How to create a method which accepts a lambda expression - c#-3.0

Question

So I find myself writing this code all the time:
[TestMethod]
[Description("Asserts that an ArgumentNullException is thrown if ResetPassword(null) is called")]
public void ResetPassword_Throws_ArgumentNullException_With_Null_Parameter( )
{
try
{
new MembershipServiceProvider( ).ResetPassword( null );
}
catch ( ArgumentNullException )
{
// ArgumentNullException was expected
Assert.IsTrue( true );
}
catch
{
Assert.Fail( "ArgumentNullException was expected" );
}
}
So instead of writing this code over and over I'd really like to create a method which accepts a Lambda expression which will execute the method in the try block.
Something like this:
public void AssertExpectedException( Action theAction ) where TException : Exception
{
try
{
// Execute the method here
}
catch ( TException )
{
Assert.IsTrue( true );
}
catch
{
Assert.Fail( string.Format( "An exception of type {0} was expected", typeof( TException ) ) );
}
}
So I can do something like this:
var provider = new MembershipServiceProvider();
AssertExpectedException(provider => provider.ResetPassword(null));
I'm really not sure if any of this is on the right track but hopefully someone can point me in the right direction.
Thanks

Answer 1

You're almost there. Here's what the test helper should look like:
public void AssertExpectedException<TException>( Action theAction )
where TException : Exception
{
try
{
// Execute the method here
theAction();
}
catch ( TException )
{
// The Assert here is not necessary
}
catch
{
Assert.Fail( string.Format(
"An exception of type {0} was expected",
typeof(TException)));
}
}
And to call it:
var provider = new MembershipServiceProvider();
AssertExpectedException<ArgumentNullException>(() => provider.ResetPassword(null));
Note the usage of () => something which means the lambda has no parameters. You also have to specify the generic argument of ArgumentNullException because the compiler cannot infer it.

Answer 2

The following should do what you need (I've added the type parameter for TException and the invocation of theAction).
public void AssertExpectedException<TException>(Action theAction)
where TException : Exception
{
try
{
theAction();
}
catch (TException)
{
Assert.IsTrue(true);
}
catch
{
Assert.Fail(string.Format("An exception of type {0} was expected",
typeof(TException)));
}
}
You can call it with the following code:
var provider = new MembershipServiceProvider();
AssertExpectedException<ArgumentNullException>(() => provider.ResetPassword(null));
You need to specify the type argument to indicate which type of exception to test for. The () => ... syntax is a lambda expression that takes no parameters.