Assigning out and ref parameters

Sometimes methods have out or ref parameters that need to be filled in when the faked method is called. Use AssignsOutAndRefParameters:

string theValue;
A.CallTo(()=>aFakeDictionary.TryGetValue(theKey, out theValue))
 .Returns(true) 
 .AssignsOutAndRefParameters(someCoolValue);

AssignsOutAndRefParameters takes a params object[], with one element (in order) for each of the out and ref parameters in the call being faked - the other arguments to the method should be omitted.

While assigning out and ref parameters, the Returns method (or some variant) should be used to specify the return value for the method - AssignsOutAndRefParameters does not do this on its own.

Assigning Values Calculated at Call Time

When out or ref parameter values aren't known until the method is called, AssignsOutAndRefParametersLazily can be used.

string theValue;
A.CallTo(() => aFakeDictionary.TryGetValue(theKey, out theValue))
 .Returns(true) 
 .AssignsOutAndRefParametersLazily((string aKey, string aValue) => new [] { aValue + aValue });

As shown above, the inputs to the method may be used to calculate the values to assign. Convenient overloads exist for methods of up to four parameters.

The type of the Func sent to AssignsOutAndRefParametersLazily isn't checked at compile time, but any type mismatch should trigger a helpful error message.

If more advanced decision-making is required, or the method has more than 4 parameters, the convenience methods won't work. Use the variant that takes an IFakeObjectCall instead:

string theValue;
A.CallTo(() => aFakeDictionary.TryGetValue(theKey, out theValue))
 .Returns(true) 
 .AssignsOutAndRefParametersLazily(objectCall => calculateValuesFrom(objectCall));

The IFakeObjectCall object provides access to

  • information about the Method being called, as a MethodInfo,
  • the Arguments, accessed by position or name, and
  • the original FakedObject

Implicitly Assigning out Parameter Values

Any Expression-based A.CallTo configuration that's made on a method that has an out parameter will cause the value of the variable used in the A.CallTo to be assigned to the out parameter when the method is actually called. For example:

string configurationValue = "lollipop";
A.CallTo(()=>aFakeDictionary.TryGetValue(theKey, out configurationValue))
 .Returns(true); 

string fetchedValue;
aFakeDictionary.TryGetValue(theKey, out fetchedValue);

// fetchedValue is now "lollipop";

If this behaviour is not desired, AssignsOutAndRefParameters (or …Lazily) can be used to provide different behaviour.