235 lines
7.7 KiB
HTML
235 lines
7.7 KiB
HTML
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<title>NUnit - ExceptionAsserts</title>
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<a id="logo" href="http://www.nunit.org"><img src="img/logo.gif" alt="NUnit.org" title="NUnit.org"></a>
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<a href="http://www.nunit.org">NUnit</a>
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<div id="content">
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<h2>Exception Asserts (NUnit 2.5)</h2>
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<p>The <b>Assert.Throws</b> method is pretty much in a class by itself. Rather than
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comparing values, it attempts to invoke a code snippet, represented as
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a delegate, in order to verify that it throws a particular exception.
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<p>It's also in a class by itself in that it returns an Exception, rather
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than void, if the Assert is successful. See the example below for
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a few ways to use this.
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<p><b>Assert.Throws</b> may be used with a constraint argument, which is applied
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to the actual exception thrown, or with the Type of exception expected.
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The Type format is available in both both a non-generic and (in the .NET 2.0 version)
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generic form.
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<p><b>Assert.DoesNotThrow</b> simply verifies that the delegate does not throw
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an exception.
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<p><b>Assert.Catch</b> is similar to <b>Assert.Throws</b> but will pass for an exception
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that is derived from the one specified.
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<div class="code" style="width: 40em"><pre>
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Exception Assert.Throws( Type expectedExceptionType, TestDelegate code );
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Exception Assert.Throws( Type expectedExceptionType, TestDelegate code,
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string message );
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Exception Assert.Throws( Type expectedExceptionType, TestDelegate code,
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string message, params object[] parms);
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Exception Assert.Throws( IResolveConstraint constraint, TestDelegate code );
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Exception Assert.Throws( IResolveConstraint constraint, TestDelegate code,
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string message );
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Exception Assert.Throws( IResolveConstraint constraint, TestDelegate code,
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string message, params object[] parms);
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T Assert.Throws<T>( TestDelegate code );
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T Assert.Throws<T>( TestDelegate code, string message );
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T Assert.Throws<T>( TestDelegate code, string message,
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params object[] parms);
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void Assert.DoesNotThrow( TestDelegate code );
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void Assert.DoesNotThrow( TestDelegate code, string message );
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void Assert.DoesNotThrow( TestDelegate code, string message,
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params object[] parms);
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Exception Assert.Catch( TestDelegate code );
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Exception Assert.Catch( TestDelegate code, string message );
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Exception Assert.Catch( TestDelegate code, string message,
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params object[] parms);
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Exception Assert.Catch( Type expectedExceptionType, TestDelegate code );
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Exception Assert.Catch( Type expectedExceptionType, TestDelegate code,
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string message );
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Exception Assert.Catch( Type expectedExceptionType, TestDelegate code,
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string message, params object[] parms);
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T Assert.Catch<T>( TestDelegate code );
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T Assert.Catch<T>( TestDelegate code, string message );
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T Assert.Catch<T>( TestDelegate code, string message,
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params object[] parms);
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</pre></div>
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<p>In the above code <b>TestDelegate</b> is a delegate of the form
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<b>void TestDelegate()</b>, which is used to execute the code
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in question. Under .NET 2.0, this may be an anonymous delegate.
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If compiling under C# 3.0, it may be a lambda expression.
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<p>The following example shows different ways of writing the
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same test.
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<div class="code"><pre>
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[TestFixture]
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public class AssertThrowsTests
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{
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[Test]
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public void Tests()
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{
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// .NET 1.x
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Assert.Throws( typeof(ArgumentException),
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new TestDelegate(MethodThatThrows) );
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// .NET 2.0
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Assert.Throws<ArgumentException>( MethodThatThrows() );
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Assert.Throws<ArgumentException>(
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delegate { throw new ArgumentException(); } );
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// Using C# 3.0
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Assert.Throws<ArgumentException>(
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() => throw new ArgumentException(); } );
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}
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void MethodThatThrows()
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{
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throw new ArgumentException();
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}
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</pre></div>
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<p>This example shows use of the return value to perform
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additional verification of the exception.
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<div class="code"><pre>
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[TestFixture]
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public class UsingReturnValue
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{
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[Test]
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public void TestException()
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{
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MyException ex = Assert.Throws<MyException>(
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delegate { throw new MyException( "message", 42 ); } );
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Assert.That( ex.Message, Is.EqualTo( "message" ) );
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Assert.That( ex.MyParam, Is.EqualTo( 42 ) );
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}
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</pre></div>
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<p>This example does the same thing
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using the overload that includes a constraint.
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<div class="code"><pre>
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[TestFixture]
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public class UsingConstraint
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{
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[Test]
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public void TestException()
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{
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Assert.Throws( Is.Typeof<MyException>()
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.And.Message.EqualTo( "message" )
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.And.Property( "MyParam" ).EqualTo( 42 ),
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delegate { throw new MyException( "message", 42 ); } );
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}
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</pre></div>
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<p>Use the form that matches your style of coding.
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<h3>Exact Versus Derived Types</h3>
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<p>When used with a Type argument, <b>Assert.Throws</b> requires
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that exact type to be thrown. If you want to test for any
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derived Type, use one of the forms that allows specifying
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a constraint. Alternatively, you may use <b>Assert.Catch</b>,
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which differs from <b>Assert.Throws</b> in allowing derived
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types. See the following code for examples:
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<div class="code">
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<pre>
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// Require an ApplicationException - derived types fail!
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Assert.Throws( typeof(ApplicationException), code );
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Assert.Throws<ApplicationException>()( code );
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// Allow both ApplicationException and any derived type
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Assert.Throws( Is.InstanceOf( typeof(ApplicationException), code );
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Assert.Throws( Is.InstanceOf<ApplicationException>(), code );
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// Allow both ApplicationException and any derived type
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Assert.Catch<ApplicationException>( code );
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// Allow any kind of exception
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Assert.Catch( code );
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</pre>
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</div>
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<h4>See also...</h4>
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<ul>
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<li><a href="throwsConstraint.html">ThrowsConstraint</a></ul>
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Copyright © 2010 Charlie Poole. All Rights Reserved.
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