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stewartrae on 05/24/11

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ExceptionAssert - a useful class for MSTest that works like Assert.Throws in NUnit

/ Published in: C#

Are you using MSTest as a unit testing framework? Do you miss the Assert.Throws method in NUnit? Never fear! You can use this class - ExceptionAssert - like Assert.Throws within MSTest unit tests.

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/// <summary>
    /// Useful assertions for actions that are expected to throw an exception.
    /// </summary>
    public static class ExceptionAssert
    {
        /// <summary>
        /// Executes an exception, expecting an exception to be thrown.
        /// Like Assert.Throws in NUnit.
        /// </summary>
        /// <param name="action">The action to execute</param>
        /// <returns>The exception thrown by the action</returns>
        public static Exception Throws(Action action)
        {
            return Throws(action, null);
        }

/// <summary>
        /// Executes an exception, expecting an exception to be thrown.
        /// Like Assert.Throws in NUnit.
        /// </summary>
        /// <param name="action">The action to execute</param>
        /// <param name="message">The error message if the expected exception is not thrown</param>
        /// <returns>The exception thrown by the action</returns>
        public static Exception Throws(Action action, string message)
        {
            try
            {
                action();
            }
            catch (Exception ex)
            {
                // The action method has thrown the expected exception.
                // Return the exception, in case the unit test wants to perform further assertions on it.
                return ex;
            }

// If we end up here, the expected exception was not thrown. Fail!
            throw new AssertFailedException(message ?? "Expected exception was not thrown.");
        }
        
        /// <summary>
        /// Executes an exception, expecting an exception of a specific type to be thrown.
        /// Like Assert.Throws in NUnit.
        /// </summary>
        /// <param name="action">The action to execute</param>
        /// <returns>The exception thrown by the action</returns>
        public static T Throws<T>(Action action) where T : Exception
        {
            return Throws<T>(action, null);
        }

/// <summary>
        /// Executes an exception, expecting an exception of a specific type to be thrown.
        /// Like Assert.Throws in NUnit.
        /// </summary>
        /// <param name="action">The action to execute</param>
        /// <param name="message">The error message if the expected exception is not thrown</param>
        /// <returns>The exception thrown by the action</returns>
        public static T Throws<T>(Action action, string message) where T : Exception
        {
            try
            {
                action();
            }
            catch (Exception ex)
            {
                T actual = ex as T;
                if (actual == null)
                {
                    throw new AssertFailedException(message ?? String.Format("Expected exception of type {0} not thrown. Actual exception type was {1}.", typeof(T), ex.GetType()));
                }

// The action method has thrown the expected exception of type 'T'.
                // Return the exception, in case the unit test wants to perform further assertions on it.
                return actual;
            }

// If we end up here, the expected exception of type 'T' was not thrown. Fail!
            throw new AssertFailedException(message ?? String.Format("Expected exception of type {0} not thrown.", typeof(T)));
        }
    }

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