
will then be consistent in format with other assertions performed in the tests.</p>
<h4>Example:</h4>
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<pre class="cs">namespace NUnit.Tests
{
using System;
using NUnit.Framework;
[TestFixture]
public class SuccessTests
{
[Test]
[ExpectedException(typeof(InvalidOperationException))]
public void ExpectAnExceptionByType()
{ /* ... */ }
[Test]
[ExpectedException("System.InvalidOperationException")]
public void ExpectAnExceptionByName()
{ /* ... */ }
}
}
</pre>
<pre class="vb">Imports System
Imports Nunit.Framework
Namespace Nunit.Tests
<TestFixture()> Public Class SuccessTests
<Test(), ExpectedException(GetType(Exception))>
Public Sub ExpectAnExceptionByType()
' ...
End Sub
<TestFixture()> Public Class SuccessTests
<Test(), ExpectedException("System.Exception")>
Public Sub ExpectAnExceptionByName()
' ...
End Sub
End Class
End Namespace
</pre>
<pre class="mc">#using <Nunit.Framework.dll>
using namespace System;
using namespace NUnit::Framework;
namespace NUnitTests
{
[TestFixture]
public __gc class SuccessTests
{
[Test]
[ExpectedException(__typeof(InvalidOperationException))]
void ExpectAnExceptionByType();
[Test]
[ExpectedException(S"SystemInvalidOperationException")]
void ExpectAnExceptionByName();
};
}
#include "cppsample.h"
namespace NUnitTests {
// ...
}
</pre>
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